Lunar eclipses always cast a circular shadow of Earth on the Moon.

During a lunar eclipse, the Earth passes directly between the Sun and the Moon, casting its shadow onto the lunar surface. Throughout history, observers have noted that this shadow is consistently circular, regardless of the Earth's orientation in space. The only object that consistently casts a circular shadow from any angle is a sphere. If the Earth were a flat disc, its shadow would vary from an ellipse to a straight line depending on the angle at which the sun illuminated it relative to the moon, which is never observed. This immutable circular shadow is powerful, ancient evidence.