two obsevation earth is not flat
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Explanation:
This story was originally published on Smarter Than That in 2008. We are republishing a lightly edited version on Popular Science in light of recent interest in the subject.
Humanity has known Earth is round for a few millenia, and I've been meaning to show more methods that prove the world is not flat. I've had a few ideas on how to do that, but recently got an interesting incentive, when Phil Plait, The Bad Astronomer, wrote about a recently published BBC article about "The Flat Earth" society. (Most recently, rapper B.o.B. went on a Twitter rant on the topic.) Phil claims it's ridiculous to even bother rebutting the Flat Earth Society—and I tend to agree. But the history of our species' intellectual pursuit is important and interesting. You don't need to denounce all science and knowledge and believe in a kooky conspiracy theory to enjoy some historical factoids about humanity's quest for space.
Earth from the ISS
Earth from the ISS
The curvature of the Earth is visible in this 2014 photo, which ESA astronaut Samantha Cristoforetti snapped from the International Space Station.NASA/Samantha Cristoforetti
On we go, to the top 10 ways to know the Earth is unequivocally, absolutely, positively, 100% not flat!
1. The Moon
Now that humanity knows quite positively that the Moon is not a piece of cheese or a playful god, the phenomena that accompany it (from its monthly cycles to lunar eclipses) are well-explained. It was quite a mystery to the ancient Greeks, though, and in their quest for knowledge, they came up with a few insightful observations that helped humanity figure out the shape of our planet.
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Aristotle (who made quite a lot of observations about the spherical nature of the Earth) noticed that during lunar eclipses (when the Earth’s orbit places it directly between the Sun and the Moon, creating a shadow in the process), the shadow on the Moon’s surface is round. This shadow is the planet's, and it’s a great clue about the spherical shape of the Earth.
lunar eclipse
Lunar eclipse
A sequential view of the lunar eclipse that occurred on April 15, 2014. You can see Earth's shadow crossing the face of the Moon, and the shadow's shape is curved because Earth is spherical.Javier Sánchez
Since the earth is rotating (see the "Foucault Pendulum" experiment for a definite proof, if you are doubtful), the consistent oval-shadow it produces in each and every lunar eclipse proves that the earth is not only round but spherical—absolutely, utterly, beyond a shadow of a doubt not flat