Felon said:
You were initially laying out a situation where a large group of people were being misled for hours by the spell. I'll grant that the issue of concealing something like the sun with what would appear to be a low-flying cloud is a bit subjective (you certainly couldn't block all of the light cast by the sun, or the resulting direction of the shadows). However, the main reason it doesn't work is because once the spell is cast the figment can only move within a relatively small area of effect (a certain number of 10-foot cubes). So right there it's not traveling with you for any significant distance.
Assume the following:
Your campaign world is the size of earth with a moon the size of Earth's moon and at the same distance. Also, the sun is the same size and distance from your world as it is from Earth.
Also, assuming that my math is correct...
The moon has a mean diameter of 3476 KM. It's mean distace from the earth is 384000 KM. If my trig is correct, if you could make a a copy of the moon that was 8.53m in diameter it would have to be 942.32m distant to look like the real moon.
Just to flesh out those numbers a bit... an 8th level caster can make a silent image that occupies 4 +1/level so 4+8 or 12 10 foot cubes. Imagine a square made of 16 10 foot squares, 4 to a side. Now remove the corner squares, you have something like a great big plus "+" sign, this is how to arrange those 12 10foot cubes. In that space an illusionist could place a 28 foot diameter (8.53m) flat image of the moon. If he did so 942.32m distant, it would look exactly like the moon.
And that is for a full moon. Imagine that you only had one third (or less) of a moon visible, creativley arranging those 12 cubes you could make the illusion of a moon that would be say 30 or more feet in diameter, you could be even further away to make that image.
Further, even though they are different sizes the sun and moon appear as the same size because the moon is closer. Proof of this is that during an eclipse, the moon completely blocks out the sun. So all the math I did above applies to making an illusion of a sun as well.
True the real sun would make shadows contrary to the illusionary sun, but moving the sun say 10 degrees could make a person, group, army walk in a different direction for a significant amount of time.