Why is the Greenwich meridian where it is and not in some other vertical position?

Answer

The Prime Meridian or Greenwich Meridian is simply determined by coincidence by convention.

Sir George Airy established the Prime Meridian in 1851 at the Royal Observatory in Greenwich, a suburb of London. It was not until 1884 at an international meeting in Washington that it was decided that the Greenwich meridian would become the official prime meridian.

The French will not accept the Greenwich meridian as the prime meridian until the early 20th century. After all, the French had been working with the Paris meridian as the prime meridian for more than two hundred years.