Latitudes and Longitudes

Every place on the Earth has an exact address, and that address is written in degrees. By drawing an imaginary grid of crossing lines over the globe, we can name the position of any city, river or mountain with precision. We can also work out the time at each place. The two sets of lines that make this grid are the latitudes and the longitudes. Reading them confidently is one of the most useful skills in geography and one of the most frequently tested.

A globe is a true small model of the Earth. Unlike a flat map, it shows the real shapes and sizes of the continents and oceans without distortion. A globe is always shown tilted, not upright, because the Earth itself is tilted.

Running through the globe is an imaginary line called the axis, passing through two fixed points called the North Pole and the South Pole. The Earth spins on this axis. Exactly halfway between the two poles, an imaginary circle runs right around the middle of the Earth. This is the Equator, and it divides the Earth into the Northern Hemisphere and the Southern Hemisphere.

The axis points at a fixed spot in the sky. Almost exactly above the North Pole sits the Pole Star, also called Polaris. Because it lies on the line of the axis, it does not appear to move as the Earth spins. It always marks due north in the night sky of the Northern Hemisphere.

This makes the Pole Star a natural compass. Face the Pole Star and you face north. East is then on your right, west on your left, and south behind you. A traveller who wants to walk east at night simply keeps the Pole Star on the left. To walk west, keep it on the right.

A latitude is the distance of a place north or south of the Equator, measured in degrees. The Equator is the starting line, so its value is 0 degrees. From there the latitude rises to 90 degrees at the North Pole and 90 degrees at the South Pole.

The lines drawn for each latitude run parallel to the Equator and to one another, so they are called parallels of latitude. These circles grow smaller as you move away from the Equator towards the poles, until the pole itself is just a point. All places on the same parallel have the same latitude.

Five parallels matter more than the rest, and their values are worth memorising.

• The Equator at 0 degrees.
• The Tropic of Cancer at 23.5 degrees North.
• The Tropic of Capricorn at 23.5 degrees South.
• The Arctic Circle at 66.5 degrees North.
• The Antarctic Circle at 66.5 degrees South.

The Tropic of Cancer is especially important for India, because it passes through the middle of the country.

These parallels matter because they divide the Earth into three broad heat zones, based on how directly the Sun's rays fall.

The Torrid Zone lies between the Tropic of Cancer and the Tropic of Capricorn. Here the midday Sun is overhead at least once a year, so this belt receives the most heat and is the hottest.

The two Temperate Zones lie between the Tropics and the Polar Circles, one in each hemisphere. The Sun is never directly overhead here, so these areas have moderate temperatures.

The two Frigid Zones lie between the Polar Circles and the poles. The Sun's rays are always slanting and weak, so these are the coldest parts of the Earth.

Latitudes tell us how far north or south a place is. Longitudes tell us how far east or west it is. A longitude is an imaginary semicircle that runs from the North Pole to the South Pole. Each such line is called a meridian of longitude. Unlike parallels, all meridians are of equal length.

The starting line for longitude is the Prime Meridian, fixed at 0 degrees. It passes through Greenwich, near London, where a famous observatory stands. From the Prime Meridian, longitude is measured up to 180 degrees east and 180 degrees west. The 180-degree meridian lies on the opposite side of the Earth, and the eastern and western counts meet there.

The most practical use of longitude is to measure time. The Earth makes one full turn of 360 degrees in 24 hours. Divide one by the other and you find that the Earth turns 15 degrees of longitude in one hour, or one degree in four minutes.

This is why the time differs from place to place. Places to the east see the Sun rise earlier, so their local time is ahead. Places to the west are behind. When it is noon at Greenwich, a place 15 degrees to the east already reads 1 p.m. A place 15 degrees to the west still reads 11 a.m. The local time of a place is set by its own meridian, taking midday as the moment the Sun is highest in its sky.

If every town kept its own local time, travel and railways would fall into chaos. The time would change with every step east or west. To avoid this, each country chooses one central meridian and sets a single standard time for the whole nation from it.

India's standard meridian is 82.5 degrees East (written 82 degrees 30 minutes East), which passes near Mirzapur. The local time of this meridian is taken as the time for the entire country and is called Indian Standard Time (IST). India is a wide country. The Sun rises in the far east nearly two hours before it does in the far west. Yet both keep the same clock.

India lies ahead of Greenwich. IST is 5 hours and 30 minutes ahead of the time at the Prime Meridian, known as Greenwich Mean Time (GMT). Very large countries such as Russia, Canada and the United States are so wide that one standard time will not serve. They are divided into several time zones.

The same logic ranks every country on the clock. The farther east of Greenwich a country lies, the more its standard time runs ahead of GMT. The farther west, the more it falls behind. A few worked examples make the ordering clear.

• Japan: lies far to the east, at about GMT+9, so its clocks are among the most ahead.
• Iraq: in West Asia, at about GMT+3.
• Greece: in eastern Europe, at about GMT+2.
• Cuba: west of Greenwich in the Americas, at about GMT-5, so it is behind GMT.
• Costa Rica: still farther west in Central America, at about GMT-6.

So when a new day begins, Japan greets it first, then Iraq, then Greece, while Cuba and Costa Rica are still hours behind Greenwich. East of Greenwich means ahead; west means behind.

Counting time by longitude creates one strange result. Move east from Greenwich and the clock runs ahead, up to 12 hours at 180 degrees. Move west and it falls behind, up to 12 hours at the same meridian. The two counts meet at the 180-degree meridian with a full 24-hour gap between them. The calendar must therefore change by one whole day at this line. The line drawn for this purpose is the International Date Line, which follows the 180-degree meridian for most of its length.

The rule of the line is fixed. The side west of the line (Asia, Australia, eastern Siberia) is always one day ahead of the side east of the line (Alaska, the Americas). A traveller crossing the line westward skips forward and loses a day. A traveller crossing it eastward repeats a day, setting the date back by one while the clock keeps running. So a ship that crosses the 180-degree meridian eastward late on 1 January simply records 1 January again on the far side, and an hour later its diary still shows the early hours of 1 January.

This produces a famous oddity in the Bering Strait. Anadyr in Siberia and Nome in Alaska lie only a short distance apart, yet they sit on opposite sides of the line. Anadyr, on the western side, is a day ahead. When people in Anadyr begin a Monday morning, it is still Sunday in Nome. The line itself is not perfectly straight. It bends, or zigzags, around land in the Bering Strait and around Pacific island groups such as Fiji, so that no single country or island cluster is split between two calendar dates.