Brief Notes on Earth Rotation

1. Spins on its imaginary axis from west to east in 23 hrs, 56 min and 40.91 sec.

2. Rotational velocity at equator is 1667 km/h and it decreases towards the poles, where it is zero.

3. Earth’s rotation results in (i) causation of days and nights; (ii) a difference of one hour between two meridians which are 15° apart; (iii) change in the direction of wind and ocean currents; (iv) rise and fall of tides everyday.

4. The longest day in North Hemisphere is June 21, while shortest day is on 22 Dec (Vice-versa in S. Hemisphere).

5. Days and nights are almost equal at the equator.

At the equator, there is a 12-hour day and a 12-hour night. North of 66Vi° N there is continuous day light; south of 66!/2°S there is conti­nuous night. Days become longer with increasing latitude north, shorter with increasing latitude south.

Revolution: The movement of the earth around the sun is called Revolution. The path on which it describes its motion is called orbit. The shape of the orbit is an ellipse and not a circle. The mean distance of the Earth from the Sun is 150 million km but because of the elliptical shape of the orbit the distance varies from time to time.

The Earth takes about 365 W days in making one revolution of the Sun. As we take 365 days for a year, difference of V* of a day requires adjustment.

This is why we add one day to February after every four years. Effects of the Revolution are (a) change of seasons, (b) length of days and nights, (c) shifting of winds, belts, and (d) determination of latitudes.

Solstices and Equinoxes: The Earth moves around the sun in such a way that its axis remains inclined towards the orbital plane and that its axis always points towards the Pole Star. As a result, the axis of the earth leans towards the sun by various degrees in the course of the revolution.

On June 21, the earth leans towards the sun at the maximum angle of 23Vi°. The earth’s northern hemisphere is tipped towards the sun and the southern hemisphere away from the sun. This condition of the earth in relating to sun is called Summer Solstice. After six months on December 22, the earth occupies equivalent position on the opposite point in its obit. In this position the Earth is again inclined to the maximum of 23’A° but this time the southern hemisphere is tipped towards the sun and northern hemisphere away from the Sun.

This position of the Earth is called Winter Solstice. In between these two Positions the earth has two other important positions. On March 21 and September 23, the earth’s axis is not inclined to the sun, i.e., neither the north nor the south pole is inclined to the Sun. The sun’s rays are vertical at the Equator.

The position on March 21 is called Spring Equinox (Vernal Equinox) and that on September 23, Autumn Equinox.

Latitude and Longitude: The latitude of a place is defined as its distance north or south of the equator, measured as an angle. Latitudes are thus lines drawn parallel to the equator which is at 0° and they are counted up to 90° north and south, the two poles.

The longitudes show the position of a point east or west of the prime meridian which is at 0° and passes through Greenwich village near London in England.

There are 360 longitudes and all longitudinal lines join at the poles. The longitude of a place can be defined as its distance east or west of the meridian of Greenwich, measured as an angle.

Some lines of latitude and longitude have great significance in geographical studies. The parallel (or latitude of) 0° is called the Equator, those of 23 V2⁰ N and S are known as The Tropic of Cancer and the Tropic of Capricorn respectively; and the 66V2⁰ N and S are known as the Arctic and Antarctic Circles respectively.

Greenwich Mean Time: The Prime Meridian (0° longitude) passing through the former Royal Observatory at Greenwich, England from which other meridians are calculated, is called the Greenwich Meridian.

The local time at Greenwich or any other place situated on the Prime Meridian is called the Greenwich Mean Time (GMT).