But, I tried something different, and the result was a more realistic sun/moon with no problems at all.
So, how do you do a more realistic sun without any problems at all?
Well, the answer to that is very simple, you use actors instead:
- Place any decoration actor on the skybox, like a "barrel" for example;
- Then set its properties, not by code, but by the actor properties window (right click on the actor, then click "properties"):
1) Display:
2) Advanced:Texture: <The sun or moon texture>
bUnlit: True
DrawType: DT_Sprite
DrawScale: <Whatever you want how big is your sun/moon (1.0 for normal size)>
DrawStyle: STY_Translucent for sun case, STY_Masked for moon case
3) Movement:bNetTemporary: False
4) Collision: Everything which is boolean set to FalsePhysics: PHYS_None
What are the advantages of this?
a) More realistic sun or moon on the sky
b) You can place it anywere without having rebuild all the map geometry/BSP
c) You can resize it without mading calculations to set the right scale and size
d) It's one less bsp to care about, one less motive to have a HOM or a BSP Hole
e) Other ones that you can find

This tutorial is originally made by me, and that is one thing I tested and worked on my own maps. Enjoy