A precise conversion is difficult because Earth is not a perfect sphere. Latitude is easier to convert. The circumference of Earth taken over the poles is 24,859.82 miles, which is equivalent to 360° of latitude. Therefore one degree is equal to 69 miles. So, the north-south distance between two cities 5° of latitude apart would be 345 miles.

Longitude is more complicated. At the equator 360° is Earth’s circumference, 24,901.55 miles. But at the poles it is zero! So the conversion of longitude to distances depends on the latitude. If you use trigonometry to find the distance, you will find that the circumference at a latitude of θ degrees is the circumference at the equator times the cosine of the angle θ. So, at the latitude 40°, the circumference is 19,076 miles, and one degree of longitude is 53 miles. This result is only approximate because Earth is not a perfect sphere. For more accurate conversions consult a website such as http://www.nhc.noaa.gov/gccalc.shtml.