We have various ways of knowing that the Earth is round – many of which involve some degree of geometry. For one, we have had to account for the curvature of the Earth while navigating and mapmaking for centuries. Going into the modern day, you can literally fly a plane in a straight line and end up right back where you started. The ancient greeks measured the roundness of the Earth by comparing the angle of the sun in the sky at different places at the same time on the same day. They used these angles to calculate the amount that the Earth curved between those two places. Another way we know that the Earth is round and spinning is foucault’s pendulum, or gyroscopes. Someone holding a gyroscope will see it rotate over the course of a day, and this rotation will tell them exactly how far and in what direction the Earth has rotated over the course of the gyroscope’s spinning. Focault’s pendulum is a pendulum which is used in such a way that it behaves somewhat like a gyroscope, and the rotation of the Earth causes it to rotate. This rotation varies based upon latitude, once again allowing us to extrapolate the curvature of the Earth. Last, but not least, we can literally send people to space who can look at the Earth and say “Huh. Looks like a ball from here.” There are a million other ways to demonstrate the Earth’s flatness, but hopefully that gives you some insight into easy ways to do it at home.

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