Start with the earth's radius. The planet's mean radius is 3.9 miles.
Determine the difference in latitude and longitude of both cities, represented by the arithmetic formula "lat = lat2' lat1 and "long = long2' long1.
Compute the results according to the following formula: sin²("lat/2) + cos(lat1).cos(lat2).sin²("long/2). In other words, the sine squared times the difference of the latitude, times the cosine of the first latitude, multiplied by the cosine of the second latitude. Multiply that by the sine squared of the difference of the latitudes divided by 2. This will yield the value "a" for the purposes of the formula.
Compute the value "c" by multiplying 2 times the tangent of 2 times the square root of 1 minus a. In mathematical terms, it will be 2 x (a)tan2(√a, √(1'a)).
Calculate the radial distance between the two cities. The distance equals "radius times c," or 3.9 miles times what you found for "c" in Step 4.