Latitude is the angular distance of a position north or south of the equator, measured in degrees, minutes and decimals of a minute from 0° at the equator to 90° at the poles. Lines of equal latitude are called parallels because they run parallel to the equator around the globe. For the beginner, latitude is the first half of every position – the number written before the N or S. For the experienced navigator it is also a measuring tool: one minute of latitude is one nautical mile everywhere on earth, which is why the latitude scale up the side of a chart is where you set your dividers to measure distance, and why you never measure distance on the longitude scale at the top or bottom.
How Latitude Is Defined
Imagine a line from the centre of the earth to your position. The angle between that line and the plane of the equator is your latitude. Because the earth is very nearly a sphere, that angle translates directly into distance along a meridian: 60 nautical miles per degree, one mile per minute, roughly 200 metres per tenth of a minute. Positions are written as, for example, 50° 48.5′ N, read as fifty degrees forty-eight and a half minutes north. Older charts and almanacs use seconds instead of decimal minutes, which trips up sailors moving between GPS formats; one second of latitude is about 31 metres.
Latitude on the Chart
On a Mercator chart, parallels are straight horizontal lines. Because the Mercator projection stretches the scale as you move away from the equator, the latitude scale on the side of the chart is not uniform: a minute of latitude is drawn longer at the top of a northern-hemisphere chart than at the bottom. The rule that follows is simple and important. Measure distance on the latitude scale level with the part of the chart you are working in, never at the top or bottom and never on the longitude scale, where a minute of arc is not a nautical mile except at the equator. A navigator who takes distance from the longitude scale at 50° N will be out by more than a third.
Finding Latitude Without Electronics
Latitude is the easiest coordinate to find by observation, which is why sailors had it centuries before they had longitude. In the northern hemisphere, the altitude of Polaris above the horizon is very nearly your latitude, with a small correction. At local noon, the sun’s altitude at its highest point combined with the declination from an almanac gives latitude directly with one line of arithmetic – the noon sight that every ocean navigator learns first. Until the chronometer solved longitude in the eighteenth century, ships would sail to the latitude of their destination and then run down the parallel until land appeared, a technique called latitude sailing.
Latitude and the Wind
Latitude also organises the world’s weather, which is the skipper’s concern. Between roughly 5° and 25° either side of the equator the trade winds blow steadily from the north-east and south-east, the sailing highway of the oceans. Around 30° lie the horse latitudes, belts of light and variable wind under the subtropical high-pressure systems. From about 40° to 60° the westerlies dominate, strongest and most consistent in the southern hemisphere where there is little land to interrupt them – the Roaring Forties, Furious Fifties and Screaming Sixties. Ocean passage planning is largely about choosing a latitude for the crossing and a season when that latitude behaves.
Latitude and the Sail Wardrobe
A sailmaker asks where a boat will sail before deciding what to build. In tropical latitudes the enemy is ultraviolet light: the sun is high, exposure is relentless, and UV strips, covers and UV-stable fibres such as polyester and Dyneema® earn their keep, while unprotected aramid laminates degrade quickly. A boat trading between 10° and 20° also spends most of its life running in 15 to 25 knots, so downwind sails, poles and preventers matter more than a heavy No. 4. In high latitudes the priorities reverse: heavy, chafe-resistant cloth, deep reefs, a storm jib that can actually be set, and hardware sized for cold, wet hands. A cruising boat planning both should tell its sailmaker so, because the ideal sail for the Caribbean and the ideal sail for Patagonia are not the same sail.
Frequently Asked Questions about Latitude
Why is one minute of latitude one nautical mile?
The nautical mile was defined to make it so: it is the length of one minute of arc along a meridian, about 1,852 metres. That definition is what lets the latitude scale double as a distance scale on every chart.
Why does my GPS show a different format from the chart?
GPS receivers can display degrees with decimal degrees, degrees and decimal minutes, or degrees, minutes and seconds. Charts use degrees and minutes. Set the receiver to degrees and decimal minutes so its numbers plot directly onto the chart scale.
Latitude is the coordinate you can measure with a sextant and a horizon, the scale you measure every distance against, and the number that tells you what wind and weather to expect. Read it off the side of the chart, set your dividers on it, and let it shape both your passage plan and the sails you carry.