Earth Latitude and Longitude
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.
- 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 ° meridian. Such points are called Antipodl 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.
- A suitable memory acronym can be: East-Gain-Add (E.G.A.) and West-Lose-Subtract (W.L.S.). So, if it is noon in London (near 0°), 15° east will be one hour ahead of London or 1 p.m. and Chennai of 80°E will be 5 hours 20 minutes ahead. To avoid confusion about having many local times within one country, a particular Meridian is chosen for the whole country whose time is known as ‘standard time’.
- Generally, the standard meridians are chosen to differ from the Greenwich meridian by the multiples of fifteen degree or seven and a half degree, i.e., by exact number of hours or half hours. The world is thus divided into a number of time zones. Larger countries like Russia, Canada, USA etc., have greater east-west extension, so they adopt several time zones. Russia has 11 time zones while USA and Canada have 5 time zones.
- 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.
- 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.