Prime Meridian

Also called: Greenwich Meridian · 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

The Prime Meridian (also called the Greenwich Meridian) is the 0° line of longitude. It is the meridian through the transit instrument at the Royal Observatory, Greenwich, near London (United Kingdom). Countries chose it by international agreement in 1884 [2].

• It is the starting line for measuring longitude. Every place on earth is counted as so many degrees east (E) or west (W) of it.
• It is also the base for world time. Standard time zones, including Indian Standard Time, are worked out from it.

Explanation

What longitude and meridians are

• Longitude is a place's angular distance (distance measured as an angle from the earth's centre) east or west of the Prime Meridian. It is measured along the equator.
• Longitude runs from 0° to 180°E and from 0° to 180°W.
• A meridian (line of longitude) is an imaginary half-circle (semicircle) from the North Pole to the South Pole [4].
• Features of meridians:
• All meridians are equal in length. Each one is half as long as the equator.
• They are farthest apart at the equator and meet at the poles.

• Every meridian crosses the equator at right angles (90°).

• Why one degree of longitude gets shorter as you go north or south:

• All meridians meet at the poles.
• So the gap between two meridians shrinks as latitude rises.
• The gap is about 111 km at the equator, about 56 km at 60°, and 0 km at the poles [4].

The Prime Meridian as the reference line

• The Prime Meridian is the 0° meridian. All other longitudes are counted from it.
• 180°E and 180°W are the same line. It is written simply as 180°, with no E or W.
• The 0° and 180° meridians together make one great circle (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.

• How it fixes time:

• The earth turns 360° in 24 hours, so every 1° of longitude = 4 minutes of time.

• India's standard meridian is 82°30'E. So India is 82.5 × 4 = 330 minutes, that is 5 h 30 min ahead of Greenwich.

• Polar view (a construction trap): if you look down on the earth from above the North Pole, the meridians appear as straight lines going out from the pole. The eastern longitudes appear on the left, which is the opposite of a normal wall map.

How Greenwich was chosen: the 1884 conference

• Before 1884: many countries used their own prime meridians, often drawn through their capital cities [5].
• The 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 zero line chosen was the meridian of the transit instrument at Greenwich. A transit instrument is a telescope used to time when stars cross the local meridian [2].
• Greenwich won by a vote of 22 to 1 [5][3].
• San Domingo (today's Dominican Republic) voted against it. France and Brazil abstained (did not vote) [3].

• France kept its own Paris meridian for some decades afterwards.

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

• What came next: the conference led to 24 standard time zones, each ideally 15° of longitude wide. In practice, local governments set the zone boundaries, and many of them depart a lot from the 15° lines [2].
• Modern update: in the 1980s scientists defined the International Reference Meridian (IRM). It is a more precise zero line based on satellite measurements [4]. For NCERT and exam purposes, the Prime Meridian passes through Greenwich.

An older Indian prime meridian: Ujjayini

• Greenwich was not the first prime meridian.
• Ancient Indian astronomy used the madhya rekhā ("middle line"). It was a prime meridian through Ujjayinī (Ujjain).
• 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].
• Ujjain was a centre of astronomy. Varāhamihira worked there about 1,500 years ago. He was one of the writers of the siddhāntas (classical Hindu astronomy books), along with Aryabhata and Brahmagupta [7].
• Why some cities listed on this line are slightly off it:
• Longitude is fixed by measuring time differences.
• Clocks were less exact then.
• So the positions could not be fixed precisely.

Where it occurs

• Greenwich, near London (United Kingdom): the Royal Observatory, where the 0° meridian is marked [2].
• Washington, D.C. (USA): where the International Meridian Conference met in October 1884 [2].
• Paris (France): home of the rival Paris meridian, which France kept for some decades after 1884 [3].
• Ujjain (Ujjayini), Madhya Pradesh, India: the ancient Indian madhya rekhā passed through here [6].
• India's standard meridian, 82°30'E: India's clock time is set to this line, 5 h 30 min ahead of Greenwich.
• Example longitudes measured from Greenwich:
• New York: about 74°W
• Delhi: 77°E
• Tokyo: 140°E

Don't confuse with

• Equator: it is the 0° line of latitude, a full circle that divides the earth into the Northern and Southern Hemispheres. The Prime Meridian is the 0° line of longitude, a half-circle. Together with the 180° meridian it divides the earth into the Eastern and Western Hemispheres.
• 180° meridian: it lies on the opposite side of the earth from the Prime Meridian. 180°E and 180°W are the same line, so it gets no E or W. The Prime Meridian is 0°.
• Indian Standard Meridian (82°30'E): this is India's own time reference line, 5 h 30 min ahead of Greenwich. It is not a prime meridian. Its longitude is itself measured from Greenwich.
• Madhya rekhā of Ujjayini: this was the ancient Indian prime meridian through Ujjain, used in astronomy. Greenwich is the modern international zero line, adopted in 1884.

Prelims Hooks

• Prime Meridian: Royal Observatory, Greenwich, near London. It was adopted at the International Meridian Conference, Washington, D.C., October 1884 [2].
• The 1884 vote was 22–1 with 25 countries present. San Domingo voted against, and France and Brazil abstained [3][5].
• Trap: all meridians are equal in length, each half the equator's length. Parallels are not equal.
• One degree of longitude is about 111 km at the equator, about 56 km at 60°, and 0 at the poles [4].
• Equator → Northern and Southern Hemispheres. Prime Meridian with 180° → Eastern and Western Hemispheres.
• West to east by longitude: New York (~74°W) → Greenwich (0°) → Delhi (77°E) → Indian Standard Meridian (82°30'E) → Tokyo (140°E).

Mains Points

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

• One common meridian gave the world a single system of longitude and led to 24 standard time zones [2][5].

• Geography and power:

• Greenwich won partly because many sea charts already used it. This reflected Britain's lead at sea and in map-making [5].
• France held out and kept the Paris meridian [3].

• So even a "scientific" standard was shaped by politics.

• Indian knowledge tradition:

• The Ujjayini madhya rekhā shows that India had its own reference meridian for astronomy and time-keeping centuries before Greenwich [6].
• Some cities on it sit slightly off the line. This is because fixing longitude needs exact timekeeping, the same problem that later made longitude hard to fix at sea.

Related concepts

• Meridians of longitude
• Ujjayini meridian

Read more

• Meridians of longitude and the Prime Meridian