GATEWAY TO CGL MAINS: GEOGRAPHY STUDY NOTES

Earth Latitude and Longitude

Earth Latitude

• Imaginary lines drawn parallel to the equator. Measured as an angle whose apex is at the centre of the earth.

• The equator represents 0° latitude, while the North Pole is 90° N and the South Pole 90° S

• 23½° N represents Tropic of Cancer while 23½° S represents Tropic of Capricorn.

• 66½° N represents Arctic Circle while 66½° S represents Antarctic Circle.

• There are total 181 latitudes including the equator. Each parallel of latitude is a circle, but they are not equal.

• The circle becomes smaller towards the poles. Equator is the ‘Greatest Circle’ that can be drawn on the earth’s surface.

• The distance between any two parallels of latitude is always equal.

1 degree lat. = 111km.

Earth Longitude

• It is the angular distance measured from die centre of the earth. On the globe the lines of longitude are drawn as a series of semicircles that extend from the North Pole to the South Pole through the equator. They are also called meridians.

• The distance between any two meridians is not equal.

At the equator, 1 degree = 111 km.  At 30°N or S, it is 96.5 km.  It goes on decreasing this way until it is zero at the poles.

• There are 360 meridians of longitude. The prime meridian is a longitude of 0⁰, passing through the Royal Observatory at Greenwich near London.

• This meridian is taken by geographers to divide the earth into the eastern and the western hemispheres.

• Each meridian of longitude is a semi-circle. 180° meridian (International Date Line) lies exactly opposite to 0° meridian. Such points are called Antipodal Points.

• The earth is divided into 24 longitudinal zones, each being 15° or 1 hour apart in time (4 minutes / degree).

Longitude and Time

• Places that are on the same meridian have the same local (sun) time. Since the earth makes one complete revolution of 360° in 24 hours, it passes through 15° in one hour or 1° in 4 minutes.

• The earth rotates from west to east, hence places east of Greenwich see the sun earlier and gain time whereas places west of Greenwich see the sun later and lose time.

• India, whose longitudinal extent is approx. 30°, has adopted only one time zone, selecting the 82.5°E for the standard time which is 5 hours and 30 minutes ahead of GMT (Greenwich Mean Time).

International Date Line

• It is the 180° meridian running over the Pacific Ocean, deviating at Aleutian Islands, Fiji, Samoa and Gilbert Islands. It is a zig-zag line

• Travelers crossing the Date Line from west to east (i.e., from Japan to USA) repeat a day and travelers crossing it from east to west (i.e., from USA to Japan) lose a day.

Important Parallels of Latitude

1. The Tropic of Cancer : It is in the northern hemisphere at an angular distance of 23 1/2° (23°30’N) from the equator.

2. The Tropic of Capricorn : It is in the southern hemisphere at an angular distance of 23 1/2° (23°30’S) from the equator.

3. The Arctic Circle : It lies at a distance of 66 1/2° (66°30’N) north of the equator.

4. The Antarctic Circle : It lies at a distance of 66 1/2° (66°30’S) south of the equator. There are two solstices each year, called the Summer Solstice and the Winter Solstice. 

Summer Solstice : The day of 21st June when the sun is vertically overhead at the Tropic of Cancer (23°30’N). 

Winter Solstice : The day of 22nd December when the sun is vertically overhead at the Tropic of Capricorn (23°30’S).

Meridians of Longitude

The semi-circles running from pole to pole or from north to south are known as meridians of longitude and distance between them is measured in degrees of longitude. Greenwich Meridian or Prime Meridian with a value of 0° longitude serves as a common base for numbering meridians of longitude lying on either side of it — east as well as west. There are 360 meridians including Prime Meridian. Each degree of a longitude is divided into sixty equal parts, each part is called a minute. Each minute is again divided into sixty equal parts, each part being called a second.

Local Time : Local time of any place is 12 noon when the sun is exactly overhead. It will vary from the Greenwich time at the rate of four minutes for each degree of longitude.

Greenwich Mean Time : The time at 0° longitude is called Greenwich Mean Time. It is based on local time of the meridian passing through Greenwich near London.

Indian Standard Time : It is fixed on the mean of 82 1/2°E Meridian, a place near Allahabad. It is 5 1/2 hours ahead of Greenwich Mean Time.