Latitude, Longitude and Time

1. The earth's shape and the geographical grid

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• The earth is nearly a sphere, but not a perfect one. Rotation makes it bulge at the equator, so the equatorial radius is larger than the polar radius.
• Geoid means the earth's true shape. It is slightly flattened at the poles and bulges at the equator, so it looks like an oblate spheroid. Beyond NCERT, the equatorial radius is about 6,378 km and the polar radius about 6,357 km.
• A sphere has no natural starting point. Rotation gives two fixed reference points, the North and South Poles.
• The geographical grid is a network of crossing lines built on these poles. Parallels of latitude run east-west and meridians of longitude run north-south.
• Geographical coordinates are a place's latitude and longitude together. They are angles, measured in degrees (°), minutes (1° = 60′) and seconds (1′ = 60″). With them you can work out location, distance and direction.
• Class 6 explains coordinates with two everyday examples: a shop found as "7th shop, 5th row" in a market, and chess moves written like "d2 to d4".
• Class 6 places Delhi at about 29°N, 77°E. These figures are rounded, because Delhi is really near 28°37′N. India runs from about 8°N to 37°N and 68°E to 97°E (details in india-location).

2. Parallels of latitude: equator, great and small circles, length of a degree

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• Parallels of latitude: latitude is a place's angle north or south of the equator, measured from the earth's centre. A parallel is a circle joining places with the same latitude. The equator is 0° and the poles are 90°N and 90°S.
• If you draw parallels at every 1°, you get 89 in each hemisphere, or 179 in all including the equator.
• Latitude can be found from how high the sun or the Pole Star stands in the sky. In the northern hemisphere, the Pole Star's height roughly equals the latitude.
• The equator is the 0° parallel and the longest one. It splits the earth into the Northern and Southern Hemispheres.
• A great circle is any circle whose plane passes through the earth's centre, so it cuts the earth into two equal halves. The equator is the only parallel that is a great circle. Any meridian together with its opposite meridian also makes one.
• The arc of a great circle is the shortest route between two points. That is why ships and aircraft follow great-circle routes, for example Vancouver to Yokohama across the North Pacific.
• A small circle is one whose plane does not pass through the centre. Every parallel except the equator is a small circle, and these circles shrink towards the poles.

Length of a degree of latitude and longitude	Equator	45°	Poles
1° of latitude	110.6 km	≈111 km	111.7 km
1° of longitude	111.3 km	79 km	0 km

• Trap: a degree of latitude is slightly longer at the poles, because the earth is flatter there.

3. Important parallels and the heat zones

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• Tropic of Cancer (23½°N) and Tropic of Capricorn (23½°S): the sun can be directly overhead at noon only between these two lines.
• Arctic Circle (66½°N) and Antarctic Circle (66½°S): these mark where the midnight sun (24-hour daylight) and the polar night (24-hour darkness) begin.
• Beyond NCERT, these lines come from the earth's axial tilt of about 23.4°. The tilt changes very slowly, so the Tropic of Cancer is now at about 23°26′.
• The eight Indian states that the Tropic of Cancer crosses are covered in india-location.
• Heat zones (Class 11 Table 3.1; Class 6):

Zone	Latitudes	Class 6 climate word
Torrid	0°–23½° N and S	hot
Temperate (two)	23½°–66½° N and S	moderate
Frigid (two)	66½°–90° N and S	cold

• Real climate belts, such as the Koeppen types, do not follow these lines exactly. Altitude, ocean currents and distance from the sea also matter. Why latitude controls heat is explained in insolation-heat-budget.

4. Meridians of longitude and the Prime Meridian

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• Meridians of longitude: longitude is a place's angle east or west of the Prime Meridian, measured along the equator. It runs from 0° to 180°E and 180°W.
• Meridians are half-circles from pole to pole, and all are equal in length. Each is half as long as the equator (a Class 6 true/false item).
• Meridians are farthest apart at the equator, meet at the poles and cross the equator at right angles.
• 180°E and 180°W are the same line, so it is written simply as 180°.
• The Prime Meridian splits the earth into the Eastern and Western Hemispheres. The equator splits it into the Northern and Southern Hemispheres.
• The Prime Meridian is 0° and passes through the Royal Observatory, Greenwich, near London. It was adopted by international agreement in 1884. Beyond NCERT, this was the International Meridian Conference in Washington. France abstained and kept the Paris meridian for some decades.
• Ujjayini meridian: Greenwich was not the first prime meridian. Ancient Indian astronomy used the madhya rekhā ("middle line") through Ujjayinī (Ujjain), where Varāhamihira worked about 1,500 years ago. Some cities that old texts placed on this line lie a little off it, because timekeeping was less exact then.
• Examples from Class 6: New York ≈74°W, Delhi 77°E, Tokyo 140°E.

5. Longitude and time: local time

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• Longitude and time: the earth rotates from west to east through 360° in 24 hours.
• So the sun seems to cross 15° of longitude per hour, or 1° every 4 minutes.

• Places to the east see the sun first, so time is ahead in the east and behind in the west.

• Local time is a place's time set by the sun over its own meridian. Local noon is when the sun is highest in the sky.

• Places on the same meridian share the same local time, whatever their latitude. Delhi and Bengaluru are both near 77°E, so the difference is zero.
• Method: find the difference in longitude, multiply it by 4 minutes, then add the result for a place to the east or subtract it for a place to the west. If the two places are on opposite sides of Greenwich, add their longitudes first.
• Worked examples (Class 11), with noon at Greenwich:
• Thimphu (90°E): 90 × 4 = 360 min = 6 h, so the time is 6 p.m.

• New Orleans (90°W): the time is 6 a.m.

• Class 6 Porbandar–Tinsukia exercise: take the difference as 30°. Then 30 × 4 = 120 minutes, about 2 hours. That is why the sun has already set in Assam while Gujarat still has daylight.

6. Standard time, GMT and UTC

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• If every town kept its own local time, one country would have many clock times. So each country adopts standard time: one uniform time, taken as the local time of a chosen meridian.
• The standard meridian is the meridian whose local time becomes the standard time. By convention it is a multiple of 7°30′ (half an hour of time), so the offset from Greenwich is a whole or half hour.

• Class 11's "divisible by 150°" is a misprint. Read it as 15° or 7°30′; Class 11, India: Location states 7°30′ correctly.

• Greenwich Mean Time (GMT) is the mean local time at 0°. It was the world's reference time from 1884.

• Coordinated Universal Time (UTC) is the world's main time standard today, kept by a worldwide set of atomic clocks and coordinated by the International Bureau of Weights and Measures (BIPM).
• It replaced GMT for civil use, starting in the 1960s and in its present form from 1972.

• IST = UTC+5:30.

• Leap second means one extra second added to UTC to keep it within 0.9 second of the earth's slightly uneven rotation.

• 27 leap seconds have been added since 1972. The last one was on 31 December 2016.
• The CGPM (General Conference on Weights and Measures) decided in 2022 to stop adding them by 2035.

7. Time zones of the world

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• Time zone: the world has 24 major zones, each about 15° wide (Class 11 Fig. 3.6). Each keeps one standard time, usually a whole number of hours from GMT.
• Zone boundaries are not straight meridians. They bend to follow international and internal borders, so "zone lines are meridians" is FALSE (Class 6).
• Countries that are wide from east to west use several zones:
• Russia uses 11.
• The USA has 6 in Class 6 and 7 in Class 11, India: Location. The count changes depending on whether overseas territories are included.

• Beyond NCERT, France has the most, 12, counting its overseas territories.

• Half-hour and quarter-hour time zones are zones offset from UTC by part of an hour (beyond NCERT):

• India and Sri Lanka +5:30
• Nepal +5:45
• Afghanistan +4:30
• Iran +3:30
• Myanmar +6:30

• Chatham Islands (New Zealand) +12:45

• China's single time zone: China spans about 60° of longitude, or about five natural zones, but uses one, Beijing Time (UTC+8). In western Xinjiang the winter sunrise can come around 10 a.m., and many locals unofficially keep a time two hours behind.

• Daylight saving time means moving clocks one hour forward in summer to make use of evening daylight. It is common in mid-latitude Europe and North America.
• India does not use it, because day length changes little at low latitudes.
• Its energy savings are disputed, and the EU has debated ending it (verify current).

8. Indian Standard Time and the two-time-zones debate

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• IST is the local time of 82°30′E, which passes near Mirzapur in Uttar Pradesh. The sum is 82.5 × 4 = 330 minutes, so IST = GMT/UTC + 5 h 30 min.
• Beyond NCERT, the 82°30′E meridian passes through five states: Uttar Pradesh, Madhya Pradesh, Chhattisgarh, Odisha and Andhra Pradesh.
• History: IST was introduced in 1906. Calcutta Time survived until 1948 and Bombay Time until 1955.
• Indian Standard Time keeping: IST is generated and kept by CSIR-National Physical Laboratory (NPL), New Delhi, using atomic clocks. Draft Legal Metrology (IST) Rules 2024 would make IST compulsory for official, legal and commercial use (verify current).
• The problem: India spans about 29° of longitude, which is nearly two hours of solar time. The sun rises in the north-east almost two hours earlier than in Kutch.
• Dong valley (Anjaw district, Arunachal Pradesh, near the India–China–Myanmar trijunction) is India's first place to see sunrise, around 4 a.m. in summer.

• Offices open long after daylight begins there, and evenings need more electricity.

• Chai bagaan time: since British times, Assam's tea gardens have kept a local time about one hour ahead of IST to use the early daylight.

• Two time zones debate:
• For a second zone: saving daylight and energy, better productivity and health, and a long-standing north-east demand (raised by Assam's Chief Minister in 2014).
• Against: risk of railway and air-traffic mistakes at the zone border, confusion in a closely linked economy, and the symbolic value of one national time.
• NPL researchers' proposal (Current Science, 2018): IST-I at UTC+5:30, and IST-II at UTC+6:30 for the north-east and Andaman & Nicobar, divided near 89°52′E. The Union government has not accepted it (verify current).
• Another suggestion is to move all-India IST forward by half an hour.

9. The International Date Line

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• The calendar day has to change somewhere. The International Date Line runs roughly along 180°, which is 12 hours from Greenwich in either direction. The +12 and −12 zones meet there.
• Rule (Class 6, which is clearer than Class 11):
• Crossing eastward (Asia to America): subtract a day, so Tuesday becomes Monday again.

• Crossing westward (America to Asia): add a day, so Sunday becomes Monday.

• Why it works:

• A traveller going east gains an hour in each zone.
• After 24 zones they have gained a whole day.

• The Date Line cancels those 24 hours. Jules Verne's Around the World in Eighty Days turns on this.

• The line zigzags so it does not split countries or island groups. It bends east of Chukotka (Siberia), west of the Aleutians, and around Kiribati, Fiji and Tonga.

• Kiribati date line shift (beyond NCERT):
• In 1995 Kiribati moved the line east so all its islands share one date. Its Line Islands (UTC+14) became the first inhabited land to see each new day.

• Samoa skipped 30 December 2011 to join the west side, for easier trade with Australia and New Zealand.

• So clock times on earth can differ by up to 26 hours (UTC+14 to UTC−12).

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Exam angles

Prelims — high-yield facts and traps

• Time moves 15° per hour, or 4 minutes per degree. East is ahead and west is behind. Places on the same meridian have the same local time, whatever their latitude.
• IST meridian is 82°30′E, near Mirzapur. It crosses UP, MP, Chhattisgarh, Odisha and Andhra Pradesh. IST = UTC+5:30, kept by CSIR-NPL.
• Standard meridians are multiples of 7°30′. Nepal (+5:45) breaks the whole/half-hour pattern; Afghanistan (+4:30) is a half-hour zone. "Divisible by 150°" in Class 11 is a misprint.
• The equator is the only parallel that is a great circle. All meridians are equal in length. Parallels shrink towards the poles. At 1° spacing there are 179 parallels.
• "A degree of latitude is longest at the equator" is FALSE: it is 111.7 km at the poles and 110.6 km at the equator. A degree of longitude falls from 111.3 km to 79 km (at 45°) to 0.
• "Time-zone boundaries are meridians" is FALSE. "In Assam, local time = IST" is FALSE.
• Date Line: crossing eastward repeats a day, crossing westward skips one. The line is not a straight meridian.
• Heat zones: torrid 0–23½°, temperate 23½–66½°, frigid 66½–90°.
• Greenwich was adopted in 1884. The Ujjayini madhya rekhā was ancient India's prime meridian.
• Leap seconds: 27 since 1972, last in 2016, to end by 2035. China spans about five natural zones but uses one time (UTC+8).
• Class 11 slip: it dates Hurricane Katrina to October 2005, but it struck New Orleans in August 2005.

Mains — GS-I themes

1. Should India adopt two time zones? Weigh the economic, social and administrative arguments, with reference to the north-east (chai bagaan time, Dong valley, NPL 2018 proposal).
2. How does longitude decide time? Why do time-zone boundaries depart from meridians? Give examples (Russia, the USA, Kiribati, Samoa).
3. One national time as a tool of unity vs regional daylight realities: compare India, China and Russia.
4. Precise timekeeping as strategic infrastructure: atomic clocks at NPL, NavIC timing, and the case for a legally mandated IST.

Current-affairs hooks

• Notification and enforcement of the Legal Metrology (IST) Rules, and NPL's time-distribution network (verify current).
• Renewed demands for a north-east time zone, in Parliament or from state governments.
• The end of leap seconds and other changes to international time standards (BIPM, ITU).
• Daylight-saving-time debates in the EU and the USA (verify current).
• Satellite navigation timing (NavIC clocks) and its link to national time (see remote-sensing-gis).