Geographic Grid: Latitudes and Longitudes

Q: Name the main latitudes of the Northern Hemisphere with their values.

The Tropic of Cancer at 23tfrac12° N and the Arctic Circle at 66tfrac12° N, with the Equator at 0° as the base and the North Pole at 90° N.

Q: What is the Prime Meridian? Why was it necessary to agree on one?

The Prime Meridian is the 0° meridian of longitude passing through Greenwich near London. Meridians have no natural starting line, unlike parallels, so a common 0° meridian was needed for maps, navigation and time to agree across countries.

Q: Why does the local time change by 4 minutes for every degree of longitude?

The Earth turns 360° in 24 hours, which is 15° in one hour, so one degree takes 60 ÷ 15 = 4 minutes.

ICSE Class 9 · Geography · Chapter 2

What this chapter covers

The geographic grid
Concept of latitudes
Main latitudes and their location
Parallels of latitude and their uses
Concept of longitudes
Eastern and Western Hemisphere
Using latitudes and longitudes to find location
Longitude and local time
Standard time, Greenwich Mean Time and time zones
Calculation of time
International Date Line
Great circles and their use

Key definitions

Latitude is the angular distance of a place north or south of the Equator, measured in degrees from the centre of the Earth.

Parallels of latitude are the imaginary circles drawn on the globe joining all places that have the same latitude, running east to west and parallel to the Equator.

Longitude is the angular distance of a place east or west of the Prime Meridian, measured in degrees from the centre of the Earth.

Meridians of longitude are the imaginary semicircular lines drawn on the globe from the North Pole to the South Pole, joining all places that have the same longitude.

Local time is the time of a place calculated from the position of the Sun over its own meridian, so that it is 12 noon when the Sun is at its highest point in that place's sky.

Standard time is the uniform time adopted for a whole country or region, based on the local time of one chosen meridian called its standard meridian.

Indian Standard Time (IST) is the standard time of India, based on the local time of the meridian of 82^° 30' E, which passes close to Mirzapur near Prayagraj in Uttar Pradesh.

Greenwich Mean Time (GMT) is the local time of the Prime Meridian at Greenwich, used as the reference from which the standard times of all countries are stated.

Time zones are the belts of the Earth's surface, each about 15^° of longitude wide, within which the same standard time is used.

Key points

The geographic grid is the network of parallels of latitude and meridians of longitude used to fix the position of any place.
Latitude is the angular distance north or south of the Equator; it runs from 0^° at the Equator to 90^° at the poles.
The main latitudes are the Equator (0^°), the Tropics of Cancer and Capricorn (23tfrac12^° N and S) and the Arctic and Antarctic Circles (66tfrac12^° N and S).
Parallels locate places north or south, measure north-south distance (about 111 km per degree), mark the heat zones and divide the globe into Northern and Southern Hemispheres.
Longitude is the angular distance east or west of the Prime Meridian (0^°, Greenwich); it runs to 180^° E and 180^° W.
Meridians are semicircles of equal length that meet at the poles; the Prime Meridian and 180^° meridian divide the globe into Eastern and Western Hemispheres.
A place is located where its parallel and meridian cross; write latitude first, then longitude, each with a direction letter.
15^° of longitude = 1 hour and 1^° = 4 minutes; places to the east are ahead in time.
Local time depends on each place's own meridian; standard time is the time of one chosen meridian for a whole country.
IST is based on 82^° 30' E and is 5 hours 30 minutes ahead of GMT, the local time of Greenwich.
The Earth is divided into 24 time zones of about 15^° each, with boundaries adjusted to political borders.
The International Date Line runs roughly along 180^°; crossing it eastward repeats a day, and crossing it westward skips a day.
A great circle divides the Earth into two equal halves; the Equator is the only great-circle parallel, and every pair of opposite meridians forms one.
Great circle routes are the shortest routes on the globe and are used by aircraft and ships.

Questions answered

Define latitude and state its value at the Equator and at the poles.

Latitude is the angular distance of a place north or south of the Equator, measured in degrees from the centre of the Earth. It is 0^° at the Equator and 90^° N or 90^° S at the poles.

Name the main latitudes of the Northern Hemisphere with their values.

The Tropic of Cancer at 23tfrac12^° N and the Arctic Circle at 66tfrac12^° N, with the Equator at 0^° as the base and the North Pole at 90^° N.

Why are parallels of latitude so called?

They run parallel to the Equator and to one another; the distance between any two of them stays the same everywhere, so they never meet.

State two uses of parallels of latitude.

They show how far north or south of the Equator a place lies, and they divide the Earth into heat zones (Torrid, Temperate and Frigid). They also help to estimate north-south distances, since one degree of latitude is about 111 km.

What is the Prime Meridian? Why was it necessary to agree on one?

The Prime Meridian is the 0^° meridian of longitude passing through Greenwich near London. Meridians have no natural starting line, unlike parallels, so a common 0^° meridian was needed for maps, navigation and time to agree across countries.

Give two differences between parallels of latitude and meridians of longitude.

Parallels are full circles that never meet and become smaller towards the poles, while meridians are semicircles of equal length that meet at the poles. One degree of latitude is about 111 km everywhere, while one degree of longitude shrinks from about 111 km at the Equator to zero at the poles.

In which hemispheres does India lie? Give a reason.

India lies in the Northern Hemisphere, because all its latitudes are north of the Equator, and in the Eastern Hemisphere, because all its longitudes are east of the Prime Meridian.

Why does the local time change by 4 minutes for every degree of longitude?

The Earth turns 360^° in 24 hours, which is 15^° in one hour, so one degree takes 60 ÷ 15 = 4 minutes.

Past board exam questions

ICSE 2019 sample paper · 2 marks

Why are the lines of longitudes also called meridians?

Show answer

Longitudes are called meridians because all places on the same line of longitude have their noon (mid-day, Latin 'meridies') at the same time: when the sun is overhead at a longitude, it is mid-day at every place along that line from the North Pole to the South Pole.

ICSE 2019 sample paper · 2 marks

(i) Why are latitudes also called parallels of latitude? (ii) Why are all the lines of longitude of the same length?

Show answer

(i) Lines of latitude run east-west and are parallel to the equator and to one another; they never meet, so they are called parallels of latitude. (ii) Every line of longitude is a semicircle joining the North Pole to the South Pole, so all of them have the same length (about 20,000 km).

ICSE 2019 sample paper · 3 marks

Give a geographical reason for each of the following: (i) Great circles are the shortest routes between two places. (ii) The Earth is a habitable planet. (iii) Kuala Lumpur rotates faster on the Earth’s axis than London.

Show answer

(i) A great circle divides the Earth into two equal halves; its plane passes through the centre of the Earth, so on a sphere the arc of a great circle between two places is the shortest distance between them. Hence ships and aircraft follow great circle routes. (ii) The Earth is at a suitable distance from the sun, so its temperature is moderate; it has liquid water and an atmosphere containing oxygen (and an ozone layer that filters harmful ultraviolet rays), which together support life. (iii) Kuala Lumpur lies near the equator (about 3°N), while London is at about 51½°N. The circumference of the parallel of latitude is far greater near the equator, but every place completes one rotation …