Definitive proof that celestial bodies are spherical is the moon. Lunar libration allows us to see 59% of the moons surface. The nights we can see features not normally visible are highly anticipated by amateur astronomers.

Libration occurs as the moons orbit takes it closer and farther from the earth causing it to speed up and slow down slightly throwing its spin rate slightly out of whack with its orbital speed. We can then see craters and mountains that are truly breathtaking in the contrast created.

Flat earthers really have not thought things through.

When my kids were little we would take a trip up north to the cabin with the telescope, make popcorn and marshmallows over the fire and moon watch all night just to catch a glimpse of some as yet unseen feature. I still have our logs almost 40 years later. Even with some margin notes in crayon by my little one ;) I keep them locked in the floor safe with all my important papers.