Meridians of longitude and the Prime Meridian

Detail

What longitude is

• 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.
• It runs from 0° to 180°E and from 0° to 180°W.
• Every place gets a letter as well as a number: E (east of Greenwich) or W (west of Greenwich).
• New York is about 74°W.
• Delhi is 77°E.

• Tokyo is 140°E.

• Longitude divides the earth into the Eastern and Western Hemispheres [4].

What a meridian is

• A meridian (line of longitude) is an imaginary half-circle (semicircle) that runs from the North Pole to the South Pole [4].
• All meridians are equal in length. Each one is half as long as the equator. This is a standard Class 6 true/false item.
• Spacing between meridians:
• They are farthest apart at the equator.
• They come closer as you move towards the poles.

• They meet at the poles.

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

• How the distance per degree shrinks [4]:

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

• Why the length shrinks:
• Meridians meet at the poles.
• So the gap between two meridians gets smaller as latitude increases.
• At 60° latitude the gap is about half of what it is at the equator.

Parallels and meridians compared

Feature	Parallels of latitude	Meridians of longitude
Shape	Full circles	Half-circles from pole to pole
Length	Not equal. The longest is the equator; they shrink to a point at the poles	All equal (each is half the equator's length)
Spacing	About the same distance apart	Widest at the equator, zero at the poles
Reference line	Equator (0°)	Prime Meridian (0°)
Range	0°–90°N and S	0°–180°E and W
Hemispheres	Northern and Southern	Eastern and Western

The 180° meridian

• 180°E and 180°W are the same line. So it is written simply as 180°, with no E or W.
• 0° and 180° together make one full great circle (a circle whose centre is the centre of the earth). This circle splits the earth into the Eastern and Western Hemispheres.
• In the same way, the equator splits the earth into the Northern and Southern Hemispheres (NCERT Class 6, Fig. 1.4).

Drawing meridians from a polar view (construction aside)

• Look at the earth from directly above the North Pole (NCERT Class 11, Fig. 3.4, 3.5).
• The equator appears as the outer circle.

• The meridians appear as straight lines (radii) from the centre (the pole) out to the circle.

• Seen from above the North Pole, the eastern longitudes appear on the left. This is the opposite of an ordinary wall map, so beginners often get it wrong.

The Prime Meridian at Greenwich

• 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).
• It was chosen by international agreement in 1884.
• The International Meridian Conference (October 1884):
• It was held at Washington, D.C. (USA) [2].
• It was called by US President Chester Arthur. Representatives of 25 countries took part [5].
• The exact zero line chosen was the meridian of the transit instrument at the Royal Observatory, 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 after this.

• Why Greenwich was chosen: many nautical charts (sea maps) and time systems already used Greenwich as their starting point, so keeping it made sense [5].

• Before 1884: many countries used their own prime meridians, often drawn through their capital cities [5].
• What came next: the conference led to the system of 24 standard time zones, each one 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]. (NCERT: the Prime Meridian passes through Greenwich.)
• Link to India's time: India's standard meridian is 82°30'E. It is 5 h 30 min (82.5° × 4 minutes) ahead of Greenwich.

Ujjayini: an older Indian prime meridian

• Greenwich was not the first prime meridian.
• Ancient Indian astronomy used the madhya rekhā ("middle line"). This 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.

• He was one of the authors who set out classical Hindu astronomy in texts called siddhāntas (astronomy treatises), along with Aryabhata and Brahmagupta [7].

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

• Old astronomy texts list several cities on this meridian (NCERT Class 6, Fig. 1.5).
• Some of these cities actually lie a little off the line.
• Why: clocks and timekeeping were less exact then, so longitude, which depends on measuring time differences, could not be fixed precisely.

Prelims Hooks

• All meridians are equal in length. Each is half the equator's length. Parallels are not equal. This is a classic true/false trap.
• 180°E and 180°W are the same meridian, written simply as 180°.
• Meridians are farthest apart at the equator, meet at the poles and cross the equator at 90°.
• One degree of longitude ≈ 111 km at the equator, ≈ 56 km at 60°, and 0 at the poles [4].
• Prime Meridian: Royal Observatory, Greenwich (near London). Adopted at the International Meridian Conference, Washington, D.C., October 1884 [2].
• Vote at the 1884 conference: 22–1. San Domingo voted against. France and Brazil abstained. 25 countries attended [3][5].
• Equator → Northern and Southern Hemispheres. Prime Meridian (0°–180°) → Eastern and Western Hemispheres.
• Ancient Indian prime meridian (madhya rekhā) passed through Ujjain (Ujjayini), the centre of astronomy linked with Varāhamihira.
• 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 a common prime meridian mattered (GS-I):
• Before 1884, each country had its own zero line, often through its capital.
• This made sea charts, telegraph timing and railway timetables hard to match up between countries.

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

• Geography and power: Greenwich was chosen partly because many sea charts already used it, which reflected Britain's naval and map-making lead [5]. France held out and kept the Paris meridian [3]. So even a "scientific" standard involved politics.

• Indian knowledge tradition: the Ujjayini madhya rekhā shows that India had its own reference meridian for astronomy and time-reckoning centuries before Greenwich [6]. Cities on it sit slightly off the line because exact timekeeping is needed to fix longitude, which is the same problem that later held up longitude at sea.
• Converging meridians and maps: a degree of longitude shrinks from about 111 km at the equator to 0 at the poles [4]. As a result, east–west distances on maps get distorted towards the poles, and high-latitude countries cover many degrees of longitude in fewer kilometres.