Let's talk about Spherical Mercator. I've made a couple of references to it throughout this chapter, but what is it? It is a term used to refer to the projection that many commercial, third party mapping APIs use. We need to use Spherical Mercator to properly overlay data and layers on top of third party map layers.
All the APIs we've used in this chapter so far (Google, Yahoo!, VirtualEarth/Bing, and OSM) are in the Spherical Mercator projection—a projection that treats the earth as a sphere (as opposed to an ellipsoid).
The official EPSG code for the projection is currently EPSG:3857, but when the code was established it was referred to as EPSG:900913. This EPSG:900913 code can still be used in OpenLayers, as the EPSG:3857 code is identical to it. Google was one of the (if not the) first to publish maps with this projection—notice how 900913 resembles 'gOOglE' (with a backwards 'E' for 3).