Shouldn't the moon always be full in equestria?

[silvadel 1,393]

I mean the moon is always opposed to the sun (the sun is set and the moon rises and vice-versa) so the moon disc should always be fully lit. 

[~Silver Essence~ 2,625]

Well, it's up to luna to take care of the moon as it is her duty after all.

 

However, maybe being full is the point, as in the beginning of the series, it's always full.

[FabulousJeremy 48]

I may not remember correctly, as I've not paid too much attention to the background while watching the show, but isn't the moon always full when they show it?  I think a couple points it's instead been a crescent.

 

Regardless, Earth science doesn't seem to apply well to the pony world.  After all, if it did Rainbow Dash would go up in flames attempting to get close to a rainboom.

[Scootalove 10,689]

The moon probably goes in order of the eclipses, and the phases of the moon. Luna takes control of the moon, so it wouldn't be full when she isn't in control of the moon.

[RazzyJam 1,519]

I may not remember correctly, as I've not paid too much attention to the background while watching the show, but isn't the moon always full when they show it?  I think a couple points it's instead been a crescent.

 

Regardless, Earth science doesn't seem to apply well to the pony world.  After all, if it did Rainbow Dash would go up in flames attempting to get close to a rainboom.

 

Not really go up in flames, more like have all of her internal organs liquify like fruits in a blender, but lets use your description as it is less gorry. 

[Winona the Dog 1,984]

I assume Luna is in charge of controlling the moon and placing it in the certain positions so it's in a certain phase.

 

Although time in Equestria is inconstant between episodes so every time we see the moon in an episode it could be a completely different time of the year and therefore a different phase of the moon.

[Forlong 1,726]

The ponies use "moon" as a measurement of time, which is how long a lunar cycle or orbit around the Earth takes (27.5 days).  They wouldn't use that measurement if the moon didn't have phases.

 

Season 4 spoilers:

 

Also, the Moon eclipsed the Sun in the Season 4 premiere.