How do we know that the earth is round?
We're often told that the Earth is a sphere, like a giant ball, but how do we really know for sure? For centuries, people looked up at the night sky and wondered about the shape of our home. Was it flat like a pancake? A giant box? Our story begins with a super-smart guy named Aristotle, who loved asking big questions, around 350 BCE in ancient Athens.
Aristotle noticed a peculiar thing about lunar eclipses in his observation and accounts of people before him. A lunar eclipse is when the Earth gets right between the Sun and the Moon, and the shadow of our Earth is seen on the surface of the Moon.
Aristotle noticed that no matter when he saw an eclipse, the shadow the Earth cast on the Moon was always round. Not square, not a squiggle, always a perfect curve!
Now, here's the big brain-teaser Aristotle faced: What kind of 3D shape always casts a perfectly round shadow, no matter how you shine a light on it? Think about it... a box makes square or rectangle shadows. A pyramid makes triangle shadows. The only shape that fits the bill is a sphere!