Can anyone solve this problem and provide steps?

1) The Ancient Greek scientist Erathosthenes is famous for measuring the circumference of the earth thousands of years before satellites. He did this by measuring a shadow in two different cities directly north and south of each other at the same time. In one city, sunlight shined straight down and formed non shadow.in the other city the shadow showed that the sun was shining down at a 7 degree angle. He hired a man to measure the distance Between the two cities and found that they were 489.33 miles apart Using this information what did he calculate the earths circumference to be? Assume the Earth is a perfect sphere?

2) The actual circumference of the earth at the equator is 24,901 miles. Why wasn't his answer exactly right? List at least three reasons why?