Given a set of points, it is frequently needed to calculate their "average" point - a weighted center of sorts. Simply creating a new point with AVG(longitude(point)), AVG(latitude(point)) may work in many places, but it could be grossly incorrect near anti-meridian.
this stackoverflow question provides a simple explanation of how it is done, by transitioning each point's WSG 84 (longitude, latitude) into Cartesian system (x,y,z), averaging on each coordinate, and converting back. If earth is assumed to be a sphere, the calculations are slightly simpler.
I found this example that uses JTS (reference library for all geo calcs) - https://gist.github.com/oschrenk/2787570
Usage
SELECT (geof:pointCentroid( ?location ) as ?center) WHERE { ?place wdt:P625 ?location . ... }