Celestia provides you with very powerful and flexible ways to control your view into Celestia's simulation of the cosmos. Once you understand how to manipulate its interactive controls, you can quickly and easily position yourself to view the astronomical phenomena that interest you. If is helpful to think of Celestia as providing a remotely-controlled camera that is at your command. The view that you see on your screen is determined by the location of the camera, the direction the camera is pointing, and the magnification of the camera's zoom lens. Each of these factors can be controlled independently.
One important thing to understand is that both the location and direction of the camera are specified relative to some frame of reference. The frame of reference may be attached to a moving object (such as a planet or moon). If so, then the actual location and direction of the camera (with respect to absolute space) will automatically change as the object moves. This is often very convenient, because it means that you do not have to continuously change the camera location and direction manually in order to keep the object in view.
Commands which change Celestia's view of the universe are described in the file Celestia/controls.txt
The magnification of the camera's zoom lens determines how large or small objects will appear on your screen, and also how much of the environment around you is visible (the field of view). If you increase the magnification then objects will appear larger, but you'll have a smaller area in view. (You can also make objects appear larger by moving the camera closer to them. The difference between zooming and moving the camera is important, as we will explain below.) Celestia displays the current magnification factor and field of view in the lower right hand corner of the window. As you zoom in, the magnification factor will increase, and the field of view will decrease. Zooming out has the opposite effect. If you zoom out to a very large field of view, the view will become distorted, much like what you would see with a fish-eye lens.
Repeatedly typing a comma (,) narrows the field of view, increasing the magnification. Repeatedly typing a period (.) widens the field of view, decreasing the magnification.