Longitude is the angular measurement of a position east or west of the prime meridian, the north–south line through Greenwich in London that is defined as 0°. Longitude runs from 0° to 180° east and 0° to 180° west, meeting at the antimeridian in the Pacific. Together with latitude it fixes any point on the surface of the earth, and every position you read from a GPS, write in a logbook or plot on a chart is a pair of these two numbers. Beginners need to know which of the pair is which: latitude is the north–south figure and always comes first. The experienced navigator adds a warning that catches people out in practice – a minute of longitude is not a nautical mile except at the equator, so you never measure distance on the longitude scale at the top and bottom of a chart.
How longitude is measured
Picture the earth sliced like an orange from pole to pole. Each slice line is a meridian, a semicircle of longitude from the north pole to the south pole. The meridians are farthest apart at the equator, where a degree of longitude spans about 60 nautical miles, and they converge until they meet at the poles, where longitude has no meaning. At 50° north, roughly the latitude of the English Channel, a degree of longitude is only about 39 nautical miles, which is why a chart’s longitude scale compresses as you go north and why distance is always taken from the latitude scale at the side.
Longitude is written in degrees, minutes and either decimal minutes or seconds, followed by E or W. A modern GPS shows 001° 17.42′ W: one degree, seventeen point four two minutes west of Greenwich. Older charts and almanacs may use seconds instead of decimal minutes, so check the format before entering a waypoint, because 17′ 42″ and 17.42′ are not the same place.
Why longitude was the hard one
Latitude can be found from the height of the sun at noon or the pole star at night, and sailors had that worked out centuries ago. Longitude is a matter of time. The earth turns through 15° every hour, so if you know the exact time at Greenwich when the sun is highest where you are, the difference gives your longitude. The trouble was carrying Greenwich time across an ocean in a damp, rolling ship where pendulum clocks were useless. After a string of shipwrecks the British Parliament offered a large prize in 1714 for a practical method, and the clockmaker John Harrison eventually produced marine chronometers that kept time to within seconds over months at sea. The story explains why GMT, now UTC, is every navigator’s time reference and why the prime meridian runs through an observatory in south London.
Longitude on the chart and in the plotter
- Reading a position: run the plotter from the point to the nearest labelled meridian, then read the minutes off the longitude scale at the top or bottom of the chart.
- Measuring distance: use dividers on the latitude scale at the side of the chart, level with your position. One minute of latitude is one nautical mile everywhere.
- Datum: charts are drawn to a horizontal datum and GPS uses WGS 84. Older charts on a local datum can differ by hundreds of metres; check the chart title and set the plotter to match.
- Entering waypoints: read the longitude back aloud, check the hemisphere letter and confirm the waypoint against the chart. A transposed digit can put a waypoint on the wrong side of a headland.
Cruising vs racing
The cruising navigator lives with longitude as one half of every position entered in the log each hour, the figure that shows progress on a westward passage, and the number read out on the VHF if help is ever needed. Writing the full position in the log at every watch change means the dead-reckoning plot can start from a known point if the electronics die. Racing navigators care less about the number and more about what it feeds – the wind and current models that predict where the boat should be – but marks are still given as coordinates that must be entered correctly before the start. Both benefit from remembering that the boat crosses a degree of longitude faster the further north it is, which changes the arithmetic of a long east–west leg.
Frequently Asked Questions about Longitude
Which comes first, latitude or longitude?
Latitude first, then longitude, always. A position is written as, for example, 50° 45.60′ N, 001° 17.42′ W.
Is a minute of longitude a nautical mile?
Only at the equator. Elsewhere a minute of longitude is shorter, shrinking to nothing at the poles. Measure distance on the latitude scale at the side of the chart instead.
Why does longitude start at Greenwich?
Because the Royal Observatory at Greenwich produced the astronomical tables most navigators used in the nineteenth century, and an international conference in 1884 confirmed it as the prime meridian.
Longitude is half of knowing where you are, and understanding how it behaves on the chart turns a set of GPS digits into real situational awareness. Read it carefully, write it down often, measure distance on the other scale, and the east–west half of your navigation will never let you down.