The prime meridian (zero longitude) was established by the Brits in the mid 1700s (i think) for map making - others adopted it. Before that, there were other systems going back to the early Greeks. I guess you could call the selection of prime meridian passing through the Royal Observatory in Greenwich objective if you wish but that selection is something that only a Brit would think of, Italians would have likely selected some landmark in Rome as where to place zero longitude.
Then there is the whole 360 degrees to complete the circuit of the Earth thingy. Why 360 degrees other than tradition? There is nothing inherent about circles that make it 360 degrees rather than, say, 100 degrees. If all the math tables had been made after adoption of the metric system then it likely would have been so designated. If it had been then the international date line would be at 50 degrees longitude instead of 180 degrees and the poles would be at 25 degrees latitude instead of 90 degrees latitude.
The French used a Paris based longitude system for a long time, in competition with the English Greenwich system. GPS has recently imposed a whole new system - the WGS84 standard used by GPS has its prime meridian located approximately 102.5 metres east of the Greenwich Meridian at Greenwich (although the mean variation from the Greenwich longitude standard worldwide is close to zero). Because the Earth is not spherical, and does not exhibit uniform gravity, the variation between WGS84 and the Greenwich longitude standard is different in different parts of the world; The two systems agree exactly* about universal time, however.
At Greenwich, the eye-piece of Sir George Airy’s Transit Circle, that since 1851 has been used to view the transits of celestial bodies to define Universal Time, is leveled based on the local gravitational plane; And the Airy standard was adopted as the world standard for both time and longitude in 1884.
However the GPS zero longitude plane established 100 years later, in 1984, is defined as passing through the Earth's centre of mass, while the North/South plane normal to the local gravitational plane at the Airy circle, does not; The GPS Zero meridian is defined as the place where a plane which passes through the Earth's centre of mass, and is
parallel to the plane defined by the Airy circle, transects the Earth's surface. As the two planes are parallel, they meet the same time standard, as transits of stars at infinity* are simultaneous for the two planes.
Essentially, because the Earth is not a perfectly homogeneous sphere, keeping the Universal Time standard as defined by Airy, meant relocating the zero longitude standard, such that it remains parallel to, and therefore
temporally identical with, the plane defined in the time standard, but now passes through the centre of the Earth. They could keep time, or location, at the Airy circle; But due to the misshapen planet, they could not keep both.
So if you go to Greenwich, there is a laser beam, and a brass strip on the ground, and a multitude of tourists, and much pomp and ceremony, that mark the zero point from which
universal time is calculated; But your GPS receiver will tell you that your
longitude is zero, only if you walk 102.5 metres to the East from those markers. The Royal Observatory staff have marked the new location of zero longitude (which falls in Greenwich Park, outside the compound of the Observatory itself) with a rubbish bin. Make of that what you will.
*Yes, I know, the stars are not actually an infinite distance away, so 'exactly' is not exactly correct; But the difference is defined by the parallax with a baseline of 102.5 metres and a range measured in light years, so it's easily good enough for government work; We are very unlikely ever to have, or to need, clocks sufficiently precise as to be able to tell the difference, which is to all practical intents, zero.
