It's obvious why the Equator is zero latitude. WHY is London considered to be zero longitude? How did that happen?

Unlike the equator (which is halfway between the Earth's north and south poles), the prime meridian is an arbitrary line. In 1884, representatives at the International Meridian Conference in Washington, D.C., met to define the meridian that would represent 0 degrees longitude. For its location, the conference chose a line that ran through the telescope at the Royal Observatory in Greenwich, England. At the time, many nautical charts and time zones already used Greenwich as the starting point, so keeping this location made sense.