Here is a simple chart showing how an ancient Greek figured out the size of our planet. remember that he did it in 250 BC using only sticks and shadows.

Here is another graphic showing the angles of the shadows in more detail. Note that he was off just a smidgen on his calculations. That 22.77508... should be 23.5. Twenty-three point five degrees is the tilt of the earth on its axis relative to the sun.