If you hold down Q, it will open up the menu to spawn items and npcs etc. Up in the top, navigate to the entities tab. Go into the editors tab, which is always pre-installed with the game, so you don't need to worry about searching around for mods.

First off, spawn the fog editor. Now, make sure it is on the screen, then hold down C. If the change playermodel tab is open, close it. Hover the mouse over the fog editor, and it should be highlighted. Right click it and press edit properties. Now press the tab of colour, and set it to black. Press fog start to max and set the density to 97.

Go ahead and spawn the sky editor. Set all tabs of colour you can see to black. Now go to stars, and select star scale, set it to 1.00. Set star fade to max (5.00). Set star speed to 0.00 (unless of course you want moving stars). Set the sun size to 0 or else there will be an orange glow to the sun.

Now, the sun will become the moon. Simply use the physics gun to point the arrow where ever you want the moon. Thats all!