Sun>>>>>>>You don't know this distance>>>>>Earth
There are a number of ways to make this calculation.
Here is the way the Greeks did it:
Around the 7th of the lunar month, you will notice that exactly 1/2 of the
Moon is lit. That means that a path from you to the Moon makes a right
angle with the path from the Moon to the Sun. Now measure the apparent
angle between the Moon and the Sun. This gives you a trigonometry problem
that let us you calculate the Earth-Sun distance in terms of the Earth-Moon
The Earth-Moon distance can be obtained in terms of the Earth's radius by
observing the apparent angle traveled by the Moon when it has traveled, say,
four hours (60 degrees as measured from the center of the Earth).
Another way to get the Earth's radius is to note the distance from a
mountain or tower of a given height when it first appears on the horizon.
Another way is to use Parallax during a 'transit of Venus' eclipse.
Another way is to use radar to determine the distance to Venus during it's greatest elongation and then use the angle of Venus/Earth/Sun to calculate the distance to the Sun.
There is yet another way...using parallax to determine the distance from Earth to Mars and then using Kepler's Thrid Law of Planetary Motion...a bit more complicated.