Hi Kenneth,

A key bit of information you need to figure out how large something is is how far away it is. This involves geometry and trigonometry, but basically the size something appears to be depends on how large it actually is and how far away it is so once you know one of those things you can figure out the other.

For the Sun this was figured out using observations of Venus, and something called parallax, which is the effect you see if you put a finger in front of your face and look at it with first you right eye and then your left. Your finger seems to jump back and forth compared to the background and the closer it is to your face the more it appears to move. This effect was measured during a transit of Venus across the Sun using Venus as a the finger, the Sun as the background, and observers on different parts of the Earth as the two eyes. From that people could tell the distance to Venus and with some more calculations, the distance to the Sun. There is more information below:

http://sunearth.gsfc.nasa.../vt_vtstory_2004.htm

http://www.astro-tom.com/...rth-sun_distance.htm

Terry