We've all heard about Longitude. Dava Sobel wrote a best-selling book about it, people flock to Greenwich to see the 0° longitude line, and clocks are involved, somewhere or other. But where's the corresponding book about Latitude? This entry will attempt to rectify this gaping hole in the published world.

Latitude is a number of degrees, shown by the symbol °, which shows how far you are from the equator and how far from the poles. The equator has a latitude of 0° and the poles have latitudes of 90° North and 90° South. The rest of the world falls somewhere in between.

For more accurate values, each degree is divided in 60 minutes, denoted by the symbol ' and each minute is divided into 60 seconds¹ denoted by the symbol ". A typical latitude might be 52°30'25" N.

All the points which share the same latitude form a line which is known as a line of latitude. This is not a straight line, but is in fact a circle. These circles are often called 'parallels', because any two lines of latitude stay a constant distance apart. Lines of longitude, on the other hand, get closer together and meet at the poles.

Lines of latitude run from West to East, or from East to West if you prefer. So if you follow the command 'Go West Young Man' to the letter, you'll be following a line of latitude.

Important Lines of Latitude

The Equator - this is the line that is equidistant between the North and South Poles. The Equator passes through South America, Africa and some of the biggest islands in Indonesia, dividing the world into Northern Hemisphere and Southern Hemisphere. Most of the world's land is in the Northern Hemisphere. Due to the motion of the earth around the sun, the sun is directly above the equator twice every year, at the equinoxes² (around 21 March and 21 September).

The Tropic of Cancer - this line is 23° 27' North. It marks the northernmost position of the sun in the sky. Further north than this, the sun is never directly overhead. On the tropic itself, the sun is directly overhead at noon on the Northern Hemisphere summer solstice, which occurs around 21 June each year. When the tropic was given its name over two thousand years ago, the sun was in the Zodiac sign of Cancer on 21 June. Due to the precession of the Earth, the Sun is now in the sign of Gemini at this time, but the name of the tropic has not changed.

The Tropic of Capricorn - this line is 23° 27' South. It marks the southernmost position of the sun in the sky. On the tropic itself, the sun is directly overhead at noon on the Southern Hemisphere summer solstice, which occurs around 21 December each year. Further south than this, the sun is never directly overhead. When the tropic was given its name over two thousand years ago, the sun was in the Zodiac sign of Capricorn on 21 December. Due to the precession of the Earth, the Sun is now in the sign of Sagittarius at this time, but the name of the tropic has not changed.

As latitude increases, the length of the day starts to vary, getting longer in the summer and shorter in the winter. Eventually we reach a point where the day is 24 hours long in the summer and the night is 24 hours long in the winter.

The Arctic Circle - this line is 66° 30' North. It marks the point where the sun does not set but just touches the horizon on the Northern Hemisphere Summer Solstice. Further north than this, there is constant daylight 24 hours a day for a period during summer³. Conversely, there is 24 hours of darkness for a period during the winter.

The Antarctic Circle - this line is 66° 30' South. It marks the point where the sun does not set but just touches the horizon on the Southern Hemisphere Summer Solstice. Further south than this, there is constant sunshine 24 hours a day for a period during summer. Conversely, there is 24 hours of darkness for a period during the winter.

Less Well-Known Latitudes

The Doldrums

The Doldrums is a name for the part of the oceans around the equator. This region has stable weather with little or no wind, which in the days of wind-powered sailing ships was a real nuisance. The Doldrums are from 5° S to 5° N in the Indian and Pacific Oceans, but slightly further north in the Caribbean and Atlantic.

The word 'doldrums' originally meant a state of depression, from the Old English word 'dol' meaning dull and foolish. This word was probably created by analogy with the word 'tantrums'. The phrase 'in the Doldrums' was first used to mean becalmed at sea in 1824, and was applied to the equatorial region from 1855 onwards. The Doldrums are also known as the Intertropical Convergence Zone.

The Roaring Forties

The parts of the ocean which lie between the latitudes of 40° and 50°, both North and South of the equator, are noted for strong westerly winds, particularly in the Southern Hemisphere where there is little land to block the path of these winds. These areas of the globe are known to sailors as the Roaring Forties, not to be confused with the Roaring Twenties, which was a time in the 20th Century famous for women wearing short skirts and feathers, drinking gin and dancing all night.

The Horse Latitudes

Closer to the equator, we have the Horse Latitudes, which lie between 30° and 35°, again both North and South of the equator. Here we have bands of high-pressure settled weather with very little wind. Sailing ships could be marooned for weeks in the Horse Latitudes without a breath of wind.

How the Horse Latitudes got their name is a bit of a mystery. The traditional story is that sailors when marooned in the Horse Latitudes would throw their horses overboard to save on provisions. This seems unlikely; why waste a valuable source of meat? An alternative, equally dubious, explanation is that when the wind does blow in the Horse Latitudes, it tends to be skittish and changeable, like a frisky horse.

Measuring Latitude

There's no great problem measuring latitude in the Northern hemisphere. The easiest way is to wait for night time, and find Polaris⁴, the North Star. Polaris is permanently fixed directly above the Earth's North Pole. Measure the angle between it and the point on the horizon directly beneath it, and that gives you your latitude. Of course this assumes that you're able to see Polaris; cloudy skies may prevent you from doing this. And Polaris is not exactly above the pole; it is nearly a degree out, so various corrections have to be applied if you want an exact figure.

In the southern hemisphere, it is not quite as simple, as there is no 'South Star'. But you can use the patterns of the constellations to identify the point in the sky directly above the South Pole. Measuring the angle between this point and the horizon gives you your latitude.

In times gone by, sailors used a different method to calculate their latitude. At noon each day, they would measure the height of the sun above the horizon, using a quadrant (a crude alignment device), an astrolabe (a quadrant with a plumb line for levelling) or a sextant (a more elaborate device with lenses and things). It didn't matter exactly when around noon they did this, as the sun tends to stay at its highest point for a while before starting to descend. Then they would consult a latitude table: given the height of the sun and the date, they could read off the latitude. The earliest such tables date back as far as 1473, when Abraham Zacuto, working in Portugal, published his 'Perpetual Almanac'.

This method has the advantage over observation of the North Star that you're more likely to be able to see the sun during the day than a star at night, and that it works in the Southern Hemisphere as well, where Polaris is not visible.

Uses of Latitude

The main use of Latitude is to specify an exact location on the Earth's surface, when combined with Longitude. For example, the city of Athens is at 37°58' N, 23°43' E. The first figure here is the latitude, and it is commonly called a Northing because it tells you how far North the location is. The longitude is called an Easting. Of course in other parts of the world, there would be Southings and Westings.

Knowing the latitude of a place is a help in estimating the likely temperature there. In general, the lower the latitude, the hotter it will be. But the continent of Europe forms an exception to this rule. Warmed by the North Atlantic Drift, a huge movement of warm water originating in the Gulf of Mexico, things are warmer in Europe than they should be. So Aberdeen, Scotland is quite habitable, despite being almost at the same latitude as Juneau, the capital of Alaska.

Lines of latitude are occasionally used as borders, as a peaceful alternative to the age-old method of all-out warfare to settle the position of a border. Most notable is the 49th parallel, or 49° N, which forms most of the border between the USA and Canada. Other borders which lie along lines of latitude include the border between Egypt and Sudan, and the border between Angola and Namibia.