If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

# Size of the sun

## Lucky match

This is an image taken from the Mir space station showing the Moon's shadow 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 sun’s diameter to moon’s diameter.

### distance to sun / distance to moon = sun diameter / moon diameter

We’ve already determined these values so we can simply plug them in!

### sun diameter = 1 352 004 km

This is very close to the actual value! The actual 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 this lesson!

You are ready to tackle some bigger questions...

## Want to join the conversation?

• In this section, we calculate the value and are told that it is "very close to the actual value". How was the "actual value" of the diameter of the sun measured? With these same methods, but better tools and higher accuracy?
• the sun is so big but the earth is so small why is that
• The Sun is a star, the Earth is a planet
• Okay, correct me if I'm wrong, but I believe the size difference between the sun and the moon is canceled out by their relative distances from Earth, which is why we can have total solar eclipses, even though the sun is so much bigger than the moon. But since our sun is expanding faster than the moon is moving away from earth, does that mean that we won't always have total solar eclipses? If so, does anyone know how much longer we'll have them for?
• How big is the sun really is?
• the radius of the sun is 696,000 kilometres and the diameter is 1.392 million kilometres in this case we can end up 110 earths
(1 vote)
• is there gonna be a geography section on khan Academy? Reall y want one thx if and hope u consider a geography section.