Determine the centroid of multiple points

Question

I'm writing a mapping application that I am writing in python and I need to get the lat/lon centroid of N points. Say I have two locations

a.lat = 101
a.lon = 230

b.lat = 146
b.lon = 200

Getting the center of two points is fairly easy using a euclidean formula. I would like to be able to do it for more then two points.

Fundamentally I'm looking to do something like http://a.placebetween.us/ where one can enter multiple addresses and find a the spot that is equidistant for everyone.

Answer 1

Have a look at the pdf document linked below. It explains how to apply the plane figure algorithm that Bill the Lizard mentions, but on the surface of a sphere.

poster thumbnail and some details http://img51.imageshack.us/img51/4093/centroidspostersummary.jpg
Source: http://www.jennessent.com/arcgis/shapes_poster.htm
There is also a 25 MB full-size PDF available for download.
Credit goes to mixdev for finding the link to the original source, and of course to Jenness Enterprises for making the information available. Note: I am in no way affiliated with the author of this material.

Answer 2

Adding to Andrew Rollings' answer.

You will also need to make sure that if you have points on either side of the 0/360 longitude line that you are measuring in the "right direction"

Is the center of (0,359) and (0, 1) at (0,0) or (0,180)?