Meridians of longitude

Also called: Longitude, Meridians · Topic: Latitude, Longitude and Time · NCERT: Class 6, Ch 1 "Locating Places on the Earth"; Class 11, Ch 3 "Latitude, Longitude and Time"; Class 11, Ch 4 "Map Projections"

Meaning

Longitude is the angular distance of a place east or west of the Prime Meridian (0°). Angular distance means the distance measured as an angle from the earth's centre. It is measured along the equator and runs from 0° to 180°E and from 0° to 180°W. Meridians are the lines of longitude: imaginary half-circles that join the North Pole and the South Pole.

Meridians help us fix the exact position of any place on the earth. They also form the basis of world time, time zones and India's own standard time.

Explanation

What a meridian is and how it is drawn

• A meridian is an imaginary half-circle (semicircle) running from the North Pole to the South Pole [4].
• All meridians are equal in length. Each one is half as long as the equator.
• Every meridian crosses the equator at right angles (90°).
• Every place gets a number and a letter:
• E if it lies east of Greenwich.

• W if it lies west of Greenwich.

• 180°E and 180°W are the same line. So it is written only as 180°, with no E or W.

• 0° and 180° together form one great circle. A great circle is a circle whose centre is the centre of the earth.
• This circle splits the earth into the Eastern and Western Hemispheres [4].

• In the same way, the equator splits the earth into the Northern and Southern Hemispheres.

• Seen from above the North Pole (NCERT Class 11, Fig. 3.4, 3.5):

• The equator looks like the outer circle.
• Meridians look like straight lines (radii) running from the pole at the centre out to that circle.
• Eastern longitudes appear on the left. This is the opposite of a normal wall map, and beginners often get it wrong.

Why meridians come closer towards the poles

• Cause: all meridians meet at the two poles.
• Process: so the gap between any two meridians gets smaller as you move away from the equator.
• Result: the length of 1° of longitude shrinks with latitude [4].

Latitude	Length of 1° of longitude
Equator (0°)	about 111 km (the same as 1° of latitude)
60°N / 60°S	about 56 km (roughly half)
Poles (90°)	0 km, because all meridians meet there

• Spacing in short: meridians are farthest apart at the equator, come closer towards the poles, and meet at the poles.

The Prime Meridian and how it was chosen

• The Prime Meridian is the 0° meridian. All other longitudes are counted east or west of it.
• It passes through the Royal Observatory, Greenwich, near London (United Kingdom).
• International Meridian Conference, October 1884:
• It was held at Washington, D.C. (USA) [2].
• US President Chester Arthur called it, and 25 countries sent representatives [5].
• The exact zero line was the meridian of the transit instrument at Greenwich. A transit instrument is a telescope used to time the moment a star crosses the local meridian [2].
• Greenwich won by 22 votes to 1 [5][3].
• San Domingo (now the Dominican Republic) voted against. France and Brazil abstained, which means they did not vote [3].

• France kept using its own Paris meridian for some decades after this.

• Why Greenwich: many nautical charts (sea maps) and time systems already used it as their starting line [5].

• Before 1884: many countries used their own prime meridians, often drawn through their capital cities [5].
• What followed: a system of 24 standard time zones, each ideally 15° of longitude wide. In practice, governments draw the zone boundaries, and many of them differ a lot from the 15° lines [2].
• Modern update: in the 1980s scientists defined the International Reference Meridian (IRM). It is a more exact zero line based on satellite measurements [4]. NCERT still says the Prime Meridian passes through Greenwich.

An older Indian prime meridian: Ujjayini

• Ancient Indian astronomy used the madhya rekhā ("middle line"). It was a prime meridian through Ujjayinī (Ujjain), in present-day Madhya Pradesh.
• Ujjain was a centre of astronomy. Varāhamihira worked there about 1,500 years ago.

• Along with Aryabhata and Brahmagupta, he was one of the authors who set out classical Hindu astronomy in siddhāntas (books on astronomy) [7].

• Britannica calls Ujjain the place on "the first meridian of the ancient Hindu geographers." As Ujjayini, it was the capital of the Avanti kingdom (6th–4th century BCE) [6].

• Why some cities on this line are slightly off it (NCERT Class 6, Fig. 1.5):
• To fix a place's longitude, you must measure the difference in time between places.
• Clocks in those days were not very exact.
• So the longitudes worked out at the time were a little wrong.

Where it occurs

• Greenwich, near London (UK): the Prime Meridian (0°) passes through the Royal Observatory here.
• Washington, D.C. (USA): the 1884 International Meridian Conference that chose Greenwich was held here [2].
• Paris (France): France kept its own Paris meridian for some decades after 1884.
• Ujjain (Madhya Pradesh, India): the ancient Indian madhya rekhā (prime meridian) passed through here [6].
• India's standard meridian, 82°30'E:
• Indian Standard Time is set by this line.

• It is 5 h 30 min ahead of Greenwich (82.5° × 4 minutes per degree).

• Cities placed by longitude:

• New York: about 74°W
• Delhi: 77°E

• Tokyo: 140°E

• The 180° meridian: the line on the opposite side of the globe from Greenwich. Together with 0°, it divides the world into the Eastern and Western Hemispheres.

Don't confuse with

• Parallels of latitude: parallels are full circles and they are not equal in length (the equator is the longest, and they shrink to a point at the poles). Meridians are half-circles, and all are equal, each half the equator's length. Parallels run 0°–90°N/S. Meridians run 0°–180°E/W.
• Spacing of parallels and meridians: parallels stay about the same distance apart. Meridians are widest apart at the equator and meet at the poles.
• Prime Meridian vs India's standard meridian: the Prime Meridian (0°, Greenwich) is the world's starting line for longitude. India's standard meridian (82°30'E) only sets Indian Standard Time.
• Prime Meridian vs International Reference Meridian (IRM): the IRM is a more exact zero line, defined in the 1980s from satellite measurements [4]. For exams, NCERT's answer is still Greenwich.

Prelims Hooks

• All meridians are equal in length, and each is half the equator's length. Parallels are not equal. This is a classic true/false trap.
• 1° of longitude is about 111 km at the equator, about 56 km at 60°, and 0 km at the poles [4].
• 180°E and 180°W are the same meridian, written only as 180°. Meridians cross the equator at 90°.
• 1884 International Meridian Conference, Washington, D.C.: Greenwich won 22–1. San Domingo voted against. France and Brazil abstained. 25 countries attended [2][3][5].
• Equator → Northern and Southern Hemispheres. Prime Meridian with the 180° line → Eastern and Western Hemispheres.
• West to east order: New York (~74°W) → Greenwich (0°) → Delhi (77°E) → Indian Standard Meridian (82°30'E) → Tokyo (140°E). The ancient Indian prime meridian passed through Ujjain, linked with Varāhamihira.

Mains Points

• Why one world prime meridian was needed (GS-I):
• Before 1884, each country had its own zero line, often through its capital [5].
• This made it hard to match sea charts, telegraph timings and railway timetables across countries.

• A single meridian gave the world one longitude system and led to 24 standard time zones [2][5].

• Science, power and tradition:

• Greenwich was chosen partly because many sea charts already used it. This reflected Britain's lead in navy and map-making [5].
• France refused to vote and kept the Paris meridian [3]. So even a "scientific" standard involved politics.

• Centuries earlier, India had its own reference meridian through Ujjayini for astronomy and time-keeping [6]. Cities on it are slightly off the line because longitude needs exact timekeeping.

• Converging meridians and maps:

• A degree of longitude shrinks from about 111 km at the equator to 0 at the poles [4].
• So east–west distances on maps get stretched out of shape towards the poles. This is a key problem in map projections.
• High-latitude countries cover many degrees of longitude in fewer kilometres.

Related concepts

• Prime Meridian
• Ujjayini meridian

Read more

• Meridians of longitude and the Prime Meridian