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The Solar System
Solar sys.jpg

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These planets are little bigger than dots in the night sky, but if you call them dots, then Earth (in the marble-sized group, topleft) is smaller than a grain of sand.

Wikijunior books welcomes you to the children's book Solar System. Outer space is perhaps the final frontier for humanity. Even though the rest of the solar system objects may seem like tiny dots from Earth, our celestial neighbors are still important to learn about. If, when you grow up, you are going to be an astronaut and travel in space, you will need to know quite a bit about the solar system. And even if you don't travel to space, the things other people do there will affect you, so you need to know about it. Also, if you meet an astronomer or an astronaut, you do not want to sound ignorant! The importance of learning about the solar system has led many experts here at Wikijunior to donate their time and talents to bring this volume together.

Wikibooks is a project of the Wikimedia Foundation, aimed at providing free, easily available, quality reading for adults and children to promote the global spread of knowledge. Traditional publishing houses make the bulk of their income from re-issues of classic books, new books by authors with long track records, or celebrities who are famous in their own right. The chances of a truly good new work being published solely on the basis of merit skyrocket when the traditional business model is overturned and the wellspring of new talent out there is tapped using the Internet.

With this project we have reached a crossroads between the books of yesterday and the encyclopedia of everything for tomorrow. Simply by reading this book and telling others about it, you have advanced the cause of free access to information and of democratizing the field of publishing. Thank you, and once again, welcome.

Studying the Solar SystemEdit

A stamp issued by the Soviet Union, showing Sputnik's planned orbit path around the Earth.

Scientists are still exploring the universe. Whether things are very tiny, like the cells of plants and animals, or very big like a solar system or a galaxy, there is still a lot scientists don’t know.

Scientists who study space are called Astronomers or Astrophysicists. They explore the solar system in two different ways. Astronomers do it by observing celestial bodies through telescopes, while astrophysicists (a specialized class of astronomers) try to explain the observed phenomena using physics, as suggested by the name, and theorize about what is still unseen or unknown.

Telescopes were invented in the early 1600s in Europe and allowed curious scientists like Galileo Galilei to look at very distant things in close-up and see details of the solar system and the universe that nobody had ever seen before. Using his telescope, Galileo was the first person to see the rings around Saturn, and draw a very detailed picture of the moon. He also saw the four largest moons of Jupiter, sometimes called the Galilean moons, and saw spots on the Sun. Telescopes on Earth and in space are still used to explore the Solar System. There are several types of telescopes. The most common ones are optical telescopes, such as Galileo's (optical means relating to light, which what these telescopes see), and radio telescopes, that pick up radio waves from outer space (radio waves occur naturally; they don't have to be made by humans).

Until the 1950s, humans were limited to exploring the Solar System from the ground. However, in 1957 the Soviet Union (now Russia and several other countries) launched the very first satellite, Sputnik 1 (pronounced like spoo-tneek). Since then, humans have been launching vehicles into space to explore the Solar System—some manned (with people) and some unmanned (without people).

Now, the Solar System is full of human-made probes, exploring the planets and moons of the solar system. The probes send back information to Earth that scientists study to figure out what it means. Every year, scientists are learning more about the Solar System. Sometimes they learn things about other worlds that remind us of Earth. Other times, the things they learn are very strange. Everything they learn helps us to understand more about Earth, Earth's history and Earth's neighborhood.

How is the Solar System measured?Edit

It’s important that scientists use measurement to tell how big, how hot or cold, or how far away something is. In science, people use the metric system, which is named after its basic unit the metre. Below is a description of all the types of measurement used in this book.

Distance or length/widthEdit

For indication of measurements, how distant something is, or how long or wide it is, scientists use kilometres or metres. Units of the metric system (a kilometre is 1000 metres, a metre is little more than 3 feet in the old imperial unit system still in use in some regions). Kilometres is often shortened to km and metres is often shortened to m. Kilometres and metres can also be spelled as Kilometers and Meters, but the International Bureau of Weights and Measurements uses the -re versions as the official spelling.

Since distances outside of the Earth get so vast, scientists have also invented new units of measurement to make it easier to measure large distances in space. They invented the Astronomical unit (㍳) which is equivalent to 149 597 871 kilometres. One Astronomical Unit is the approximate distance between the Earth and the Sun. The mean distance between the sun and Neptune (the farthest planet from the sun) is 30.1 ㍳, or 4.503 billion kilometres. This is why it's good to use ㍳ for a big distance like that: 30 distances from the Earth to the sun is easier to understand than four and a half billion kilometres. You might not realize there was something wrong if someone told you the distance from the sun to Neptune was 45.03 million kilometres, but if you thought of it as 0.301 ㍳, you would know it couldn't be right.

In astronomy, they have leveling-up scales similar to the metric (10 mm in 1 cm, 100 cm in 1 m) and customary (12 in. in 1 ft., 3 ft. in 1 yd.) scales. Usually these distances are not used within the solar system, but they are important to know if you want to be an astronomer or astrophysicist.

1 Light-year (ly)=63241.077 ㍳
1 Parsec (pc)=3.26 ly
1 Kiloparsec (kpc)=1000 pc
1 Megaparsec (mpc)=1000 kpc
1 Gigaparsec (gpc)=1000 mpc

To help you visualise just how big some of these are,

4.22 ly=The distance from Earth to the nearest star (Proxima Centauri) other than the sun
1.3 pc=The distance from Earth to Proxima Centauri
34 kpc=The length of the Milky Way
0.76 mpc=The distance from Earth to the nearest Galaxy, the Andromeda Galaxy
14 gpc=The radius of the observable universe


In order to measure how big something is, scientists measure the mass of an object in kilograms or grams. There are 1000 grams in a kilogram. Scientists do not use weight, because weight is a measurement that means how hard gravity is pulling on an object. An object’s mass is the same wherever you are in the solar system because it measures how much stuff, or matter, a thing is made up of. Your weight will change because the amount of gravity there is varies from place to place.

On Earth, mass and weight are the same. If you weigh 30kg (short for kilograms) on Earth, your mass is 30kg. If you are floating around in space, your weight if you try to stand on a set of scales will be 0kg, but your mass is still 30kg. You are still made up of the same amount of matter.


Temperature is a numeric reference, in a scale of degrees, on how hot or cold something is in relation to a "constant" reference. There are several scales. In our everyday lives we measure temperature in degrees Celsius, written °C for short (the little circle ° means "degrees"); or degrees Fahrenheit, written °F for short. But scientists, especially astronomers, use degrees Kelvin to measure temperature, written K for short (with no °). Don't use Celsius or Fahrenheit for temperatures in astronomy!

Some important temperatures to know in Kelvin:

  • 0K (−273.15°C) is maximum coldness, also called absolute zero. This is a great thing about measuring temperature in Kelvin, that the number is always positive, and tells you how much warmer things are than the coldest they could possibly be.
  • The freezing point of water is 273.15K (0°C), and the boiling point of water is 373.15K (100°C).
  • A sunny day of 30°C, 86°F, would be 303.15K. This is 273.15 + 30, because degrees change equally in Kelvin and Celsius.

Introduction for Parents, Guardians, and EducatorsEdit

The Solar System is a Wikijunior book written by a group of volunteers and made freely available to Internet users, printers, and distributors under the terms of its license. It is the result of cooperation between The Beck Foundation, The Wikimedia Foundation, and volunteer writers and editors.

The volunteer writers and contributors thank you for obtaining this book. By making it available to a young person you complete the goal of the Wikijunior project to encourage reading and literacy among young people.

The original text and graphics are available at and printed versions may be available from many different entities under license.

Again, thank you, and enjoy.

Next Topic: Solar System

Solar SystemEdit

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The Hubble Space Telescope. This telescope is in space. It takes pictures of things that are too far away to be seen with a regular telescope.
The eight planets of the Solar System, and the sun. Sizes are to scale, but distances are not.

Do you ever wonder about the things in the sky—the Sun, the Moon, the stars? People have been watching the sky for a long time, trying to figure out what is out there. We keep coming up with new ways to learn more about outer space.

Planets are big balls of rock or gas that move around stars. We live on one we call the Earth, which moves around a star we call the Sun. There are at least seven other planets moving around the Sun and a lot of other smaller things as well. All these things together are called a system. The Latin word for the Sun is Sol, so we call this system the Solar System. Far beyond our own Solar System are stars, bodies like the Sun but in some cases much bigger.

Thousands of years ago, a man called Aristarchus said that the Solar System moved around the Sun. Some people thought he was right but many people believed the opposite; that the Solar System moved around the Earth, including the Sun (and even the other stars). This seems sensible, because the Earth doesn't feel as if it's moving, does it?

About 500 years ago another man named Copernicus said the same thing as Aristarchus; that all the planets moved around the Sun and the stars were fixed in space.[1] This time more people agreed but there were still people who thought the opposite. Then, about 100 years later, a man called Galileo Galilei began looking at the sky with a new invention: the telescope. He showed that it was very likely that all the planets moved around the Sun. This time even more people thought Galileo may be right and that the Earth really did move around the Sun. Soon, more and more people started using telescopes to study the sky. However there were still some people who thought Galileo was wrong and he was even arrested and taken to court for lying. All the people who believed him began to learn how the planets and the other things in the Solar System moved so they could prove that he was not lying. Thousands of years after Aristarchus, people finally said "Okay, the Earth does move around the Sun". Galileo couldn't be called a liar anymore.[2]

We can use very large telescopes to see what has happened to other stars. We can compare pictures of distant stars with pictures of our own star, the Sun. We live in exciting times because for the first time we have sent people into space and we also have telescopes in space. These telescopes in space take thousands of pictures of the planets, our sun and the distant stars. On Earth people use the photos to learn about all the different things in the Solar System and they try to explain how the Solar System began. We even have a robot on the red planet Mars that moves around and people on Earth tell it where to go and what to photograph. We also want to know what will happen to the Earth and the Solar System in the future.

What is in the Solar System?Edit

The Solar System, showing the Sun, inner planets, asteroid belt, outer planets, an outer dwarf planet, and a comet. (Not to scale!)

At the center of the Solar System is the Sun. It is a star, like the billions of other stars in the sky. The other stars are very, very far away, so they look tiny. The Sun is important to us because it gives us heat and energy that allows life. None of the life on Earth could exist without the Sun.[3]

The rest of the things in the Solar System orbit (travel around) the Sun. The planets are the largest of these. Each planet is a little like the Earth. But the planets are also very different from each other.

Many of the planets have moons. A moon orbits a planet. Mercury has no moons[4] and nor does Venus. Earth has one. Saturn has more than 80![5]

The planets closest to the Sun are called the inner planets. These are Mercury, Venus, Earth, and Mars. Then comes a big ring of asteroids, chunks of rock much smaller than planets. This ring is called the asteroid belt. Within the asteroid belt, there is a dwarf planet (smaller than a normal planet) named Ceres. Then come the outer planets: Jupiter, Saturn, Uranus, and Neptune. Farther out there are two dwarf planets, Pluto and Eris.

The planets have the names of Roman gods that were worshipped by people thousands of years ago though no one believes in them now. Did you know that some days of the week are also the names of ancient gods? Saturday means "Saturn Day". Thursday means "Thor Day". Thor was a Viking god, the son of Odin. Monday and Sunday simply mean "Moon Day" and "Sun Day". Some of the months are also named after Roman gods. The month of "March" is named after the Roman god "Mars" - he was the god of War!

Beyond the orbit of Neptune is another big ring of things like the asteroids, called the Kuiper belt. Kuiper (said "KYE-per") was the last name of the person who first wrote about it. Most of the things in the Kuiper belt are hard to see through telescopes.

After the Kuiper belt comes the Oort cloud. Scientists think this is where comets come from. It is very far away, many times farther away than Pluto is from the Sun (over a thousand times). It is near the edge of the Solar System.[6] (Yes, "Oort" was the last name of the person who first wrote about it.)

Zodiacal light.

In between all the other things is dust. The pieces of dust are very far apart, but they shine in the light of the Sun. Before dawn, in September or October, they glow in the East. We call this the zodiacal glow, or zodiacal light.[7]

When pieces of space dust hit the Earth's atmosphere, they burn brightly. We call them shooting stars or meteors.

The Sun creates solar wind—a kind of gas that blows away from the Sun into space. This gas travels out past the planets into outer space. The edge, where the solar wind meets the wind from other stars, is called the heliopause. That is about 100 times as far from us as the Earth is from the Sun.[8] Beyond that is a lot of empty space. The nearest star to our Sun is thousands of times farther away than the size of the entire solar system. The Universe is a really huge and empty place![9]

What holds it together?Edit

Sir Isaac Newton, the discoverer of gravity. It is said that he came up with the concept of gravity when an apple fell on his head.

Why do all of the planets orbit the Sun? Why do moons orbit planets? Why doesn't the Sun move off and leave the planets behind? The answer to all of these questions has to do with gravity. Gravity is a force that is a property of mass. It pulls things together.

We don't notice the pull from the Sun because it also pulls on the Earth by the same amount. But the Sun's gravity is strong enough to keep the Earth from shooting away. Even though the Earth is going fast, it keeps turning to go around the Sun. It is like they were tied together with an invisible string. In the same way, moons orbit many of the planets. They are kept there by gravity. The Sun itself does not sit still in space. The entire Solar System is orbiting the center of our galaxy. The whole thing stays together because of the force of gravity[10].

About massEdit

Everything is made out of matter. The amount of matter is called mass. Two apples have twice the mass of one apple. The more mass a thing has, the more gravity pulls it and the more its gravity pulls other objects. We don't notice the pull from an apple because it is so much less than the pull from the Earth. If you stand on the ground and let go of an apple, gravity will pull it down towards the center of the Earth. It will hit the ground. If you could throw the apple hard enough at the right angle, it would go into orbit around the Earth. That is how rockets put astronauts into orbit. If you threw the apple really, REALLY hard in the right direction, it would fly away from Earth and never come back; but our arms are not that strong.

The force of gravity from anything is strongest when very close to that thing, and weaker when further from it. Scientists use weight to mean how hard gravity pulls us. Astronauts weigh less on the moon because it has less mass. It does not pull as hard. We actually weigh a tiny bit less on top of a tall mountain than we do in a lower place. This is because we are farther from most of the Earth.[11]

Who discovered the Solar System?Edit

Anyone who looks up at the sky enough can see seven bright objects. These are the Sun, our Moon, Mercury, Venus, Mars, Jupiter, and Saturn. People have known about them for a very long time. Ancient people thought they were related to gods. In Babylon, they named the days of the week after them. Almost everyone was sure that all these things were orbiting the Earth. They did not know we lived in a Solar System.

In about 1500, Nicolaus Copernicus figured out that the planets orbit the Sun. Only the Moon orbits the Earth. But he was afraid to say so for most of his life and only published a full account of his ideas in 1543, the year of his death.[12] Then Galileo Galilei pointed a telescope at the sky. He found moons orbiting Jupiter. He was certain Copernicus was right, and he got in trouble for saying so. It took seventy years to convince scientists that the planets orbit the Sun.[13] Now, almost everyone on Earth understands that we live in a Solar System.

The largest objects at the farthest reaches of the Solar System, even farther than Neptune!

People made better telescopes and found more things in the sky – moons,[14] new planets,[15] and asteroids.[16] More things, like the dwarf planet Eris, are being found today.[17]

How have we explored the Solar System?Edit

The Voyager 2 spacecraft.
An artist's impressions of Spirit.

Before the telescope, people explored the sky with their eyes. They saw how the planets seemed to "wander" through the sky. They learned to predict where the Sun, the moon, and planets would be in the sky. They built some observatories -- places for watching the sky. Observe is a more scientific word for watch. They observed the Sun and stars to tell the time of year. In China, they even knew when the moon would block the Sun[18]. Most people thought that celestial bodies could cause war or peace on Earth.[19]

After telescopes were first made, in the early 17th century, people kept making them better. Astronomers saw that planets are not like stars. They are worlds, like the Earth. They could see that some planets have moons.[20] They began to think about what these worlds were like. At first, some thought that the other planets and moons had people or animals living on them. They thought about how it would be to live on these other worlds.[21] Then they made telescopes better, and sent spacecraft into space, and found that there were no plants or animals on the Moon[22] or on Mars.[23]

Now, we can explore by going to some of the other worlds. Twelve Astronauts walked on the Moon about 35 years ago. They brought rocks and dirt back to Earth.[24] Spacecraft flew by Venus, Mars, and the outer planets. The pictures they took showed us a lot of what we know about these worlds.[25] Robots landed on Mars in 1971, 1976, and 1997. They took thousands of pictures of the planets. They send photos and movies back to Earth. They also check rocks to find out what the rocks are made of.[26]

So far, we have not found any life except on Earth. Maybe tiny one-celled life once lived on Mars. Maybe there is life under the ice on Jupiter's moon Europa. New spacecraft are being planned to look for life on these worlds.[27]

How was our Solar System formed?Edit

Our Solar System is part of the Milky Way galaxy. Galaxies are big mixes of dust, gas, stars, and other things. Inside our Milky Way galaxy are clouds of dust and gas where stars are born. Our Solar System was created in this kind of cloud. A part of the cloud began to get smaller and less spread out. It formed a big, spinning disk of gas and tiny pieces of dust. This disk was thickest at the middle. The middle slowly collapsed until it became the Sun. We are still trying to learn how the planets were formed. Most scientists think that they were formed from the left over gas and dust.

An artist's illlustration of how the Solar System began.

This is how it could have happened. The rest of the disk continued to spin around the Sun. The tiny pieces of dust hit each other and some of them stuck together, next the bits of dust slowly collected to form grains, these in turn joined to form lumps the size of gravel, then pebbles, and then rocks. The rocks crashed together into mountains. The mountains crashed together to make bigger things. These big things swept up most of the rest of the disk to form the planets, moons, and asteroids.[28]

The Sun got hotter as it collapsed. It began to glow. The temperature at the center reached a million degrees Celsius. The Sun started to make a lot of light and heat. This light and heat swept away most of the leftover dust and gas between the inner planets. This light and heat are the sunlight we see and feel every day on Earth. [29]

What will happen to the Solar System?Edit

In another five billion years, the Sun will use up most of its hydrogen fuel. It will enter the final stages of its life. The middle of the Sun will shrink down and become even hotter. The outer layer of the Sun will grow much bigger than it is now. It will form a red giant.

It will be so big that Mercury and Venus, probably Earth and maybe even Mars will be inside it. These planets will burn away. Which planets get destroyed will depend on how much mass the Sun has left.[30] A strong solar wind will blow some of the outer layers of gas away from the Sun. The Sun will have less mass. The Sun’s gravity will be less. All of the planets will move further away from the Sun.[31]

The massive, rapidly-aging star Eta Carinae throws off a giant cloud of gas, forming a planetary nebula.

After it has been a red giant for a while, the Sun will start to burn helium. It will shrink down and will not be a red giant any more. It will use the helium up in about a billion years. Then it will become a red giant once again. More gas will blow away for a few hundred thousand years.

A planetary nebula[Note 1] will form. The nebula could last for a few thousand to a few tens of thousands of years. It will glow in the light of the Sun.[32]

At the center, the Sun might shrink into a tiny star called a white dwarf. That kind of star is about the size of Earth. It would take about 100 of these white dwarfs, stacked end to end, to be as wide as the Sun is today. The Sun will not have any more fuel to burn. It will have lots of heat left over and will keep getting cooler and dimmer. Then its light will go out in a hundred billion years from now.[33]

Next Topic: The Sun


  1. A planetary nebula was named this because through the earliest telescopes, astronomers thought that they looked like planets. The name stuck — but they really have nothing to do with planets.


    See also Drake, Stillman (translator) (1957). Discoveries and Opinions of Galileo (1610 Letter to the Grand Duchess Christina). Anchor, NY. ISBN 0385092393. 
  9. "Outside Our Solar System" in
  10. "Gravity is the force responsible for keeping the Earth and other planets in our solar system in orbit around the Sun." from Cosmic Glue,
  11. Definitions of Mass, Gravity, and Weight from
  14. Calinger, Ronald S. "Huygens, Christiaan." World Book Online Reference Center. 2004. World Book, Inc.;
  18. Eclipse2001 museum
  19. Raman, Varadaraja V. (2000). Glimpses of Ancient Science and Scientists. Xlibris Corporation. ISBN 073881363X. 
    page 339 "The Chaldeans ... were also the first to suspect... that the Sun, the moon, the planets and the constellation of stars, all affect human life and destiny.... These beliefs gradually spread .. to Egypt, China, Greece, India, and Rome, for example ... astrology is still very popular."
    Sagan, Carl (1973). Mars and the Mind of Man. Harper and Row. ISBN 0060104430. ;
    Verne, Jules (1995). From the Earth to the Moon. North Books. ISBN 1582871035. ;
    From the Earth to the Moon on Project Gutenberg --;
  23. (bottom of page)
  30. Which planets may get destroyed
  31. Outline of Sun's death
  32. Planetary nebulae
  33. Has information on white dwarf stars


Mercury from the MESSENGER spacecraft.

What is Mercury?Edit

Template:User:Robert Horning/Whole Book/Coolfacts

Mercury is the closest planet to the Sun. It is a terrestrial planet; that means a planet made from rock like Earth. It does not have a gas atmosphere, so there is no weather. For a long time, only one spacecraft, Mariner 10, had visited Mercury. In January 2008, the MESSENGER spacecraft went by Mercury. It has gone by Mercury two more times, and has started to go around the planet in 2011.

How big is Mercury?Edit

Comparison of the size of Mercury to the Earth

Mercury is 4879 km across. Mercury's diameter is just less than half the diameter of the Earth. It is the smallest planet in the Solar System. Only dwarf planets like Pluto are smaller. Because Mercury is much closer to the Sun than the Earth, it can only be seen just after the sun goes down at night or shortly before it rises in the morning.

What is Mercury's surface like?Edit

View of the surface of Mercury

Mercury has craters like those on the Earth's moon. The largest crater on Mercury is the Caloris Basin. It is about 1300 km wide. It was created by a huge asteroid hitting Mercury. The asteroid was probably 100 km wide, but it hit Mercury's surface so hard that it made a much bigger hole.

Template:User:Robert Horning/Whole Book The surface also has big cliffs called scarps. They were made long ago when Mercury cooled down. It shrank, causing the surface to get wrinkled in some places. This wrinkling created the scarps.

There may also be ice on the top and bottom of Mercury. Like the Earth, these areas (called poles) don't get much warmth from the Sun. Any ice there won't melt.

It is very hot during the day (over 400°C) because Mercury is so close to the Sun. At night it is very cold because Mercury loses almost all its heat since there is almost no atmosphere to keep the warmth there. The temperature can fall to almost -175°C.

How long is a day on this planet?Edit

Mercury rotates (spins around) much more slowly than the Earth. It takes Mercury 58 days to spin once as viewed from a distant star. Because Mercury orbits the sun very quickly, a day on Mercury lasts longer than 58 days. If you were standing on Mercury at the equator and timed how long it took the sun to go from directly overhead to sunset to sunrise and then rise directly overhead again, it would take 176 Earth days. These long days and nights allow for the temperatures to rise as high, and fall as low, as they do.

How long is a year on Mercury?Edit

Mercury from Mariner 10

Mercury has the shortest year in the Solar System. It is about 88 Earth days long.

It used to be believed that the same side of Mercury always faced the Sun. In order for this to be true, Mercury would have to take the same amount of time to rotate (spin around) as it does to circle the sun. From watching it carefully we now know that Mercury's rotation is somewhat faster than its orbit. Because of the way the orbit and rotation work together, on Mercury, a day (the time from one sunrise to the next) is actually almost twice as long as a year.

What is Mercury made of?Edit

The center of Mercury is made of iron in partly-molten (liquid) form. We know that there is iron at the center because the planet generates a magnetic field. It contains more iron for its size than any other planet in the Solar System. The rest of Mercury, its thick crust, is made of a special type of rock called silicate rocks. There are craters near the poles that are constantly in shadow. Some of these craters contain ice. There is a huge crater on Mercury called Caloris Basin. It was formed when a comet hit the planet and lava or molten rock filled the impact crater. The round wall of this crater is over 2km tall.

How much would Mercury's gravity pull on me?Edit

If you were on Mercury, it would pull you down less than half (38%)as much as the Earth. An item that weighs 100 pounds on Earth, would only weigh 38 pounds on Mercury.

Who discovered Mercury?Edit

A computer-generated simulation of MESSENGER orbiting Mercury.

Nobody really knows who first discovered Mercury, but the first known recorded observations of it are from tablets in Assyria about three and a half thousand years ago, in the 14th century BCE. This was probably something like an informational article about Mercury's movements. In these tablets, Mercury is called (in translation) The Jumping Planet. Almost every ancient civilisation had their own written records and names for Mercury.

In 1639, an Italian astronomer named Giovanni Zupi observed that Mercury has phases, like the phases of the Moon, as it orbits around the Sun. This is evidence that it does orbit around the Sun, an idea that was fairly new at the time, suggested less than a hundred century earlier by Copernicus.

Before the twentieth century, it was a mystery to all the astronomers how long it took for Mercury to rotate. They solved this mystery in 1962, when some astronomers sent radar signals to Mercury, which then bounced back to Earth: it takes 59 days for Mercury to rotate.

It isn't easy to sent a space probe from Earth to Mercury, because Mercury is much closer to the Sun and therefore orbits the Sun much faster than Earth does. So a space probe would have to burn a lot of fuel to match its speed with Mercury, in order to orbit—or land on—the planet. In 1973, a space probe Mariner 10 was sent to take measurements of Mercury and map its surface. Because orbiting Mercury would be so expensive, Mariner 10 orbited the sun instead, and would take pictures each time it got close enough to Mercury. Unfortunately, when Mariner 10 finally ran out of fuel, it had only mapped about 45% of the surface. However, it also discovered that Mercury has an iron-rich core and a magnetic field. 29 years later, in 2004, another probe was launched, named MESSENGER. MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. MESSENGER followed a complicated path that would slowly match speeds with Mercury without using much fuel. It passed by Mercury three times, and finally settled into orbit around Mercury more than six years after it was launched. By March 2013, MESSENGER had mapped 100% of the surface of Mercury.

Who is this planet named after?Edit

A statue of Mercury.

In Roman mythology, Mercury (Latin Mercurius) was the messenger of the gods. He wore a hat and sandals with wings on them, allowing him to travel around the world very quickly. The planet Mercury was named after him because it moves faster than any other planet in the Solar System. It moves nearly 48 km every second! (Of course, the Romans didn't know that, but they could see how fast it moved across the sky.)The Greeks called this god Hermes.

Next Topic: Venus


NASA's Solar System Exploration[1]
Arnett, Bill. The Nine Planets[2]
Worldbook Online [3]
Worldbook@NASA, "Mercury"[4]
NASA Planetary Fact Sheet [5]
Hamilton, Calvin J. ., "Mercury" [6]
Encyclopedia Mythica, "Mercury" [7]
Col, Jeananda. Enchanted Learning/Zoom Astronomy [8] 1998-2005
Usborne Internet-Linked Science Encyclopedia, Usborne Publishing Ltd. ISBN 0794503314[9]
Dickinson, Terrence. The Universe and Beyond . Firefly Books ISBN 1552093611


  Venus Facts:

  • Venera 7, the first space probe to land on Venus, was destroyed by the hostile conditions on Venus after only 23 minutes.
  • Almost all of the surface features on Venus are named after women.
  • A day on Venus is equal to 117 days on Earth.
  • It has been proposed that some microbes could live in Venus's clouds.

Venus is the second closest planet to the Sun. It is a terrestrial planet. This means that we think it was created in a similar way to our planet Earth and is made of rock.

Venus. You can't see the surface in this picture; the white is the clouds.

Template:User:Robert Horning/Whole Book

How big is Venus?Edit

Earth and Venus Comparison

Venus is only slightly smaller than the Earth. This is one reason why Venus is sometimes thought of as Earth's "twin". Venus has a diameter of about 12,100 km. It has also been visited by many probes.

What is the surface like on this planet?Edit

Maat Mons on the Surface of Venus, by radar imaging

The surface of Venus is very different from the surface of the Earth. It is very dry and hot enough to melt lead. The pressure on the surface is very high. It is the same pressure as being 1 km (3,280 feet) below the surface of the sea on Earth.

Channels on the surface of Venus, looking like river channels on Earth

Channels that look like rivers have formed on Venus. Scientists think these channels are formed from erupting lava. The lava flows along as it cools down, creating the channels. One feature only Venus seems to have is unusual volcanoes called arachnoids. These are volcanoes that have formed differently from other volcanoes we have found in the Solar System. We don't know exactly how they were formed. Venus also has volcanoes like those on Earth.

Parts of the surface of Venus look something like continents. The largest of these areas is called Ishtar Terra (land of Ishtar, from the Babylonian goddess who was similar to Venus). Deep basins like those under the Earth's oceans have also been discovered. On Venus, though, they have no water. Features like mountain ranges and meteor craters have also been found on Venus. One of the highest mountains on Venus, Maxwell Montes, is about 11 km taller than Mount Everest, the highest mountain on Earth.

On the night side of the planet, there is a strange effect called Ashen Light. For some reason, the dark side of Venus has a subtle glow. There are various theories about this. One of the earliest theories — now disproved, of course — was that there were aliens on Venus and they were celebrating a new Venusian emperor. Currently, one of the more believed ones is this: There is a high concentration of carbon dioxide. When it is hit by the ultraviolet rays of the sun, they change into carbon monoxide and oxygen, and emit green light. The entire chemical procedure is CO2 → CO + O.

The Venera 13 Lander as depicted on a stamp, which made scientific measurements and sent back pictures from the surface of Venus

How long is a day on Venus?Edit

Venus rotates (spins around) even more slowly than Mercury. One full rotation of Venus takes about 243 Earth days. Venus also rotates in the opposite direction to most of the other planets in the Solar System. One day on Venus, from noon to noon, depends on the length of the year as well as the rotation time, and is about 117 earth days.

How long is a year on Venus?Edit

One year on Venus is almost 225 Earth days long. This is less time than it takes Venus to rotate on its axis and less than two Venus days.

What is Venus made of?Edit

The surface of Venus, its crust, is covered in nothing but rock. But, the core of Venus is made of nickel-iron. The atmosphere around Venus is very thick and is made of carbon dioxide, nitrogen, and poisonous gasses that create high pressure and trap in heat.

How much would Venus's gravity pull on me?Edit

If you were on Venus, it would pull you down almost as strongly as Earth. The atmosphere exerts a pressure at the surface more than 90 times Earth's normal sea-level pressure.

Who discovered Venus?Edit

Because Venus is closer to the sun than we are on Earth, we always see it close to the sun in the sky. So it only appears for us just before sunrise in the eastern sky, or just after sunset in the western sky. Many cultures thought Venus in the morning and Venus in the evening were two separate things. The ancient Romans called the evening object Venus (after the goddess of love) and the morning object Lucifer (which means light bearer — a servant who walked ahead of the sun's chariot with a torch, to light the way). Nobody knows who first thought the two were a single object. The first known written description of them as one object was the Venus Tablet of Ammisaduqa, from about three and a half thousand years ago — 1581 BCE.

A map of where the Veneras 9-14 and Vegas landed.

About three thousand years later, in 1610, Italian astronomer Galileo Galilei used a telescope to observe that Venus has phases, just as the moon does. Phases happen because only the side of Venus (or of the Moon) facing the Sun is lighted. The phases of Venus supported the theory of Copernicus that the planets go around the Sun. Then, a few years later in 1639, an English astronomer named Jeremiah Horrocks observed a transit of Venus. That's what it's called when Venus passes right between the Earth and the Sun, so that Venus is visible from Earth during the day as a tiny dot passing across the Sun. In 1761 a Russian astronomer, Mikhail Lomonsov, watching another transit of Venus, saw that Venus has an atmosphere.

Not much more was discovered about Venus until the 1920s. Then, a United States astronomer, Frank Ross, observed Venus using ultraviolet light — the light that causes sunburn — and for the first time saw the structure of the clouds on Venus.

However, there is only so much that can be learned about Venus by looking at it from Earth. The first successful pictures of Venus by a space probe were taken by Mariner 2 in 1962. Mariner 2 was the first space probe successfully sent to observe another planet. It showed two important things: Venus has practically no magnetic field, and Venus has temperatures of 490 to 590 K — that's as hot as the inside of a working oven on Earth!

The first written record of Venus. It describes Venus' movement over 21 years.

Who is this planet named after?Edit

Venus is named after the Roman goddess of love. Sometimes it can be seen shining brightly just before dawn or just after sunset, when it is called the Morning Star or Evening Star. Some people, like the Aztecs and the Greeks, gave Venus two names – one for the morning and one for the evening.

Because Venus and the Earth are the same size, scientists call Venus "Earth's sister planet". For a long time most scientists thought that Venus had plants, animals, and possibly even people. However because Venus is so hot we now know that it is impossible for anything to live there.

How long would it take people to get there?Edit

It could take around a year and a half to get there. But it is very unlikely someone would go to Venus.


  • Volcano World, "Lava Flows"[10]
  • Astronomy Picture of the Day [11]
  • Alder Planetarium[12]
  • Mount Everest [13]
  • Windows to the Universe[14]
  • NASA's Solar System Exploration[15]
    Arnett, Bill. The Nine Planets[16]
  • Worldbook Online [17]
  • Worldbook@NASA, "Mercury"[18]
  • NASA Planetary Fact Sheet [19]
  • Hamilton, Calvin J. ., "Mercury" [20]
  • Encyclopedia Mythica, "Mercury" [21]
  • Col, Jeananda. Enchanted Learning/Zoom Astronomy [22] 1998-2005
  • Usborne Internet-Linked Science Encyclopedia, Usborne Publishing Ltd. ISBN 0794503314[23]
  • Dickinson, Terrence. The Universe and Beyond . Firefly Books ISBN 1552093611

Next Topic: Earth Earth is the planet we live on. It is the only planet in the Solar System with liquid water on its surface. It is also the only planet we know to have life on it. Earth is also known as Terra.

  Earth Facts

  • The Earth is the only planet we know to have life on it (such as ourselves.)
  • There is oxygen on Earth, and oxygen is necessary for life.
  • The Earth is the third planet from the Sun.
  • The Earth is the only planet we know that has liquid water on the surface, but scientists are trying to find others.
  • The Earth's axis is tilted which is why we have four different seasons.
  • The Earth is 4.6 billion years old.
  • The Hoba meteorite in Namibia is the biggest known meteorite to crash on Earth in one piece.
A picture taken by Apollo 17, a space mission.

How big is the Earth?Edit

The Earth is nearly 13,000 km wide. It's the largest terrestrial planet in the Solar System.

The Earth's mass is about 5,973,700,000,000,000,000,000,000 kg. That's a lot. But it is little compared with Jupiter (319 Earths) and tiny compared with the Sun (335,789 Earths) or other stars!

What is the Earth's surface like ?Edit

The Earth's surface is made of rock. Most of it is underwater, but not all. Islands of rock rise up out of the water. The biggest islands are called continents, of which there are seven: North America, South America, Europe, Asia, Africa, Australia, and Antarctica. The largest bodies of water are called oceans, of which there are five: Pacific, Atlantic, Indian, Arctic and Antarctic or Southern.

Anawhata beach, west of Auckland, New Zealand

The Earth's surface is made up of huge plates. They are like huge jigsaw pieces made of rock. These plates move very, very slowly, carrying the continents with them. They can rub beside each other, push against each other, or even move away from each other. If there are gaps between them, hot molten rock can rise up and make volcanoes. Where the plates rub or push against each other, earthquakes may happen. When two plates push each other's rock upwards, mountains are formed.

Zabriske Point, Death Valley National Park, California

Earth has many kinds of environments. It is cold and icy in places like Antarctica. It is hot and dry in deserts like the Sahara in Africa and Death Valley in the United States. It is cold and dry in deserts like Siberia in Russia. Where it is warm and wet, rainforests grow.

Why is there life on Earth?Edit

Wherever we have looked on Earth, we have found living things. They may be very small, like bacteria, but they are there. We have found bacteria where it is very cold, very hot, very deep, very high or very dark.

Galileo being deployed after being launched by the Space Shuttle Atlantis

What all living things on Earth seem to need is liquid water. Wherever you can find some water, there are almost always living things there too, even if you can't see them. If we find liquid water somewhere else in the Solar System, scientists think we might find some living things there too. If we don't, there is always the rest of the universe to explore!

There is another possibility. All the living things we know need liquid water. But maybe somewhere else there are living things that don't need water. Perhaps we will need to learn how to recognize them.

What about the Earth's moon?Edit

Earth has one moon we call... the Moon!  Sometimes it is called by its name in Latin, Luna, so we don't get confused with other planets and their moons. The Moon has also been called Selene (pronounced "suh-LEE-nee") which is Greek for moon and was the name of the Greek moon goddess.

Recently we have also found some other objects that are said to go around the Earth. The largest one, called Cruithne (pronounced "cru-EE-nyuh"), is three miles wide. In fact, it orbits (goes around) the Sun in a way that makes it keep coming close to Earth.

There are various ideas about where the Moon came from (no-one was around to see it happen, after all), but the most widely held theory is that when Earth was young, a large body hit Earth and split off a section of the Earth that is now the Moon.

How long is a day on this planet?Edit

A day on Earth is 24 hours long. That's daytime and night time. A 24 hour day is how long it takes the Earth to spin around once. On the half of the Earth that is facing the sun it is daytime and on the half of the Earth that is facing away from the sun it is nighttime.

The spin of the earth is also the reason why the sun appears to rise in the east and to set in the west. Although it looks like the sun is moving from the surface of the earth, it is really the surface of the earth that is moving. The reason we do not feel like we are spinning is because the earth is so big compared to the size of people.

Also, the Earth is tilted at about 23°, so there are times that the North or South pole is always facing or turned away from the sun. If you live at one of the poles of Earth it can be light or dark throughout the whole day!
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How long is a year on this planet?Edit

A year on Earth is about 365 and 1/4 days long. That's how long it takes the Earth to orbit the Sun once. Approximately every four years we have a leap year. A leap year contains an extra day in our calendar on February 29th in order to account for the 1/4 of a day left over each year.

What is the Earth made of?Edit

When a planet is made of rock, we call its surface the crust. Below the Earth's crust is hot rock, some of which is molten. It is in a layer called the mantle. The hot molten rock is what comes out of volcanoes. It's then called lava.

Under the mantle is the core of the Earth. We think it is made from solid iron and nickel, surrounded by hot molten iron. The temperature there is very very hot!

The Earth's crust is very thin compared to the mantle and the core. But it is very thick to us. Nobody has drilled all the way through it yet. Even the deepest underground mines are far away from reaching its deepest base.

How much does the Earth's gravity pull on me?Edit

It's easy to find your weight on Earth by using a scale. You have weight because the Earth's gravity pulls you towards its center. Normally, the ground or the floor get in the way, making you feel 'stuck' to them.

There are several kinds of scales:

1) Comparing of 2 masses (weights). You put the thing(s) you want to weigh on one pan (like some marbles), and then you put several "weights" on the other pan until the pointer shows that both pans have equal weights on them. Then you look at the pan with the known weights on it, and add them all up. The total is the mass of the thing(s) you want to weigh.

2) A spring balance usually has a hook on it, with a pan. You put the thing(s) you want to weigh on the pan, the spring is pulled, and the greater the weight, the further the spring is pulled. That distance, calibrated in pounds or kilogram (or whatever), is usually shown either on a dial or on a linear scale.

3) There are also electronic scales that give a properly calibrated reading—grocery stores, for example, use these.

NOTE: Gravity varies slightly depending on the location where you want to get the weight; spring balances and some electronic scales can, in theory, read slightly different weights at different places because of that, but usually in practice that difference is too small to be noticed. But, because the balance type of scales work differently from the spring or electronic types, they will always read the true, correct mass. They would even give the same mass on the Moon, where gravity is much less than on Earth.

Did you know? that Sir Isaac Newton was the first person to realize that the force pulling you down to the ground was the same force that keeps the planets going around the Sun? The story goes that he thought of this when he saw an apple fall from a tree.

Gravity is a very important force. As well as keeping you firmly stuck to the Earth, it keeps the Moon going round the Earth, The Earth going around the Sun and the Sun going around the center of the Milky Way galaxy. Gravity also makes stars and planets a nice round ball shape. In fact without gravity there wouldn't even be a Sun, Moon or Earth because the material that they are made of would just float away into space.

Who is it named after?Edit

The Earth seen from the Moon

The word earth is used for both planet Earth and soil. Other names had been used for Earth such as Gaia, Tellus and Terra. Gaia is the Greek goddess (meaning Mother Earth) and Terra is the Roman name of the same goddess. Tellus is the Latin for "Earth", many scientific words dealing with Earth will be derived form Latin.

Next Topic: Moon


"It is also the only planet we know..." [24] [25]
"The Earth is nearly..." [26] [27]
"The Earth's mass..." [28]
"Earth's surface is made of..." [29] [30]


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The Moon is our nearest neighbour in space.

How big is the moon?Edit

Comparison of the size of the Moon and the Earth

Most of the planets in the Solar System are much bigger than their moons, but the Earth and the Moon are much closer in size. The Moon is just under 3,500 kilometers (km) wide. That's over a quarter of the size of the Earth (about 12,600 km wide) as you can see in the picture below. Because of this, the Earth and Moon together are sometimes called a binary or double planet system.

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What is the moon's surface like?Edit

Astronaut Harrison Schmitt collecting rocks from the Moon during the Apollo XVII mission.
A map of the moon.

The Moon has no atmosphere. It also has no liquid water on its surface. During the day it becomes very hot, but at night it is icy cold. A person visiting the Moon needs an air supply and a special suit.

The Moon has many craters on its surface. The largest one is called the South Pole-Aitken Basin and is roughly 2500 km across.

We think nearly all the craters on moons or planets were made by huge rocks hitting them a long time ago. These collisions are called impacts.

Some of the craters on the Moon look as if they have rays coming out of them. These rays are rocks thrown across the Moon by the impacts that made the craters. Some of the craters around the poles of the Moon may have ice in them.

There are also darker areas called maria (said "MARR-ee-ah"). These are large pools of lava that cooled a long time ago. Most maria are on the side of the Moon we see from Earth. The lighter areas on the Moon are highlands.

How long is the Moon's revolution?Edit

The Moon takes just over 27 Earth days to rotate (rotate means spin around) once.

How long is a year on the moon?Edit

The Moon also takes just over 27 days to orbit (move around) the Earth. This is why we always see the same side of the Moon when we look from the Earth. We call this side the near side. The other side we call the far side. In 1959, a probe called Luna 3 sent back pictures of the far side. That was the first time anyone saw what it looked like.

What is the Moon made of?Edit

The surface of the Moon is made of rocks and dust. The outer layer of the Moon is called the crust. The crust is about 70 km thick on the near side and 100 km thick on the far side. It is thinner under the maria and thicker under the highlands. There may be more maria on the near side because the crust is thinner. It was easier for lava to rise up to the surface.

We think the Moon has a small core (center) about 300 km across. The core is composed of solid iron. Because the core is solid, the Moon does not have its own magnetic field.

How much would the Moon's gravity pull on me?Edit

If you were on the Moon, it would pull you down about a sixth as much as the Earth does, so you'd weigh a sixth as much. So would anything else. That's why it was much easier for the astronauts visiting the Moon to pick up rocks there.

Who is the moon named after?Edit

The goddess Diane, the goddess of the Moon in Roman mythology

The names Moon and month both come from the ancient Greek name for the Moon, Mene. There have been other names for the Moon, like Selene and Luna. Selene was the Greek goddess of the Moon. Luna was the Roman goddess of the Moon. The Roman people also associated their goddess Diana with the Moon.

Who discovered the Moon?Edit

Ancient Greece and Ancient China noted over 2000 years ago that the light from the Moon is reflected from the Sun. Also, Ancient Greece noted that the Moon causes tides on Earth. More recently, Luna 1 was the first spacecraft to perform a flyby of the moon. Luna 2 was the first spacecraft to land on the moon, and Luna 3 was the first to photograph the far side of the Moon which you can not see from Earth. Luna 1 through 3 all were launched in 1959. Surveyor 3, in 1967, was the first to examine the soil of the Moon. It dug to 17.5 cm. In 1969, Apollo 11 was the very first spacecraft to land people on the moon.

Next Topic: Mars


"The Moon is just under..." [31] [32]
"So the Earth and the Moon together..." [33] [34]
"The Moon does not have any atmosphere." [35] [36]
"During the day it becomes..." [37] [38]
"The largest one is called..." [39] [40]
"These rays are rocks..." [41]
"Some of the craters around the bottom..." [42] [43]
"There are also darker areas..." [44] [45]
"The lighter areas..." [46]
"The Moon takes just over 27..." [47] [48]
"We call this side..." [49] [50] [51]
"The other side we call..." [52] [53]
"The surface of the Moon..." [54] [55] [56] [57]
" would pull you down..." [58] [59]
"The names "Moon" and..." [60] [61] [62]


Wikijunior Solar System/Mars


  Jupiter Facts:

  • Due to its magnetic field trapping particles from the Sun, Jupiter is surrounded by very powerful radiation belts which would kill anyone who entered them.
  • Jupiter's moon Europa is thought to have a giant ocean below its surface.
  • Jupiter's moon Ganymede is the largest moon in the solar system.
  • Jupiter is the third brightest object in the night sky, second being Venus and first being the Earth's moon.
  • Jupiter has more than twice the mass of all the other planets in the solar system combined.
  • Jupiter is considered an almost-star. The smallest red dwarf star was only 30% bigger than Jupiter.
  • Jupiter's red spot and the Earth are close to the same size. That means that there is a storm going on in Jupiter that is as big as the Earth!
  • Jovian in English means anything relating to Jupiter — the god or the planet. Sometimes the four outer planets are called the Jovian planets.

Jupiter is by far the largest planet in our Solar System: two and a half times larger than all of the other planets put together. It is the fifth planet from the Sun and one of the brightest planets as seen from Earth. Jupiter, along with Saturn, Uranus and Neptune, is sometimes called a "gas giant" because most of these planets are made up of liquid and gas.

How big is the planet?Edit

Comparing the sizes of the Sun, Jupiter, Earth, and the Moon. Notice the red spot is about the size of the Earth!

Jupiter is 142,984 km or about 11 Earths in diameter at the equator. That makes it about one tenth as big in diameter as the Sun! You could fit about 1,400 Earths into the volume of Jupiter. It is 133,709 km or 10 Earths in diameter from pole to pole. Jupiter's rapid rotation (it spins around in under ten hours compared with 24 hours for Earth) makes it bulge out at the equator.

Jupiter's magnetic field is the largest single planetary thing in the Solar System. It is 26 million kilometers across, making it about 20 times bigger than the Sun. It has a tail that extends past Saturn's orbit. If it could be seen from Earth, it would appear to be five times the size of the full moon.

What is the surface like on Jupiter ?Edit

The surface we see is not solid. This enormous planet has a relatively small solid and rocky core. Liquids and gases surround this core and blend with the atmosphere.

Jupiter is a cloudy, windy and stormy planet. It is always covered by a layer of clouds, and wind speeds of 600 km/h are not uncommon. The storms are visible as swirls, bands and spots. A particularly violent storm, about three times Earth's diameter, is known as the Great Red Spot. This storm has been in existence since at least 1831, and maybe since 1665. If the storm has existed since 1665, that would make it more than 300 years old!

The layer of clouds is divided into several bands. The lighter colored bands are called zones and the darker bands are called belts. The colors are caused by small changes in the temperature and chemistry. Each band rotates in the opposite direction from its neighbors. Along the edges where the bands meet, these winds collide and create swirling patterns.

The stormy atmosphere of Jupiter has flashes of lightning just like on Earth. However, while Earth's lightning strikes may be hotter than 50,000 °C, Jupiter's lightning strikes may go over 5,000,000 °C, which is a hundred times more than Earth lightning, and is more than the temperature of the sun's corona. The lightning is made by water near the tops of the clouds.

What are its rings like?Edit

Jupiter's rings are dark and hard to see. They are made of tiny particles that meteors knocked off Jupiter's small inner moons and debris left over from comets and other objects that came close to the surface of Jupiter. In fact, until the Voyager spacecraft arrived near Jupiter and took closeup pictures of the rings of Jupiter, scientists didn't even know that it even had rings at all. Two rings are clearly from material that can be associated with two sets of the inner moons of the planet.

These are the names of the rings and their sizes:

Rings of Jupiter
Ring Name Inner Radius Outer Radius
Halo 100,000 km 122,000 km
Main 122,000 km 129,000 km
Gossamer (inner) 129,000 km 182,000 km
Gossamer (outer) 182,000 km 225,000 km

Gossamer means in English anything that is delicate, light, and flimsy.

Template:User:Robert Horning/Whole Book

What is the planet's atmosphere made of?Edit

Jupiter as seen by the space probe Cassini. This is the most detailed color portrait of Jupiter ever assembled.

In the outermost layer of Jupiter lie frozen Ammonia crystals. (Ammonia is a compound of hydrogen and nitrogen; its scientific designation is NH3.) crystals.

Jupiter's atmosphere is mainly made of hydrogen(H2). Near the surface, there is almost 90% hydrogen. Apart from this, the atmosphere has helium(He). Because of the high pressure, helium becomes a liquid further down in the planet. In addition, Jupiter has methane(CH4)(0.3%), hydrogen deuteride(HD)(0.003%), ethane(C2H6)(0.0006%), and least of all, water(H2O)(0.0004%).

Jupiter's temperature is very high. Because of this, scientists cannot tell all the materials the planet is made of. The outer core of Jupiter has hydrogen. The pressure present can make the gas solid. However, because of the very high temperature, the gas melts, and becomes liquid.

What are its moons like?Edit

A simulation of the orbit paths of the moons of Jupiter.

Jupiter has 63 known moons. There are four major moons that were discovered by Galileo in 1610, the first moons ever discovered around another planet. Those moons are Io, Europa, Ganymede and Callisto; they are named from characters in mythology closely associated with Jupiter. They are called the Galilean moons. There are often eclipses on Jupiter's cloud tops by the Galilean moons.

Amalthea GroupEdit

There are four small moons orbiting inside Io's orbit. That group is called the Amalthea group because Amalthea is the largest one. They are all small and potato shaped. Amalthea is very red. The material of Jupiter's rings came from meteors knocking it off of those moons. Amalthea(Greek:Ἀμάλθεια) in Greek mythology is a foster mother of Zeus.


The four largest moons of Jupiter. From left to right with increasing distance: Io, Europa, Ganymede, Callisto.

Io (pronounced EYE-oh) is Jupiter's closest major moon. It is 3643.2 km across, slightly larger than Earth's Moon. It has the most spectacular volcanoes in the solar system and molten sulfur lakes. Any craters formed by asteroids hitting the surface are quickly covered up by the volcanic activity. Io's core is made of molten iron and is surrounded by a rock shell. Unlike Jupiter's other moons, there is very little water on Io. Scientists think that when Jupiter was forming, it was hot enough to dry out Io, but not the other major moons. In Roman mythology, Io(Ancient Greek:Ἰώ) was a beautiful young nymph(Nymphs are divine spirits that animate nature and generally portrayed as beautiful young women.) that Zeus loved.


Europa (pronounced Eurṓpē) is 3,121.6 km across, about 10% smaller than Earth's Moon. It is made of silicates and has a layer of smooth water ice 10 to 30 km thick. The ice has long cracks in it and very few craters. It looks like the sea ice on Earth. The ice has slid around at the cracks. We believe there is liquid water under the ice as much as 100 km below the surface. There are also some large spots on the surface. In Roman mythology Europa was courted by Jupiter in the form of a bull. Europa (Ancient Greek:Ευρώπη) in Greek mythology was a Phoenician noblewoman who was abducted by Zeus to Crete (An island of Greece).


Jupiter and its four largest moons, as seen through a telescope.

Ganymede is 5262.4 km across, making it 380 km wider than Mercury. It is Jupiter's largest moon and the largest moon in the Solar System. It has plate tectonics like Earth. There are older, darker regions and newer areas with grooves where the plates have moved. Newer craters have bright rays around them from material thrown up by impacts. Older craters look flat and faded because the icy surface does not hold the shape of the crater as well as rock does over long periods of time. Ganymede may have an iron and sulfur core with a silicate mantle and an icy shell. It may be similar to Io except with a layer of ice on it. In Roman mythology Ganymede was a beautiful young man who Jupiter kidnapped and made cupbearer to the gods on Mt. Olympus.


The orbit paths of the 4 largest moons.

Callisto is 4820.6 km across, about the same size as Mercury. It has many craters. Like craters on Ganymede, the older craters have faded. The largest crater is Valhalla. It has a bright center 600 km across with rings around it up to 3000 km across. Callisto is made of silicates and ice. There is a 200 km thick icy crust with a liquid water sea under it. In Roman mythology Callisto was turned into a bear by Jupiter's jealous wife Juno. Later Jupiter placed her in the stars as The Great Bear.

Other moonsEdit

The other moons are smaller in several groups outside the orbits of the major moons. There is also a small moon, Themisto and four groups of small moons that orbit very far from Jupiter.

How long is a day on this planet?Edit

One Jupiter day is about 10 Earth hours long. You have to say "about" because different parts of Jupiter rotate about its axis at different speeds. This is caused by the fact that Jupiter is mostly gases that are in constant motion and sometimes going in different directions. Some efforts have been made to try and measure the rotation speed of the inner rocky core of Jupiter, but that has proved to be quite difficult to accomplish due to the magnetic fields that surround Jupiter and the very active radio energy that is generated by the atmosphere of Jupiter, which interferes with measuring techniques like radar that has been used to measure the surface of Venus and Mars.

How long is a year on Jupiter?Edit

The Great Red Spot
Jupiter Eclipses

One year on Jupiter is 4,335 Earth days or 11.87 Earth years long.

A Jupiter year is about equal to four-tenths (or two-fifths) of a Saturn year. Thus after every two Saturn years, Jupiter has completed five full orbits about the Sun. So after 59 years, Saturn and Jupiter will be back in nearly the same position. When the orbits of two planets are simple ratios of each other like this, it is called a resonance.

How much would Jupiter's gravity pull on me?Edit

If someone were floating close to the cloud tops of Jupiter, it would pull them down with a force about two and a half times as strong as the force of Earth's gravity.

Jupiter's rapid rotation causes the equator to bulge out. This would also cancel out about 10 percent of gravity's force on them if they were at the equator. The amount of this counteraction becomes lower the closer they get to the poles.

Who is it named after?Edit

Statue of Zeus (Jupiter) in Olympia, Greece

Jupiter (Latin Iuppiter) is named after the king of the Roman gods, also called Zeus in ancient Greece. The god Jupiter was known for causing lightning strikes on Earth. He is associated with the eagle and the oak tree.

Next Topic: Saturn


  Saturn Facts:

  • If you could find a bathtub big enough, Saturn would float in it.
  • Some of Saturn's moons control the width of its rings. These are known as shepherd moons.
  • Although it is made mostly of gases, scientists believe Saturn has a small rocky core.

Saturn is the sixth planet from the sun, and is one of the gas giants.

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How big is the planet?Edit

Comparison of the size of Saturn and the Earth

Saturn is 120,536 km or 9.449 Earths wide at the equator.[1]

What is its surface like?Edit

Saturn is mostly gas and liquid.[2] Saturn may have a small core of rock and ice.[3] The atmosphere has bands, but they are not as colorful as Jupiter's.

What are its rings like?Edit

Saturn casts a shadow on its rings

Saturn's rings are composed of rock and ice particles ranging in size from specks of dust to the size of a house. Some particles might even be a few kilometers wide! The particles in the rings are actually spaced far apart. It would be easy to pass through the rings.[4]

What are its moons like?Edit

Map of the Saturn system (NASA)

Saturn has 56 moons, and many of them have names.[5]The size of Saturn's moons and the size of the chunks of ice in its rings are similar, which means that we can never know the exact number of moons. [6] New moons are still being discovered. Saturn's biggest moon is named Titan, and is large enough to be a planet in its own right!

Shepherd moonsEdit

There are small potato-shaped moons in or near Saturn's rings. They control the ring particles with their gravity. That is why they are called shepherd moons. Six of them are known, and there may be more.[7]


Mimas is mostly made of water ice with a small amount of rock.[8] It has a large crater for its size called Herschel. It is 130 km across, making it about a third as big as Mimas.[9] The crater makes Mimas look like the Death Star from the Star Wars movies.


Enceladus is made of ice. It is denser than the other icy moons. That suggests it also has some rock inside.[10] It has smooth areas, cracks and some craters. The smooth areas are younger. Craters there have been erased within the past 100 million years. Water vapor was found over a smooth area around the south pole. The cracks and grooves suggest tectonics similar to Ganymede's. Some ridges similar to Europa's ridges were also found. Those suggest oceans like Europa's under some areas of Enceladus.[11]Tidal forces from Dione could be powering some of this activity. It is because Enceladus orbits Saturn twice for every orbit by Dione. This makes Dione and Saturn tug on Enceladus. This is similar to how Europa and Ganymede's tidal forces on Io power Io's volcanoes.[12]


Tethys is an icy moon that has many craters, including the huge Odysseus. It is 400 km across, 1/5th as big as Tethys is. The crater had become flattened because the icy material does not hold its shape as well as rock would. There is also a large valley called Ithaca Chasma. It is 3 to 5 km deep, 100 km wide and 2000 km long, three fourths of the way around Tethys.[13]

There are two moons, Telesto and Calypso, which share Tethys' orbit. Telesto is ahead of Tethys and Calypso is behind it.[14]


Dione is made of lots of ice and there may be some rock in its core. It has lots of craters. The craters are flattened because the ice does not hold their shape as well as rock. One side has bright white lines that are fractures. Two moons share Dione's orbit. Helene is ahead of Dione and Polydeuces is behind it.[15]


Rhea is an icy moon similar to Dione with some rock in the core. It has many craters mostly on one side, and the other side has some bright white icy areas.[16]


Titan is the largest moon of Saturn and the second largest one in the solar system.[17] It is the only moon in the Solar System that has a thick atmosphere. The atmosphere is made of nitrogen, argon, methane and various organic compounds.[18] Its surface has light and dark areas and few craters. The Cassini probe discovered a huge crater 440 km wide.[19]


Hyperion is made of water ice with a little rock. It is potato shaped. It wobbles instead of rotating in the same way other moons do.[20]


Iapetus is almost entirely ice.[21] It has a light area called Roncevaux Terra that has craters.[22] There is a big dark area called Cassini Regio that covers half of Iapetus. The dark material may be from Phoebe. Some of it is on the bottom of craters. Some huge craters and a ridge have been discovered in Cassini Regio by the Cassini probe. The ridge stretches 1300 km along the equator. It is up to 20 km high, which is over 2.26 times higher than Mount Everest.[23] More huge craters were found in Roncevaux Terra when Cassini went by Iapetus again.[24]


Phoebe is made of ice and rock. It looks dark because it has a layer of dark material on the outside. It also looks rough.[25]

Other moonsEdit

There are two groups of small outer moons. Phoebe is part of the second outermost group.[26]

How long is a day on this planet?Edit

One day on Saturn is about 10 hours and 39 minutes in Earth time.[27]

How long is a year on this planet?Edit

One year on Saturn is about 29.46 Earth years long. That is 10,760 Earth days![28]

What is it made of?Edit

Saturn has a rocky core. Around the core, there is ice. Above the ice is liquid metallic hydrogen. On top of that is gaseous hydrogen. There is no place where the hydrogen suddenly turns from a gas to a liquid.

The gaseous hydrogen makes up most of Saturn's atmosphere. Other gases there include helium and some other gases. There may be rain made of helium falling through the hydrogen. There is also ammonia on the surface.[29]

How much would Saturn's gravity pull on me?Edit

If you were floating close to the cloud tops of Saturn, it would pull you down with a force only a little stronger than the force of Earth's gravity.[30] The effects of Saturn's large radius and its mass almost cancel each other out, making the force only a little bigger. So, if you weighed 100 lbs. on Earth, you would weigh 106 lbs. on Saturn.

Who is it named after?Edit

Saturn is named after the most important Roman god of agriculture and harvest time. He taught people how to farm. He was the father of Jupiter. Saturday is named after him.[31]




Uranus, the seventh planet from the Sun, was discovered by William Herschel on March 13, 1781.[1]

Uranus as seen from Voyager 2

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How big is Uranus?Edit

Comparison of the size of Uranus and the Earth

Uranus is 51,118 kilometers or about four Earths wide. It is the third widest and fourth heaviest planet in the Solar System.

What is the surface of Uranus like?Edit

Uranus does not have a surface that you could stand on without going deep into the atmosphere. Under the atmosphere, there may be an even mixture of rock and ice.[2]

What are the rings around Uranus like?Edit

Uranus has eleven rings. They are dark in colour and very hard to see. They were discovered by accident in 1977. Scientists were studying a bright star near Uranus. However, the star's light was blocked before and after it disappeared behind Uranus. From this, they figured out that Uranus has a ring system.[3]

What are its moons like?Edit

Uranus's rings and moons

Uranus has 27 known moons, which places it third in the Solar System for number of moons. The five main ones are Miranda, Ariel, Umbriel, Titania and Oberon.[4]


Miranda is the smallest and closest of Uranus's major moons. It is mainly made of ice and rock. Miranda's surface has grooves, cliffs, and valleys. The moon was named after a character in The Tempest, a play by Shakespeare.[5]


Ariel is made of rock and ice. Ariel has many valleys, but not many craters. Ariel was named after a character in the poem The Rape of the Lock by Alexander Pope. Ariel is also a spirit in The Tempest by Shakespeare.[6]


Umbriel is made of lots of ices and some rock. It is also the darkest of Uranus's major moons. Umbriel was named after a character in the poem The Rape of the Lock by Alexander Pope.[7]


Titania is the largest moon of Uranus. It is mostly ice and rock. The surface is covered with canyons. It was named after the Queen of the Fairies in A Midsummer's Night Dream, a play by Shakespeare.[8]

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Oberon is the outermost of the major moons of Uranus. It is made of the same things as Titania. It has many craters. Some of them have white rays around them and dark crater floors. It was named after the King of the Fairies in A Midsummer's Night Dream.[9]

Other MoonsEdit

There are 13 tiny moons known to be orbiting Uranus inside Miranda's orbit. Nine more tiny moons are known to be in big orbits beyond Oberon's orbit.[10]

How long is a day on Uranus?Edit

One day on Uranus is about 17.24 Earth hours long. Uranus spins on its side, maybe because of a big impact early in the history of the Solar System.[11]

How long is a year on Uranus?Edit

One year on Uranus would be 30,708 days or 84 years on Earth.[12]

What is Uranus made of?Edit

Unlike Jupiter and Saturn, Uranus is thought to be made mostly of rock and ice. The gases in its atmosphere are 83% hydrogen, 15% helium, and 2% methane. Other gases found in smaller amounts are ammonia, water, and methane.[13] Uranus' blue color comes from methane clouds, which absorb red light and reflect blue light.[14]

How much would Uranus's gravity pull on me?Edit

If you were floating close to the cloud tops of Uranus, you would be pulled down with a force about 89% of Earth's gravity.[15]

Who is Uranus named after?Edit

Uranus was named after Ouranos, the Greek name for the sky. Ouranos was the ancient Greek deity of the heavens, the earliest supreme God. According to Greek mythology, Ouranos was the husband and son of Gaia, Mother Earth.[16]

Next Topic: Neptune


  11. Gierasch, Peter J., and Philip D. Nicholson. "Uranus." World Book Online Reference Center. 2004. World Book, Inc.;
  12. Gierasch, Peter J., and Philip D. Nicholson. "Uranus." World Book Online Reference Center. 2004. World Book, Inc.;
  15. Gierasch, Peter J., and Philip D. Nicholson. "Uranus." World Book Online Reference Center. 2004. World Book, Inc.;


Neptune as seen from Voyager 2

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How big is this planet ?Edit

Template:User:Robert Horning/Whole Book

Comparison of the size of Neptune and the Earth

Neptune is very similar to Uranus in size. Its diameter is only slightly smaller, at 49,528 km wide. [1] It is almost as big as four Earths in length.[2]

What is its surface like?Edit

The atmosphere of Neptune has some dark blue spots. When the Voyager probe went by Neptune in 1989, it saw a large one called the Great Dark Spot. In 1994, it vanished, but later reappeared.[3] There is also a large white cloud nicknamed "Scooter." It goes around Neptune every 16 hours.[4] The winds of Neptune are very fast, blowing at up to 2000 km per hour(the fastest in the entire solar system).[5] That is about four times faster than the fastest recorded tornado on Earth.

What are its rings like?Edit

Neptune has some faint rings that are dark and hard to see. There are clumps in some parts of the rings where the material is denser.[6]

What are its moons like?Edit

Neptune has 13 moons. There could be more.[7]

Inner MoonsEdit

There are five small potato-shaped moons orbiting close to Neptune.


Proteus is a dark moon about 418 km across. It has an irregular shape. In Roman mythology Proteus was a sea-god who could change into any shape he wanted.[8]


Triton is the largest moon of Neptune. Scientists think that it is a lot like Pluto. It is 2700 km across. It is made of rock and ice. It has a surface temperature of −235 °C Triton has a very thin atmosphere made up of nitrogen and a little methane.

There are volcanoes that have eruptions of liquid nitrogen, dust or methane compounds. The eruptions happen because of the seasons. There are few craters because the eruptions cover them up. There are ice caps that change sizes with the seasons. There are also ridges and valleys. They may have formed because of repeated freezing and thawing.

An interesting thing about Triton's orbit is that it goes around Neptune in the opposite direction that Neptune's rotates. Because of this, scientists think that Triton was captured by Neptune long ago. In Roman mythology, Triton was the son of Neptune.[9]


Nereid is an irregularly shaped moon about 340 km across. Its orbit is very eccentric or noncircular. It may have been captured by Neptune or moved into the eccentric orbit by Triton's gravity when Triton got captured. In Roman mythology Nereids were sea nymphs.[10]

Outer MoonsEdit

There are five other known moons. They are small potato-shaped moons far from Neptune. There might be more we haven't seen yet.

How long is a day on this planet?Edit

A day on Neptune lasts 16 hours and 7 minutes.[11]

How long is a year on this planet?Edit

One year on Neptune is about 165 Earth years, or 60,265 days.[12]

What is it made of?Edit

Neptune is made of rock and metal in the core. The core is probably bigger than Uranus's because Neptune weighs more, but is the same size. Around the core is rock, water, ammonia and methane. The atmosphere is made of hydrogen and helium. Lower down in the atmosphere, there is methane and ammonia too. The methane makes Neptune look blue-green.[13]

Artistic impression of the mythological Neptune

How much would Neptune's gravity pull on me?Edit

If you were floating close to the cloud tops of Neptune, it would pull you down with a force only a little stronger than the force of Earth's gravity.[14] The effects of Neptune's larger radius and its mass almost cancel out, making the force only a little bigger.

Who is it named after?Edit

Neptune is named after the Roman god of the seas, also known as Poseidon in ancient Greece. [15]

Next Topic: Pluto


  1. Smith, Bradford A. "Neptune." World Book Online Reference Center. 2004. World Book, Inc.;
  3. Smith, Bradford A. "Neptune." World Book Online Reference Center. 2004. World Book, Inc.;
  6. Smith, Bradford A. "Neptune." World Book Online Reference Center. 2004. World Book, Inc.;
  11. Smith, Bradford A. "Neptune." World Book Online Reference Center. 2004. World Book, Inc.;
  12.; Smith, Bradford A. "Neptune." World Book Online Reference Center. 2004. World Book, Inc.;
  13.; Smith, Bradford A. "Neptune." World Book Online Reference Center. 2004. World Book, Inc.;
  14. Snow, Theodore P. (1996) "The Outer Planets." In The Dynamic Universe: An Introduction to Astronomy. pp. 281. West Publishing Company. ISBN 0-314-64212-9
  15. Snow, Theodore P. (1996) "The Outer Planets." In The Dynamic Universe: An Introduction to Astronomy. pp. 282. West Publishing Company. ISBN 0-314-64212-9


Pluto is a dwarf planet that was discovered by the astronomer Clyde W. Tombaugh in Arizona on February 18, 1930.[1]

Pluto, as seen from the New Horizons spacecraft.

  Pluto Facts:

  • Pluto is a dwarf planet, smaller even than Earth's Moon.
  • Pluto sometimes moves closer to the sun than Neptune.

How big is Pluto?Edit

Comparison of the sizes of Pluto, Charon, the Moon and Earth

Pluto's mass is about 12,500,000,000,000,000,000,000 kilograms.[2] While this may seem large, it's only about 1/500th of the Earth's mass. Pluto is between 2200 and 2400 kilometers across.[3] Its surface area is about 17,950,000 square kilometers (or 1/30th of the Earth's).[4] Its volume is 7,150,000,000 km3 (or 1/150th of the Earth's).[5]
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What is its surface like?Edit

The surface of Pluto.

Pluto's surface is covered with ice.[6] It is very cold, at about -230 °C.[7] Pluto also has a very thin atmosphere which freezes when Pluto moves far away from the Sun.[8]

The image above on the right shows Pluto's color.

What are Pluto's moons like?Edit

Pluto has three known moons. The largest is called Charon. Charon is about half as wide as Pluto. Because Pluto and Charon are so close in size, they are sometimes called a "double planet".[9] Charon's surface is covered in water ice.[10] Two other moons were discovered in 2005. They have been named Nix and Hydra.[11]

How long is a day on Pluto?Edit

One day on Pluto is about 6.487 Earth days long. Like Uranus, Pluto also spins on its side.[12]

How long is a year on Pluto?Edit

One year on Pluto would be about 90,613 days or 248 years on Earth![13]

What is it made of?Edit

Scientists believe Pluto is made mostly of rock and ice,[14] but they will not be sure until more research is done. The discovery of Charon helped scientists estimate the density of Pluto. The information collected told them what Pluto was and was not made out of. If Pluto were made out of heavy solids, it would have a very high density. If it were made of gases, it would have a low density. Pluto is somewhere in between, so it is probably made of rock and ice.

How much would Pluto's gravity pull on me?Edit

If you were on Pluto, gravity would be only 0.06 times as strong as it is on Earth. [15] This means you could do really high jumps—even more than people could on the Moon!

Who is Pluto named after?Edit

Pluto was named after the Roman god of the underworld. In Roman mythology, he kidnapped Proserpina (Persephone) so he could marry her. This made her mother, Ceres, the goddess of agriculture, very sad, causing winter. To end winter, Jupiter, the king of the gods and her brother, decreed that Proserpina could return to the surface as long as she hadn't eaten any food of the Underworld. However, she had eaten six pomegranate seeds, so Jupiter decided she had to spend six months in the underworld each year. This is the Roman myth of winter. When she goes to the Underworld, everything stops growing. When she comes back, her mother is happy again, and life returns.[16]

In Roman mythology, Charon took dead souls across the river Acheron to the land of the dead.[17]

Is Pluto a planet?Edit

Pluto has been officially classified as a dwarf planet, which is different from a regular planet. One reason is its small size - although it is the tenth largest known object that revolves around the sun, it is smaller than many moons, including Earth's moon. Scientists used to think that Pluto was a lot larger than it actually is,[18] and it was thought of as the ninth planet for many years.

Another key reason is that Pluto is part of a large group of objects called the Kuiper Belt, which all revolve around the Sun in the area beyond Neptune. In January 2005 another object "Eris" was discovered in the Kuiper Belt. Eris is larger than Pluto. Scientists think there are even more Pluto-sized objects in this part of the solar system, as well as millions of smaller objects. Because of this, the International Astronomical Union (IAU) defined the term 'planet' for the first time. Under the definition, both Eris and Pluto (along with Ceres, Haumea and Makemake) are dwarf planets.

In spite of this, some people continue to hold on to the idea that Pluto is a regular planet because of tradition. Also, some textbooks and references are not up to date, and still list Pluto as the ninth planet.

Next Topic: Comets


  2. Snow, Theodore P. (1996) "The Outer Planets." In The Dynamic Universe: An Introduction to Astronomy. pp. 285. West Publishing Company. ISBN 0-314-64212-9;
  3. Spinrad, Hyron. 2004 "Pluto." World Book Online Reference Center. 2004. World Book, Inc.;; Snow, Theodore P. (1996) "The Outer Planets." In The Dynamic Universe: An Introduction to Astronomy. pp. 285. West Publishing Company. ISBN 0-314-64212-9;,eds,astronomy-basics.php&a=,eds
  6. Snow, Theodore P. (1996) "The Outer Planets." In The Dynamic Universe: An Introduction to Astronomy. pp. 287. West Publishing Company. ISBN 0-314-64212-9;
  7.; Snow, Theodore P. (1996) "The Outer Planets." In The Dynamic Universe: An Introduction to Astronomy. pp. 285. West Publishing Company. ISBN 0-314-64212-9;
  8. Snow, Theodore P. (1996) "The Outer Planets." In The Dynamic Universe: An Introduction to Astronomy. pp. 287. West Publishing Company. ISBN 0-314-64212-9;
  15. Snow, Theodore P. (1996) "The Outer Planets." In The Dynamic Universe: An Introduction to Astronomy. pp. 285. West Publishing Company. ISBN 0-314-64212-9;
  18. Sobel, Dava (2005) "The Planets." pp. 220. Harper Perennial Publishing Company. ISBN 1-84115-621-3