On Calculating the Circumference of the Earth

There is an approximate delay of 4 minutes from Atlantic City to Philadelphia--a 60 mile Earth distance--for the sun to set.  So... the Earth rotates at about 15 miles per minute.  And since there are 1440 minutes in a day, I calculate the Earth's circumference to be ~21,600 miles, which is a far more accurate estimation than Eratosthenes.