Celestia has full support for game controllers. When your device is paired with a supported game controller, Celestia starts receiving inputs from the controller automatically with no action required (On Win32, you can use F8 to toggle game controller control).
While vanilla Celestia for Win32 only uses a limited number of buttons (see here), Celestia on Android/iOS/macOS/Windows(UWP) adds a layer of customization which enables uses of more buttons. You can find out about them in Settings -> Game Controller (on Windows, it is File -> Settings -> Game Controller).
For the buttons listed in the settings, you can assign an action to each of them. The Menu/Select button is kept for showing the menu.
|Button action will be ignored
|Increase the speed of traveling (default for button X / Square)
|Decrease the speed of traveling (default for button A / X)
|Set the speed of traveling to 0
|Reverse Travel Direction
|Travel in the opposite direction, current orientation is kept
|Reverse Observer Orientation
|Change orientation to the opposite of current
|Simulate a click at the center of the view
|Go to Object
|Go to the current selected object
|Simulate Esc key
|Change pitch (default for D-pad up, down)
|Change roll (default for LT RT / L2 R2, D-pad left, right)
For example you can custommize and apply the settings from below to get an experience closer to other games that supports game controllers.
|A / X
|B / Circle
|LT / L2
Thumbsticks are used for Pitch Up/Down and Yaw Left/Right. You can invert them in the settings if necessary.