What is the simplest way to calculate the great circle distance between two place?

On your desktop or laptop, please open the Google Maps. If you're using Maps in Lite mode, you’ll see a lightning bolt at the footer and you won't be able to measure the distance between points.

Right-click on your starting point.

select Measure distance.

Click anywhere on the map to create a path to measure. Please add another point, click anywhere on the map.

Optional: Drag a path to move it, or click a point to remove it.

At the bottom, you’ll see the distance in miles and kilometers.

Easiest way for calculating distance visit: https://distanciaskm.com/

Remember the shortest distance across any sphere is a line  across the surface, that if made into a plane through the sphere, would cut the sphere into two equally sized  hemi-spheres. . 

On the Earth such lines are all the meridian/longitude lines and the equator.  Any other line forms a longer distance. 

Yes, Google Maps.