[danieltillett]

Western hemisphere, Eastern hemisphere. Ultimately down to the 0 meridian running through Greenwich and for this you can thank English naval power.

[omellet]

Or you can thank the English for figuring out how to measure longitude.

[danieltillett]

Well yes :)

Greenwich is actually not a bad location to place the 0 meridian since it makes the international date line run through the least populated region of the world.