eli5 : How did we measure the distance between the sun and the earth?
In: Physics
The ancient Greeks were the first to measure the distance, and the way they did it was very clever.
First, Eratosthenes measured the size of the Earth, which you may have learned about in middle/high school. He knew that on a certain day, the sun would be directly overhead in Egypt… and in Greece he could measure the length of a shadow when the sun was at its highest point. He then used the length of the shadow along with the distance between Egypt and Greece to calculate the circumference of the Earth.
Next, the Greek’s figured out the size of the moon. They did this during a lunar eclipse, when the Earth cast its shadow on the moon. By measuring the size of Earth’s shadow, they could determine the size of the moon. And if you know how big the moon is, and how big it appears, you can calculate how far away it is.
The final step is to calculate the distance to the sun. On a perfect half-moon, the Earth, Moon, and Sun form a right triangle (with the 90 degree angle at the half-moon). Now, since you know the distance from the Earth to the moon, you can measure the angle between the sun and the moon, and use a little SOHCAHTOA to calculate the distance from the Earth to the sun. Sadly, this method is not super accurate, but it does give some sort of answer.
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