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.