As opposed to the northern/southern hemisphere that has:
- a clear reference point (equators)
- traditionally we don’t model a rotation north/south
Western/eastern hemisphere
- doesn’t have a clear reference (Greenwich?)
- traditionally we model rotation on the north/south axis so western/eastern would depend from the time
Yes, that’s the prime meridian, so if you’re looking for exactitude in your hemispheric definition it is the one to go with. Any system of coordinates is an arbitrary choice imposed on the universe for our convenience. It may seem attractively rebellious to reject the one we have, but you’re not going to gain anything but confusion by picking a different one.