It's completely arbitrary, and people at the time it became standard were very aware of this. Before, each country had its own prime meridian centered on its capital. In 20,000 Leagues Under the Sea, Prof. Aronnax tries to find out Captain Nemo's country of origin by getting him to specify which meridian he uses. I can't remember how Nemo avoided this, I think it was by using the American prime meridian centered on DC, when it's very obvious Nemo isn't American.

The prime meridian is a completely arbitrary line and the "western hemisphere" (defined in terms of it) is equally arbitrary. We need a prime meridian though, if only in order to draw the international date line on the opposite side of it.

north/south is just as arbitrary, maybe even more so.

technically the antarctic pole is the North (since the magnetic field lines exit the Earth here) and the arctic pole is technically the South (here the lines enter the Earth).

we decided to name them in reverse because it was simpler to say that "north is where the north part of the magnetic hand is pointing to".

Greenwich?

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.

Perhaps "clear reference" isn't the issue. Both the equator and prime meridian are clearly defined.

The equator is kind of a natural landmark, arising from the tilt of the planet. Greenwich is a place built by humans. I think that's the rub.

Western hemisphere is an invention by US exceptionalists to justify their economic, political and military domination over the Americas

I'm not sure why you're getting downvotes, I'm sure I've thought about this is the shower too at some point

Phrases like the the West or Middle East make sense when all relative to Great Britain in geography.

The British had colonies all over and the terms like middle east and the west stuck. It's also a reason for the most common world map, the Mercator projection.

I feel like the pacific and Atlantic Oceans are at least as real as the equator.

The only explanation I have is dog whistle for some sort of “western supremacy“ 🙄