In Copernicus's heliocentric model the planets orbit in circles around the sun. One may wonder how Copernicus determined the order of the planets and the size of their orbits – after all, he had no benefit of pictures from spacecraft or even a telescope. He determined these things by measuring elongation. Elongation is the angle between the sun and a planet as seen by an observer on earth. Imagine that you could see Venus and the sun at the same time. To measure the elongation of Venus, you could hold a piece of paper so that its flat edge is in line with the two objects. Then you would draw a line pointing to Venus and a line pointing to the sun. Measuring the angle between the two lines gives you the elongation of Venus. (Looking at directly at the sun is not recommended!)