Screencast

Read first latitude Remind user to enter degrees
Read first longitude
Read second latitude and longitude
Convert all inputs to radians (× π / 180)
arc length = acos ( sin (lat1) sin (lat2) + cos (lat1) cos (lat2)
cos (long2 - long1) )
distance = arc length × radius of the earth
Test Case 1
Starting point latitude 45°, longitude 30°
Ending point latitude -45°, longitude 30°
Expected distance: 1/4 of the circumference of the earth
= 1/4 × 2 × π × 3958 ≈ 6217 miles
Test Case 2
Starting point latitude 40.759°, longitude -73.984° Times Sq NY
Ending point latitude 37.794°, longitude -122.395° San Francisco
Expected distance: Somewhat less than 2919 miles