OUR PLANET - THE EARTH 

  • Our earth is a member of the solar family. 
  • It has been able to develop and retain an atmosphere, a hydrosphere and, a biosphere, because of its appropriate size and distance from the sun. 

Origin of The Earth 

  • A number of theories have been put forward to explain the origin of the earth and solar system.
  • These theories can be classified into two categories: (i) evolutionary or natural or monistic hypothesis which suppose that the system of planets have evolved from one star or stars; and (ii) the cataclysmic or catastrophic dualistic which believe in some sudden and violent event in space, like the collision or close approach of two stars. 

Shape of The Earth 

  • Equatorial diameter —12,757 km (7,927 miles). 
  • Polar diameter —12,714 km (7,900 miles), less by 43 km (27 miles). 
  • The earth is not a perfect sphere. Its shape is like the shape of an orange which is a little flattened on both ends and made to bulge slightly around the centre. The earth too is flattened at the poles and bulges slightly at the equator. This shape is an oblate spheroid. It is also called a 'geoid' meaning earth-shaped.

Movement of The Earth 

The earth has the following important movements: 

(i) Rotation 

  • It is the spinning movement of the earth from west to east on its axis once in 24 hours. The axis is an imaginary line joining the two poles. 
  • The phenomenon of day and night is the most important consequence of rotation. The one half of the earth facing the sun experiences daylight, while the other half away from the sun experiences night. 
  • The axis of the earth is tilted at an angle of 23.5° to the vertical. The velocity of the rotation varies, from about 1,700 km per hour at the equator, its half at 60° parallel (850 km/hour) to nearly zero at the poles. 

(ii) Revolution 

  • The movement of the earth around the sun is a fixed elliptical path or orbit. It is called revolution, completed by the earth in 365 Y4 days. This one-fourth day of four consecutive years is added as an extra day on the fourth year as 29th February, making that year a 'leap year'. 
  • Due to the elliptical shapes of the orbit, the earth is closest to the sun on January 3 (at a distance of about 197 million km) and is said to be at Perihelion. 
  • Around July 4 the earth is at farthest from the sun (152 million km away) and is known to be at Apehelion. 
  • Average speed of the earth's revolution is 107,000 km/hour (29.72 km/second). 
  • The revolution of the earth coupled with the tilt of the earth's axis causes different seasons to occur on the earth.

(iii) Eccentricity 

  • The shape of the earth's orbit changes in a cyclic period between 90,000 and 100,000 years. Thus, the orbit of the earth may become more elliptical sometimes, and form a more circular path around the sun at other times. 
  • Scholars propose that the Ice Age and Global Warming are effects of such eccentricity of the earth's orbit. 

Inclination of The Earth's Axis

  • The imaginary axis of the earth has a constant angle of inclination of 66 ½°  h with the plane of the ecliptic, i.e. the plane in which the earth orbits around the sun.
  • Parallelism is another characteristic of the earth's axis. As the earth revolves around the sun, the earth's axis remains parallel to its preceding position.
  • This leads to changes in the altitude of the mid-day sun at different times of the year, in varying lengths of day and night at different times of the year, and in the four seasons.

Varying Lengths of Day And Night 

  • Owing to revolution and constant angle of inclination of earth's axis, the sun is vertically overhead at the tropic of Cancer on 21 June each year. 
  • Towards the North Pole, the length of daytime increases, and beyond 66 ½° N the region has 24 hours light for six months. 
  • On 21st June, the Northern Hemisphere has its longest day and shortest night. This is known as Summer Solstice. 
  • By 22nd December, exactly the same conditions prevail in the Southern Hemisphere when the Sun is vertically overhead at the tropic of Capricorn. This is Winter Solstice, when the Southern Hemisphere experiences its longest day and shortest night. 
  • On 21st March and 23rd September, the sun is directly over the equator and all parts of the world have equal days and nights. Hence 21st March and 23rd September are called Vernal and Autumnal Equinoxes respectively. 

Latitude 

  • Some important parallels are: 
    (i) 0° latitude — Equator. 
    (ii) 23 % ° N — Tropic of Cancer. 
    (iii) 23 % °S — Tropic of Capricorn. 
    (iv) 66 % °N — Arctic Circle. 
    (v) 66 % °S — Antarctic Circle. 
  • The parallel of 600 is half of the Equator in length and 750 is 1/4th of the Equator.

Longitude 

  • It is the angular distance measured in degrees along the equator, east or west of the Prime Meridian. 
  • The Prime Meridian (0°) is that meridian which passes through Greenwich, near London, and from which all other meridians radiate eastwards and westwards upto 180°. 
  • 1° of latitude or longitude represents approximately) 11 km. This distance is true for the longitudes along the equator only, since the distance between the longitudes gradually decreases towards the poles to 0 km. 

Longitude and Time 

  • Places on the same meridian have the same local (sun) time. Since the earth makes one complete revolution in 24 hours, it passes through 15 degress in one hour (360 ÷ 24 = 15) or I° 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 is: 'East-Gain-Add' (E.G.A.) and 'West-Lose-Subtract' (W.L.S.). So, if it is noon in London (near 00), at a place located at 15°E, the time will be one hour ahead of London and the time will be 1 pm. Whereas, at Chennai located on 80°E, the time will be 5 hours 20 minutes ahead of Greenwhich. (80 x 4 = 320 ÷ 60 = 5.20 hours) i.e. 5.20 p.m. 
  • To avoid confusion of having many local times within one country, usu-ally a country chooses a particular meridian to determine the time for the entire country. This meridian represents the 'standard time' followed in that country.
  • Generally, the standard meridians are chosen to differ from the Greenwich meridian by multiples of 15 degrees or seven and a half degrees, i.e. by exact number of hours or half-hours. 
  • Thus, the world is divided into a number of time zones. 
  • Both USA and Canada have five time zones each. India has adopted only one time zone, selecting the meridian of 82.5°E for the standard time which is 5 hours 30 minutes ahead of G.M.T. 

Earth's Satellite - The Moon 

Our moon, the only satellite of the earth, is a fascinating object for the poets as it appears as a beautiful celestial body in the sky. It is a very distinct celestial body in the satellite system, for all other satellites are very small in relation to their mother planets. Whereas, our moon is about 'A of the size of the earth. Hence moon is referred to by some as a sub-planet. Another interesting fact is that the period it takes for rotation and for revolution are the same (27% days). As a result, the same lunar hemisphere is always seen from the earth. 

  • The moon has no atmosphere as its gravitational power is too weak to hold gases (1/6th of the Earth).
  • The front side of the moon shows (i) the bright pans which are mountains and highlands; and (ii) the darker patches of lowlands which were once thought to be seas and named accordingly as Marias, though the moon has no water at all. 
  • The moon has a cratered surface produced by the bombardment of meteors. 
  • The different phases of the moon are produced by the changing position of the moon vis-a-vis the sun and the earth. 
  • A rare celestial event, the Blue Moon, is said to occur when the second full moon appears within the same month. Still unusual is the occurrence of two blue moons in the same year (as in January and March, 1999). 
  • The ocean of tranquillity is the name given to the area where Neil Arm-strong and Edwin Aldrin landed on the moon in 1969, to become the first and only humans to set foot on the moon's surface. 

Lunar Eclipse 

  • It occurs when the earth comes between the sun and the moon, and the moon is not able to get sunlight due to the shadow of earth cast on it. 
  • Lunar eclipse occurs always on full moon, but not on every full moon, since the orbital planes of the earth and moon are inclined at 5° to each other.

The International Date Line 

  • A person who travels east of Greenwich gains time till he reaches 180°E where he will be 12 hours ahead of Greenwich and he loses 12 hours when he reaches 180°W. Hence, there is a difference of 24 hours or one day between the two sides of 180°. 
  • So, 180° meridian is called the International Date Line which, when crossed, results in change of the date by exactly one day. 
  • A traveller crossing this date line from east to west will lose a day, while going from west to east he will gain a day. 
  • Thus, when it is midnight on Monday on the Asiatic side, it is Sunday midnight on the American side (as one gains a day going eastward). 
  • To avoid such confusion of day and date in some of the island groups that are cut through by this meridian, the International Date Line has been curved from the normal 180 degrees meridian at Berring Strait, Fiji, Tonga, and other islands.