In Camera Control -> Flight Mode (on Windows, Navigation -> Flight Mode), you can set different flight modes/reference frames so that you can change the way you move in Celestia.
In this mode, you do not set any reference object or target object. In this mode you can do free flights not bound to any object. You can press Esc to enter this mode.
In this mode, you need to set a reference object. In this mode, the relative position to selected object is kept. This is the default mode for a Go to command.
This is the same as Follow, but in addition synchronizes observer's orbit of the object to the rotation of the object.
In this mode, you need to set a reference object and a target object. Thie mode binds the two objects together in space. You will remain stationary above both objects, even as they move in space.
This is the same as Phase Lock, except that you only need to provide one object. The other object will be the object's parent.