<human>:  I have a question about the life of the Egyptian astronomer, Ahmad ibn Mahmoud. What was his most famous discovery?
<bot>: He was the first to calculate the circumference of the earth. He did this by measuring the length of the shadow of a gnomon, a rod that casts a shadow, at the summer and winter solstices. He found that the length of the shadow was different at the two solstices, and that the difference was due to the fact that the earth was not a perfect sphere. He also calculated the distance to the sun, which was a major breakthrough in astronomy.