By Thomas McGuire | september 30, 2015

The actual size of the moon

You may be surprised that ancient astronomers were able to calculate both the size and the distance to the moon. In fact you can use your own observations of last Sunday evening’s lunar eclipse, or my photograph of Earth’s shadow moving across the moon shown here to help you yourself determine both values. See the URL below.

Note the curved shape of Earth’s shadow as it partly covers the moon. It is an arc. (part of a circle) By using this arc to draw a full circle, you can compare the diameters of the Earth and moon. Which is larger?

The Greek scientist and mathematician Eratosthenes calculated Earth’s circumference more than two millennia ago; about 200 years before the birth of Christ. Based on that figure, it was relatively easy for him to determine both the size (diameter) of the moon as well as its distance from Earth.

But here’s an interesting question...

Did Eratosthenes actually believe that Earth is round, or were his calculations simply an interesting exercise in mathematics?

How anyone can calculate the size of Earth and our moon, as well as the distances to the moon and the sun:
http://www.astro.washington.edu/courses/astro211/CoursePack/cp03a_distance_sun.pdf

Eratosthenes: https://en.wikipedia.org/wiki/Eratosthenes

photo by Thomas McGuire: Eclipse