Maarg Manthan · Topic 1.2

Longitudes, Meridians and Time Zones

Geography › Earth, Latitudes, Longitudes and Motions · Topic 1.2

Longitude is the angular distance of a place east or west of the Prime Meridian, which passes through Greenwich and is fixed at 0 degrees. Meridians are semicircles from pole to pole of equal length, and because the Earth turns 15 degrees every hour, longitude decides local time, standard time, time zones and the International Date Line.

6 min read · UPSC Prelims · Geography · Earth, Latitudes, Longitudes and Motions

At a Glance

Meaning Angular distance east or west of the Prime Meridian
Zero line Prime Meridian through Greenwich
Range 0 to 180 degrees east and west
Time link 15 degrees equals one hour
IST 82.5 degrees E, GMT plus 5 hours 30 minutes
Exam link UPSC Prelims, Geography: Earth, Latitudes, Longitudes and Motions

Where Does It Fit?

This is the second topic of the category on the Earth, latitudes, longitudes and motions. The previous topic covered latitudes, and this one covers longitudes and their link with time. The next topic covers the rotation of the Earth. Previous topic: Latitudes and Important Parallels. Next topic: Rotation of the Earth and Its Effects.

What Is Longitude?

In short: longitude tells how far east or west a place is from the Prime Meridian.

Longitude is the angular distance of a place east or west of the Prime (First) Meridian, measured in degrees from the centre of the Earth. On a globe, longitude is shown as a series of semicircles that run from pole to pole through the equator; these lines are called meridians. In 1884 the meridian that passes through the Royal Observatory at Greenwich, near London, was chosen as the zero meridian or Prime Meridian. All other meridians radiate eastwards and westwards from it up to 180 degrees. Unlike parallels of latitude, all meridians are of equal length, and they converge at the poles. Together, latitude and longitude form a grid that gives the coordinates of any place.

How Are Longitude and Time Linked?

The Earth makes one complete rotation of 360 degrees in about 24 hours, so it turns 15 degrees in one hour, or 1 degree in 4 minutes. The Earth rotates from west to east, so for every 15 degrees one goes eastwards, local time advances by one hour, and for every 15 degrees westwards it falls behind by one hour. Places east of Greenwich therefore gain time and places west of it lose time. Greenwich Mean Time, sometimes called World Time, is the base for the time zones of the world.

Movement	Effect on time
15 degrees east	One hour ahead of the starting place
15 degrees west	One hour behind the starting place
1 degree of longitude	Four minutes of time
180 degrees east	12 hours ahead of GMT
180 degrees west	12 hours behind GMT

What Are Standard Time and Time Zones?

Local time changes with every meridian, which would be confusing in daily life. Standard time is the time of a chosen longitude, adopted for a whole country or region. Most countries take their standard time from the central meridian of the country. Countries with a very large east-west span find a single time inconvenient and use several time zones: Russia has eleven, Canada six and the United States six main zones. A country with a large north-south extent but a small east-west extent, such as Chile, does not need multiple time zones.

Indian Standard Time is the time at 82.5 degrees E longitude, which passes close to Prayagraj in Uttar Pradesh and runs through Mirzapur. IST is therefore 5 hours 30 minutes ahead of GMT. India extends from about 68 degrees 7 minutes E to 97 degrees 25 minutes E, a span of nearly 30 degrees, so the Sun rises about two hours earlier in Arunachal Pradesh than in Gujarat, even though the whole country follows one time.

What Was Chaibagaan Time?

About 150 years ago, British planters introduced Chaibagaan time, or Bagaan time, in the tea gardens of Assam, which ran one hour ahead of IST so that daylight could be used better. After Independence Assam followed IST like the rest of India. The Assam administration has proposed returning to the earlier time to save energy and raise productivity, but the Government of India has not accepted such a proposal. The debate shows how a country with a wide east-west span can face pressure for more than one time.

What Is the International Date Line?

The International Date Line is an imaginary line that runs through the Pacific Ocean, roughly along the 180 degree meridian. The date changes by exactly one day when it is crossed. A traveller going from east to west across the line loses a day, and a traveller going from west to east gains a day. The reason is that 180 degrees E is GMT plus 12 hours and 180 degrees W is GMT minus 12 hours, so the two sides differ by 24 hours.

The line is not straight; it bends around island groups so that places of the same country or island group keep the same date. It cuts across Oceania, which includes Australia, Melanesia, Micronesia and Polynesia. Samoa, Kiritimati (Christmas Island) in Kiribati and Tonga are among the first places to welcome a new year, while Baker Island and Howland Island of the United States are among the last. As an analytical point, the 180 degree meridian was chosen because it passes mostly through open ocean, which keeps disruption to settled land to a minimum.

How Do Latitude and Longitude Differ?

Both are used to fix the location of a point, but they measure different things.

Feature	Latitude	Longitude
Measured along	North or south of the equator	East or west of the Prime Meridian
Lines called	Parallels	Meridians
Zero line	Equator	Prime Meridian
Length of lines	Equator longest, shorter poleward	All equal in length
Important lines	Equator, Tropics, Polar Circles	Prime Meridian and 180 degrees
Effect	Controls heat and climate	Controls time

The earlier ideas are in Latitudes and Important Parallels, and the cause of the time difference is the rotation covered in Rotation of the Earth and Its Effects.

Why Is This Topic Important?

Time questions Frequent Prelims calculation
Date line Common confusion
India One time for a wide country

What Came Next?

The next topic describes the rotation of the Earth and its effects. Next topic: Rotation of the Earth and Its Effects.

Key People and Terms

Meridian Line of equal longitude
Prime Meridian Zero degree longitude at Greenwich
GMT Time at Greenwich
Standard time Time of a chosen meridian
International Date Line Date changes when crossed
Chaibagaan time Assam tea garden time

Exam Corner

Points to Remember

Prime Meridian chosen in 1884 at Greenwich.
15 degrees is one hour and 1 degree is four minutes.
IST is at 82.5 degrees E, GMT plus 5:30.
East to west across the date line loses a day.
All meridians are of equal length.

Do Not Confuse With

180 degrees E and 180 degrees W The same meridian, differing in direction of travel
Local time and standard time Time of one meridian versus time of a chosen meridian
Meridians and parallels Pole to pole versus parallel to the equator

Memory Hook

Fifteen degrees, one hour; east gains, west loses; cross the date line eastward and gain a day.

Mains Angle

Questions ask about time zones and India’s single time. These points add depth.

Time Longitude decides local time
Zones Large countries need several
India Two-hour solar gap, one time

A Question You May Face

An original practice question, not a past paper question.

“Discuss the case for and against more than one time zone in India.”

How to Answer

Basis Longitude and time.
Case for Assam and the Northeast.
Case against National coordination.
Verdict Balance of both.

GS Relevance

Prelims: longitude, IST, date line. GS Paper 1: world geography and time calculations.

Frequently Asked Questions

What is longitude?

Longitude is the angular distance of a place east or west of the Prime Meridian, measured in degrees from the centre of the Earth. It runs from 0 degrees at Greenwich to 180 degrees east and west, and lines of equal longitude are called meridians.

Why is the Prime Meridian at Greenwich?

In 1884 the meridian through the Royal Observatory at Greenwich, near London, was chosen as the zero or prime meridian. All other meridians are measured eastwards and westwards from it up to 180 degrees, and it is the base of Greenwich Mean Time.

How is longitude linked to time?

The Earth rotates 360 degrees in about 24 hours, so it turns 15 degrees in an hour and 1 degree in four minutes. For every 15 degrees east, local time advances by one hour, and for every 15 degrees west it falls behind by one hour.

What is Indian Standard Time?

Indian Standard Time is the time at 82.5 degrees E longitude, which passes close to Prayagraj. It is 5 hours 30 minutes ahead of Greenwich Mean Time. India follows this single time across the whole country, although it extends over nearly 30 degrees of longitude.

What happens when you cross the International Date Line?

The date changes by exactly one day. A traveller going from east to west across the International Date Line loses a day, and one going from west to east gains a day, because the two sides of the 180 degree meridian differ by 24 hours.

PYQ Practice — Statement Analysis

1 All meridians of longitude are equal in length.

True

They are semicircles from pole to pole.

2 Indian Standard Time is based on 82.5 degrees E longitude.