Aristarchus noticed that a Lunar eclipse lasts two hours.

This helped him compare the Moon's diameter with the Earth's diameter.

 

 

 

 

Aristarchus thought the Moon is fully in the shade of the Earth when the total eclipse starts (t=0) : it's already run once its diameter through the shade. It makes the first of the three "moons" (see animation above).

Then the moon keeps sliding for the first hour, up to the second of the three moons.

Then it slides for one more hour until it reaches the third "moon".

Afterwards, it goes out of the shadow of the earth, it is at this moment that Arsitarque stops counting because the total eclipse ends.

That way, he determined that the Moon's diameter was three 3 times smaller than the Earth's diameter.

 

Nowadays, we know that the Earth's diameter is equal to 12742 km. So the Moon's diameter is close to : 12742 divided by 3, which gives approximately 3474 kilometres.

 

In reality, due to the conic shape of the shade of the Earth, the Moon's diameter is equal to the Earth's diameter divided by 3.66. The actual diameter of the Moon is equal to 4247.4 kilometres.

Olivia