I'm not sure how astronomically correct FSX is.
There is an eclipse coming up on July 22, and FS9 shows it happen, so I thought it would look better in FSX, but nothing, the moon wasn't even where it was supposed to be.
The moon is fairly correct. I double checked a few moon/passing in front of" events, and except for the solar (and lunar, for that matter) eclipses, it seems dead on. For example, 1 April, 2006, the moon passing in eclipsing the Pleiades star cluster. It's dead on, but there is one little detail that makes my teeth grind:

The stars are still visible, even though they are behind the moon. I hate that. Grrrrr!
I think the only way around that is a black disk in the "image" with the moon phase, then the Alpha mask to match the phase. Sort of like the same way the "strange moons" were made. I'll experiment over the weekend.
Hi Rez Moon V3.0? :P
Maybe an overlooked feature?
Like masking out the stars. Aaaaargh!