This question came to mind after reading a comment on another question comparing Equestria to a Ptolemaic/geocentric universe.
Since the first episode, Celestia and Luna have been described as the ones who raise and lower the sun and moon - without them Equestria would be stuck in an eternal night/day (exactly what Nightmare Moon tried to do).
The show intro sequence claims that:
Once upon a time, in the magical land of Equestria, there were two regal sisters who ruled together and created harmony for all the land. To do this, the eldest used her unicorn powers to raise the sun at dawn; the younger brought out the moon to begin the night.
The use of the word "raise" and the fact that the planets couldn't simply be left to orbit made fans argue if the planet Equestria is on is shaped like a sphere or is flat like Discworld.
For example, one comment at https://tvtropes.org/pmwiki/pmwiki.php/Headscratchers/MyLittlePonyFriendshipIsMagicSetting argues that it's flat:
Observation and deduction time: The world of Equestria is self-evidently Geocentric, and the fact that the moon and sun have to be raised DAILY, rather than simply left to orbit... and that they can be stopped dead in their tracks without catastrophic environmental consequences (such as the Sun and Moon 'falling out of the sky')... implies it's a flat world as well, a la Terry Pratchett's Discworld.
Is there any concrete evidence to either theory to be found in the show?