What is the shortest path between two points on Earth's surface?
If you don't want to do the calculation yourself, you can use an online calculator like the one provided by NOAA.

Alternatively, if you do want to do the calculation yourself you can use the haversine formula. 


  This uses the ‘haversine’ formula to calculate the great-circle distance between two points – that is, the shortest distance over the earth’s surface – giving an ‘as-the-crow-flies’ distance between the points (ignoring any hills they fly over, of course!).
  
  $$a = \sin^2\left(\frac{\Delta\phi}{2}\right) + \cos \phi_1 ⋅ \cos \phi_2 ⋅ \sin^2\left(\frac{\Delta\lambda}{2}\right)$$
  
  $$c = 2 ⋅ \text{atan2}( \sqrt a, \sqrt {1−a} )$$
  
  $$d = R ⋅ c$$ 
  
  where $\phi$ is latitude, $\lambda$ is longitude, $R$ is earth’s radius (mean radius = 6,371km)