In 1884 somebody decided that the Prime Meridian runs through Greenwich, England, near London. You can’t see it, smell it, or hear it, but there it is, just the same.
From there, all the Longitudes are determined somehow, and then everybody knows something or other. It has to do with keeping track of Time.
Wherever We go (or you go), the time is different than in Greenwich, but that’s helpful, Rhinos are told. Of course, We are still trying to figure out the Meridian question, so We are already running behind.