Image via Wikipedia
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 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.
Did you know: 38th Parallel line divides north korea and south korea.