Returns the great-arc distance in radians between the two points a and b.
Each point must be specified as a two-element array [longitude, latitude] in degrees.
Parameters
a: [number, number]
Point specified as a two-element array [longitude, latitude] in degrees.
b: [number, number]
Point specified as a two-element array [longitude, latitude] in degrees.
Returns the great-arc distance in radians between the two points a and b. Each point must be specified as a two-element array [longitude, latitude] in degrees.