An orthodromic or great-circle route on the Earth’s surface is the shortest possible real way (distance) between any two points.
Ortho(A, B) = r x acos[[cos(LatA) x cos(LatB) x cos(LongB-LongA)] + [sin(LatA) x sin(LatB)]]
Where: r is the radius of the Earth (6371 kilometers, 3959 miles).
NOTE (Geonames Database)
The Geonames lat & lon data are in Decimal Degrees (wgs84)
Decimal Degrees to Radians = RADIANS(DecimalDegrees) or DecimalDegrees * Pi/180