It is next to impossible measuring the size of the Earth using a tape measure. The Earth is too big. To measure the size of the Earth accurately, this can be done with the help of mathematics. One way of doing it is to look at a star from two places on Earth. If the distance is known between these two places, we can get answers to how big the Earth really is.

## Measuring the size of Earth from a satellite

Satellites which are used for mapping is another way to measure the size of the Earth. These satellites keep a watch on Earth from 35,900 kilometers away in space. These satellites can measure the angle between themselves and the two places on Earth. Once scientists know what these angles are, it can be used to get a measure of the Earth’s radius; the distance from the center of the Earth to its surface. If the radius of a circle is known to you, you can use mathematics to work out the length around the circle.

## How ancient Greeks measured the size of Earth

- The Greek scholar Eratosthenes studies the shadow made by a tower in Alexandria, Egypt. He measured the angle that the shadow made at noon on the longest day of the year. He knew that on the same day at the same time in Syene, the sun was at its highest point. It shone straight down into a well, so it must be directly overhead.
- The angle Eratosthenes measured at Alexandria was 7.2 degrees, or a fiftieth of a circle. So, he multiplied the distance between Syene and Alexandria by 50 to get the size of the Earth. He found that it was about 40,000 kilometers around.

### Interesting facts –

In the 6th century BC, Pythagoras claimed that the Earth is round because both the sun and moon were. It was Eratosthenes however in the 3rd century BC whose estimate came very near the 40,075 kilometers, now known to be the circumference of the planet at the equator. Today’s sophisticated and advanced equipment shows that the Earth is 42 kilometers thicker at the equator than around the poles.

**Things to do**

- The Greek scholar Eratosthenes was known for a few other discoveries. What are they?
- Eratosthenes made use of which other great scholars works while discovering the size of the Earth?
- Make a circle and then calculate the length around it.

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