How do we know that the Earth is round?

Space

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by Mya Kagan (whyzz writer) >> more about the author

Have you ever looked off into the horizon, to where it seems that the Earth meets the sky, and assumed that must be one of the “edges” of the Earth?? It would seem that if you walked all the way to that point, you would reach some kind of a corner…!

For many years, people on Earth thought our planet was a flat shape with edges. The first known proposals that the Earth was round (or, more specifically, spherical like a ball) were made by philosophers and astronomers in Ancient Greece.

One piece of evidence used by the Ancient Greeks was found during something called a “lunar eclipse.” This refers to special times when the Earth lines up directly between the sun and moon and casts a shadow on the moon. When this would happen, people could see the round, curved edge that the Earth’s shadow created! (Only something with a round shape could create that kind of shadow.) Another type of proof had to do with the way the sun cast shadows at different angles on the same day of the year in different cities. If the Earth was flat, the sun’s rays would have hit the objects at the same angle! (Check out the experiment below to see the way this works!) People in Ancient Greece also defended the idea that the Earth was round by observing the way the stars in the sky were different in certain locations; they knew that on a flat Earth, everyone would be able to look up and see the same stars at the same time!

In spite of these discoveries, lots of people still continued to believe that the Earth was flat for many, many years! Eventually, explorers like Christopher Columbus would help provide further proof by sailing to “the end of the Earth” and showing that it didn’t exist! When a ship reached the horizon, those on land could see that instead of dropping off the “edge” and disappearing completely, it would continue to get smaller and smaller until only the tallest point of its sail was visible. This meant that instead of falling off a corner, it was slowly curving away!

Today, we know about the Earth’s shape with even more certainty than ever before – thanks to technology that takes astronauts into space, we can simply look at pictures and see the spherical shape!

Shadow Shapes!

To see for yourself the way that shadows helped prove the Earth’s shape, try out these two experiments!—All you need is a round ball or globe, a flashlight, some pins, and a grown-up to help you!

1. Mock Lunar Eclipse: Have someone hold out the round ball and stand a couple feet away from a blank wall. Pretend like the blank wall is the moon. (You can even cut out a circle and tape it to the wall to represent the moon.) Position yourself so the person holding the ball is between you and the wall, and then turn off all the lights. Pretend like you’re the sun and shine the beam of your flashlight directly at the ball! What is the shape of the ball’s shadow wall??

2. Have a grown-up help you put two push-pins into different parts of your ball or globe. (Use other objects with tape if push-pins will pop your ball!) Put one of the pins into the “center” of the ball, like where the equator would be. Put the other one an inch or two above the first. Have your grown-up helper act like the sun and shine the flashlight directly at the ball. Position yourself close to the ball or globe and observe the shadows made by each pin, and the way they’re cast at different angles! To see how it would be different on a flat surface, ask the grown-up to help you place both pins on a flat table-top and then shine the flashlight down on them from above: They make shadows now at the same angle!