Why bother with longitude? Do we really need to know how far east or west we are? If you were travelling across an ocean you might want to know how far you’d come but for day to day you don’t need to know that. However, there is an aspect of longitude that we use all the TIME!
The zero degrees line is also called the Greenwich Meridian and why was Greenwich so special?
Well in the park, on the hill, you find the Royal Observatory. In there is a particular telescope, the Airy Transit Circle, which is lined up with, and can only move along the line of the meridian. Except the telescope came first. Its position defines the position of the Greenwich Meridian.
So what’s so special about that? Well the telescope looks along the meridian and is able to record when objects in the sky cross (or transit) the meridian line. One of those objects is the sun. The sun crossing the Greenwich meridian marks midday and is taken as midday for the whole of Britain, even though the sun doesn’t reach its highest point in the sky in Cornwall until nearly half an hour later.
The actual length of time from one transit to the next varies slightly through the year. So the average (or mean) is used – hence Greenwich Mean Time (GMT).
Way back it didn’t really matter that Lowestoft on the far eastern tip of East Anglia reached midday an entire half hour before Land’s End in the west. But the Great Western Railway, running trains from London to Penzance in Cornwall, couldn’t have an accurate timetable unless everyone agreed on the time. The railway adopted the time set by Greenwich and this soon spread across Britain, becoming legal across the whole of Britain in 1880.