The lines for longitude are called meridians and these are relatively easy to visualise and understand. They are virtual circles around the Earth passing through the North and South Poles that allow us to divide the parallel lines, or circles of latitude, into equal divisions. They look like the lines dividing a peeled orange into segments. Each meridian of longitude is the same diameter, whereas circles of latitude decrease in diameter away from the equator.
By convention, the position of any point on Earth is located by its meridian of longitude and its parallel of latitude. Each meridian is perpendicular to all circles of latitude. As noted, each meridian is also the same length and equals the circumference of the Earth, as each meridian is a great circle dividing the Earth into two equal hemispheres.
The term ‘meridian’ comes from the Latin word for midday, meridies, highlighting again the connection between the sun and how it can give an exact time at one point each day. When the sun crosses directly above each meridian midway between the times of sunrise and sunset, it is midday for that meridian. Midday became a datum as it appeared to remain constant, whereas the length of the day between sunrise and sunset quite obviously varies throughout the year, so those times are also variable. Midday can be determined each day and timepieces can be checked against the time of midday and reset, or a correction can be made.
A datum meridian is needed as an origin to work from, and to be universally manageable that datum needs universal acceptance.
In 1851, George Airy began observing at Greenwich with a new transit circle, an accurate telescope aligned north-south. From then on, this instrument defined the prime meridian at Greenwich, replacing a previous one used (with other instruments) by his predecessors. The meridian defined by Airy’s transit instrument also became the prime meridian for the UK (and the basis of national time). It is used to locate the time at which a star crosses the meridian, as it travels from east to west across the sky, and the angle of the star above the horizon. From these measurements the transit of the sun (which is a star), and of other clock stars, across the meridian can be used to set the clocks at an observatory.
In 1884 delegates from 25 nations met in Washington DC for the International Meridian Conference, and there the Greenwich Meridian was recommended as the official prime meridian.
For decades the prime meridian was then understood to pass through the Airy Transit and its line was indicated with a brass (later stainless steel) strip in the courtyard of the Royal Observatory. From late 1999, it has been marked by a green laser shining north across the Thames and London at night.
However, Global Positioning System (GPS) receivers now show that the marked prime meridian at Greenwich is not exactly at 0 degrees, 0 minutes and 0 seconds but at approximately 5.3 seconds to the west of this mark. The discrepancy is because the GPS meridian is based on the earth’s centre of gravity. The historic Prime Meridian is based on vertical measurements defined by local gravity at Greenwich. As a result, the historic Prime Meridian does not pass through the Earth’s centre of gravity (that is, the vertical at Greenwich does not pass through the centre of the Earth), hence the misalignment of the two meridians. The GPS meridian is defined so that the time measured from the Prime Meridian using astronomical methods is exactly the same as time measured from the new (GPS) meridian using satellite technology.
A further complication for meridians and time is posed by the time zones created around the globe, 12 each east and west of the prime meridian. The time zone divisions running north and south are often lines that are related to borders between and in many cases within countries, but they cover about 15 degrees of longitude each. In each time zone the actual time correct to that of the sun occurs on only one meridian.
The 360-degree division of circles was the obvious means of dividing the equator into meridians, and these were denoted east and west of the meridian from 0 to 180 degrees, rather than running around continually and numbered in 360 degrees.
Now, the point was made earlier that midday became a datum as it appeared to remain constant. The problem is that it’s not exactly constant. Midday actually moves by 14 to 16 minutes or so either side of an average or mean time. Why? The Earth’s motion relative to the sun is not a piece of perfect geometry. Earth’s path around the sun is not truly circular, nor at an absolute constant speed, and the sun is not exactly at the centre of this path, and neither does it take exactly 24 hours to do one rotation, or exactly 365 days to do one orbit of the sun. In other words, there is a host of tiny upsetting things that together become significant, and so the fractional parts have to be added in every now and then, such as a leap year with an extra day.
So, as well as having the meridian slightly out and in the wrong place, the actual time the sun crosses the meridian has its own variation to contend with too, and with some patience and maths the variation can be accounted for — so our elegant time solution actually has some rough edges to contend with.
Time and the sun clock
Going back to our elegant solution, it’s quite simple to figure out your longitude relatively accurately, which is why the method was so appealing — you need to know a time at a datum point, and your own time as a comparison.
Having agreed on London as the location for the prime meridian of 0 degrees, that location also became the datum for time, which was initially Greenwich Mean Time, but is now the almost super-accurate UTC.
Here is how it works: if you know what time it is in London and also what time it is where you are, you know how many hours (or parts thereof) you are away from London. Then you convert this time difference into degrees of longitude to see how many degrees away from London you are. It takes 24 hours for the sun to go through the full 360 degrees around the world, so an hour of time is equal to 15 degrees of longitude, and the rest of maths is simple, even the fractions.
In the pioneering days of the chronometer on board ships, working out the exact time where you were could be done, but only once a day, which was good enough at this point. The sun rises and reaches its maximum height in the sky at midday. If you observed the sun through a sextant, and noted the time it was at its maximum height above the horizon, you had midday at your location. You could then refer to the chronometer carrying your datum time to see what time it was at the location to which the timepiece was set.
If it was midnight on the timepiece, you were on the opposite side of the world. If it was midday then you were at that same place, or somewhere on a line of longitude to the north or south of the location.
In these days of GPS, with moving dots on our smartphone maps guiding us on foot and automated voices directing us in our cars, we take precise location for granted. Ships, Clocks & Stars explains the high technology of a previous age through complex and beautifully engineered machines, and reveals the intellectual sophistication and dogged persistence that solved the vexing question of longitude.
Ships, Clocks & Stars is produced by Royal Museums Greenwich and is on at the Australian National Maritime Museum in Darling Harbour, Sydney, from 5 May until 30 October.