30,000 miles a bit to high. 3000 miles is what most agree is correct. About the distance from east coast to west coast USA.

We can triangulate the height of the sun by using the angels of the shadow it cast. Preferably by taking measurements midday, when the sun is at its highest position to your relative sky.

There is always a place where you are directly under the sun, so a object standing straight up, cast no shadow. So that way we can know where the 90° of the triangle is.