Latitudes, Longitudes and Time: Grid, IST and Date Line

A place on the earth cannot be described by distance alone, because the earth is a sphere with no edge to measure from. Geographers therefore draw an imaginary network of lines on the globe, the parallels of latitude running east to west and the meridians of longitude running north to south. Every point on the surface lies at the crossing of one parallel and one meridian, and that pair of angles is its address. The same network fixes time, because the earth rotates through a known angle in a known time. Questions from this chapter are short and numerical, and almost all of them come back to two numbers: one degree of latitude is about 111 kilometres, and one degree of longitude is four minutes of time.

Parallels of latitude

Latitude is the angular distance of a place north or south of the equator, measured at the centre of the earth. The equator is 0 degrees, the North Pole 90 degrees north and the South Pole 90 degrees south. If a parallel is drawn at every degree there are 181 of them in all, 90 in the northern hemisphere, 90 in the southern and the equator itself. Parallels are true circles that never meet, they grow shorter towards the poles, and the equator is the longest of them; it is also the only parallel that is a great circle, that is, a circle whose plane passes through the centre of the earth. The distance between two consecutive parallels is nearly constant, about 111 kilometres, because it depends only on the angle at the centre.

These parallels divide the earth into three heat zones. The torrid zone lies between the two tropics, where the sun is overhead at least once a year. The two temperate zones lie between a tropic and the corresponding polar circle, where the sun is never overhead. The two frigid zones lie beyond the polar circles, where the sun's rays are always slanting and the cold is extreme.

The Tropic of Cancer in India

The Tropic of Cancer passes through eight Indian states: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram. It roughly halves the country and is the reason India is described as lying half in the tropical and half in the subtropical belt. India's latitudinal extent runs from about 8 degrees 4 minutes north to about 37 degrees 6 minutes north, and its longitudinal extent from about 68 degrees 7 minutes east to about 97 degrees 25 minutes east.

Meridians of longitude

Longitude is the angular distance of a place east or west of the Prime Meridian, the zero meridian that passes through the old Royal Observatory at Greenwich near London. Meridians are semicircles of equal length that run from pole to pole, so unlike parallels they all meet at the poles. Counting one meridian per degree there are 360 in all, 180 to the east and 180 to the west of the Prime Meridian, and the 180 degree meridian is the same line whether approached from the east or the west. The distance between two meridians is greatest at the equator, about 111 kilometres, and shrinks to nothing at the poles.

Longitude and time

The earth turns once, through 360 degrees, in twenty-four hours. It therefore turns 15 degrees in one hour and one degree in four minutes. Places to the east see the sun earlier, so their local time is ahead. Local time differs from town to town, which is unworkable for railways and telegraphs, so every country fixes a standard meridian and keeps the local time of that meridian as its standard time. Indian Standard Time is the local time of 82 degrees 30 minutes east, the meridian that passes near Mirzapur in Uttar Pradesh, and it is five and a half hours ahead of Greenwich time. Because India stretches through nearly 30 degrees of longitude, the sun rises in Arunachal Pradesh about two hours before it rises in Gujarat, yet both keep the same clock.

The International Date Line

A traveller going east gains time and one going west loses it, so a line is needed where the date changes. The International Date Line runs roughly along the 180 degree meridian in the Pacific, but it bends to keep island groups and the tip of Siberia on one side, so that a single country does not have two dates on the same day. A traveller crossing it from east to west adds a day, and one crossing from west to east repeats a day.