# Size of theÂ sun

## Lucky match

This is an image taken from the Mir space station showing the Moon'sÂ shadowÂ during a Solar Eclipse.

Why do you think a total solar eclipse is important in terms of geometry?

When the Moon passes perfectly in line with the Sun it blocks it out completely. This means they have the same angular size in the sky. This geometric relationship allows us to apply the properties of similar triangles.

Thanks to similar triangles we know the

**ratio of distances**from earth to sun and earth to moon must equal the**ratio of sunâ€™s diameter toÂ moonâ€™s diameter**.### distance to sun / distance to moon = __sun diameter__ / moon diameter

Weâ€™ve already determined these valuesÂ so we can simply

__plug them in__!### 149 600 000 / 384 400 = __sun diameter__ / 3474

### sun diameter = 1 352 004 km

This is very close to the actual value! The actualÂ diameter is closer to

**1 391 000 km**How cool is that? We measured the size of the sun (about 1.3 million kilometers) using some basic observations and a little geometry! This illustration shows the approximate size of Earth compared to the Sun if they were put

**directly beside**each other.## Congratulations, you have completed thisÂ lesson!

You are ready toÂ tackle someÂ some bigger questions...