For you to calculate the distance between two points on a great circle, one has to know the positions of the two points. E.g point A(300N, 700E) and point B(400S, 700E), then follow the procedure below.

• Get the longitude difference,. i.e x= 30 + 40=70
• Apply the formula for calculating arc length, L= x/360 * 2*22/7*R , where R=6370km and x longitude difference =70
• The distance obtained above is in km.

• https://www.youtube.com/watch?v=eRIFM1bojO8