What is longitude?
Latitude and longitude are the means by which a ship (or person) is able to discover its location and as a result navigate safely. They are imaginary lines drawn on the face of maps of Earth. Latitude was discovered by measuring the height of the sun, but longitude is a different matter. It was discovered if a person compared the time shown on two clocks, one adjusted to keep showing local time and the other remaining unaltered, the longitude could be calculated. But the problem was the clocks of centuries past were not accurate enough, especially those capable of running aboard ships. Errors of a few minutes could cause mistakes which led to shipwrecks.
Greenwich Observatory in Greenwich, England, was established by King Charles II in 1675 to study means of fixing longitude and became the acknowledged world authority on the subject. The telescopes and other instruments there determined the exact position of the meridian, and, in 1884, an international
conference in Washington agreed Greenwich should be the site at zero longitude.
As a result, all time zones across the world are expressed as being plus or minus so many hours Greenwich Mean Time. The prime meridian is at zero degrees longitude where it passes through Greenwich. In the courtyard of the observatory, and just outside, are brass strips set in the ground and walls marking the exact site. It is therefore possible to stand astride the line, with a foot in each hemisphere.