SHAPE AND SIZE OF THE EARTH
 

According to the observation made from satellites in 1970, the Earth is not a perfect sphere but resembles an ellipsoid (also called Geoid).
The difference between the Equatorial Diameter and the Polar Diameter is about 43,524 meters. That is why the Earth cannot be called a perfect sphere. The flattening at the poles is caused by the centrifugal force produced by the rotation of the Earth along is axis. This centrifugal force causes flattening at the poles and a bulge at the Equator. The flattening and bulging causes the equatorial and polar diameters to differ.
 

Plane of Ecliptic, Inclination and Parallelism
 

The Earth in its orbit around the sun moves in a constant plane. This plane is called the PLANE OF THE ECLIPTIC. The plane of the Earth’s equator makes an angle of 23½° with the plane of the ecliptic. Thus the imaginary Earth axis , being perpendicular to the equator, has a constant ANGLE OF INCLINATION, as it is called, of 66½° with the plane of the ecliptic.
In addition to a constant angle of inclination, the Earth’s axis maintains another characteristic called PARALLELISM. As the Earth revolves around the Sun, the Earth’s axis remains parallel to its former position. That is, at every position in the Earth’s orbit the axis remains pointed towards the same spot and is close to the star we call the NORTH STAR or POLARIS. Thus, the Earth’s axis is fixed with respect to the stars system, but not with respect the Sun.
 

Latitude and Longitude

The latitude of a place is defined as its distance north or south of the equator, measured as an angle from the centre of earth. Latitudes or Parallels are thus lines encircling the earth drawn parallel to the equator, which is at O^o and they are counted up to 90^o north and south, the two poles.

It is, therefore, clear that the length of parallels decreases from Equator to Poles but the distance between them is constant. Some parallels are important

• 23 1/2⁰ North - Tropic of Cancer
• 23 1/2⁰ South- Tropic of Capricorn
• 66 1/2⁰ North- Arctic Circle
• 66 1/2⁰ South- Antarctic Circle

The longitudes show the distance of a point east of west of the prime meridian, which is at O^o and passes through Greenwich Village near London in England. There are 360 longitudes.
Some lines of latitude and longitude have great significance in geographical studies. 

Among the longitudes, the 0^o or the meridian of Greenwich is known as the prime meridian and the line just opposite it on the globe, i.e. the 180^o longitude, is called the International Date Line. For each degree of longitude there is a difference of four minutes in time and when one crosses the International Date Line one loses or gains a day.

Zones between Parallels
 

Areas of the Earth which are situated between certain parallels are called ZONES.
1. TROPICAL ZONE or TORRID ZONE - It is situated between 23 ½° North and 23 ½° South Torrid or Tropical Zone is warm or better a hot area. The Sun is vertical at least once in a year at every place in this zone.
2. TEMPERATE ZONE - It is situated between 23 ½° to 66 ½° in both the Northern and Southern Hemisphere. Temperate means neither hot not cold. The Sun is never vertical at this zone.
3. FRIGID ZONE – It is situated between 66 ½° to 90° (Pole) in both the Hemispheres. The length of day or night at least once in a year is of 24 hour duration. The poles have days and nights of 6 month duration each. It is a very cold area.
 

MOTION OF THE EARTH AND THEIR EFFECTS
 

Galactic Movement
This is the movement of the Earth with the sun and the rest of the solar system in an orbit around the centre of the Milky Way Galaxy. This movement has little effect upon the changing environment of the Earth.
The earth has two main motions (i) Rotation and (ii) Revolution.
 

Rotation
The Earth rotates (Spins) around its axis. The earth completes one rotation in 24 hours (23 hours, 56 minutes, 4.09 seconds to be exact) and varies by a small amount on a daily basis; a matter of a few milliseconds. Additionally, it is slowing down due to tidal friction so that the length of a day has actually increased over the history of the Earth. (Due to gravitational pull of the moon, there is a bulge in earth along the malleable oceans. The bulge leads to increased frictional resistance along the ocean floors thereby retarding earth’s rotation. On an average this retardation adds 1.7 seconds to a day every century.)

If we look down upon the Earth from outside, the Earth is found to rotate in anticlockwise direction, i.e., from west to east.
 

Effect of the rotation of the Earth

• Formation of days and nights
• Change in the direction of winds, ocean currents, etc. (concept of Coriolis force)
• Occurrence of sunrise, noon and sun set.
• Flatness at the poles and bulge at the equator
• Concept of time and dates
• Concept of directions
• Apparent movement of the sun, the moon and the stars
• Magnetic field of the earth
• Changes in the positions of direct and indirect tides
• The deflection in the fall of an object

Revolution
The Earth has another motion besides rotation. It moves around the Sun. This motion 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. On or about January 3, the Earth is closet to the Sun. At this time the Earth is said to be in Perihelion (nearest) and its distance from the Sun is 147 million km. Similarly, the Earth is at its farthest point from the Sun on or about July 4. In this position the Earth is said to be aphelion and is at a distance of 152 million km. It should appear that there is summer in perihelion because the Earth is the closest to the sun. But reverse is the case. It is summer in aphelion and winter in perihelion. Earth takes about one year (365-1/4 days) in making one revolution of the Sun. As we take 365 days for a year, difference of 1/4 of a day requires adjustment. That is why we add one day to February after every four years.
 

Effect of the Revolution of the Earth

• Change of seasons
• Length of days and nights
• Shifting of wind belts

THE SEASONS
 

It has been made clear that the Earth revolves around the Sun with two characteristics:
(i) Its axis of rotation is inclined to the orbital plane at an angle of 66½ degree.
(ii) The northern end of the axis of rotation points towards the pole star wherever the Earth is in the orbital path.
There is one important effect of this type of revolution. The northern and southern hemispheres in turn are tilted towards the Sun while at two places both the hemispheres are equally inclined to the Sun.

Duration of Seasons
From the point of view of the Earth’s inclination, there are four positions of SOLSTICES and EQUINOXES. Hence, there are the following four seasons according to the positions of the Earth in one complete revolution of the Earth around the Sun.
(i) SUMMER SOLSTICE: On June 21, the northern hemisphere is ‘inclined towards’ the sun while the southern hemisphere is ‘inclined away’ from the sun. The Sun rays are vertical at 23½ degree north and the northern hemisphere receives sunlight for the longer part of the day. As a result the northern hemisphere becomes hot. The season is called SUMMER SEASON. In the southern hemisphere, the conditions are opposite to that in the northern hemisphere. It is winter season there. Nights are longer than days and the number of nights with a duration of 24 hours increase as we move farther towards South Pole.

(ii) AUTUMN EQUINOX: On September 23 the northern and southern hemisphere are equally inclined towards the Sun. The Sun rays are vertical at Equator. As a result, the season is neither hot nor cold. It is a situation between summer and winter seasons. It is called AUTUMN SEASON. In the southern hemisphere similar conditions prevail except that the transition is from winter to summer.
 

(iii) WINTER SOLSTICE : On December 22, the conditions are just like those on June 21 except that the southern hemisphere is ‘tilted towards’ the sun and the northern hemisphere is ‘away from’ the sun. The Sun is vertical at 23½ degree South, on the line of Capricorn. It is winter season in the Northern hemisphere and summer season in the Southern hemisphere.
 

(IV) SPRING EQUINOX: On March 21, the northern and southern hemisphere is equally inclined towards the Sun. The conditions are similar to that of autumn equinox. From March 21 to June 21 for a total of 93 days, the Earth is moving on its path round the sun so that the Sun gradually appears to move from the Equator to its northern limits. During this period there is SPRING SEASON in the Northern hemisphere and AUTUMN SEASON in the Southern hemisphere. Between March 21 and September 22, the North Pole enjoys a 6 month long day. The length of the day and night the area between the pole and the Arctic Circle varies according to the distance from the pole. For the next six months there is night at the North Pole and the South Pole is having the same period of daylight.

It is this traveling of the Earth on the Ecliptic that makes the seasons and put things into circulation.
 

Phenomenon of Midnight Sun
The midnight sun is a natural phenomenon occurring in summer months at latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight. Given fair weather, the sun is visible for a continuous 24 hours, mostly north of the Arctic Circle and south of the Antarctic Circle. The number of days per year with potential midnight sun increases the farther poleward one goes.
There are no permanent human settlements south of the Antarctic Circle, so the countries and territories whose populations experience it are limited to the ones crossed by the Arctic Circle, i.e. Canada, USA (Alaska), Denmark (Greenland) Norway, Swede, Finland, Russia and extremities of Iceland. A quarter of Finland’s territory lies north of the Arctic Circle and at the country's northernmost point the sun does not set for 73 days during summer. In Svalbard, Norway, the northernmost inhabited region of Europe, there is no sunset from approximately 19 April to 23 August. The extreme sites are the poles where the sun can be continuously visible for a half year.
Phenomenon of Polar Night
The opposite to the phenomenon of midnight sun is polar night that occurs in winter when the sun stays below the horizon throughout the day.

TIME ON THE EARTH

 

SOLAR AND SIDEREAL TIME
The system in which we measure time is based on the concept of the solar day. It can be defined as the average time period required for the successive passages of the sun over a given meridian, and it is exactly 24 hours. This is known as the mean solar day. On the other hand, the time required for a 360 rotation of the earth, causing a given star in the sky to return to the same position in relation to the earth is known as the Sidereal day, and this period is about four minutes less than the mean solar day.
 

Time Zones
It is very inconvenient to use local time for every different meridian. The solution therefore is to fix the time for a meridian passing through the central part of a country and use the same for the whole of the country. This time is called Standard Time. In India, the standard time is the one, which is fixed for the whole country by taking the local time of the meridian of 82-1\2o E passing through Allahabad, which is situated roughly in the middle part of the country.
 

INTERNATIONAL DATE LINE
Halfway around the world from Greenwich is the International Date Line, at approximately 1800 longitude. It runs down the mid-Pacific Ocean, veering to keep all of the islands in a group, such as the Aleutians, together. Although the time is the same on both side of the line (it is in the centre of a time zone), it is one day earlier on the east side of the line than it is on the west side. Thus when it is Sunday just east of the line, it is Monday west of the line.

• In India, the standard time is the one, which is fixed for the whole country by taking the local time of the meridian of 82-1\2o E passing through Allahabad, which is situated roughly in the middle part of the country.

Jet Lag
A state of exhaustion experienced by air travelers when they cross several time zones in a relatively short time as when one flies from India to U.S.A. The condition arises from disturbances in the body's habitual rhythms of 'internal clock' by suddenly requiring it to adapt to a strange new 24-hour time-scale. Time zones are imaginary lines that run from north to south on the map. These help standardize time throughout the world since it is only possible to cross them by travelling from east to west or visa versa, jet lag is, therefore, experienced only when long distances are travelled along the east-west axis. Jet lag generally includes a feeling of fatigue, hunger at unexpected times, irregular bowel and bladder movements. The only way to combat it is adequate rest at the end of the flight.

THE EARTH IN SPACE
 

Life did not come into being with the birth of the Earth. Scientists agree that the life of the Earth may range from 3 to 5 billion years. The Earth cooled from its gaseous state into liquid one. It later covered itself with a solid layer on further cooling. The formation of its atmosphere must have simultaneously taken place. Life began in the sea in the form of one cellular organism which belonged to both animal and vegetation kingdom. Later, the animals and vegetations both evolved into more complex life forms.
The age of the Earth.... 5,000,000,000 years (Approximately)
The dawn of life on Earth..... 500,000,000 years (Approximately)
Appearance of Man on Earth....500,000 years (Approximately)
 

Basic Planetary Data
The mean distance of the Earth from the Sun is approximately 1.5 X 108 kilometres. The planet orbits the Sun in a path that is presently more nearly a circle than are the orbits of most other planets. The direction of the Earth’s revolution—counterclockwise as viewed down from the north—is in the same sense (direction) as the rotation of the Sun; the Earth’s spin, or rotation about its axis, is also in the same “direction” sense i.e. west to east or anti-clockwise. The length of a day (23 hours, 56 minutes, and 4 seconds) is typical of other planetary objects; Jupiter and most asteroids have days less than half as long, while Mercury and Venus have days more nearly comparable with their orbital periods. The tilt (inclination) of the Earth’s axis to its orbit (23.5º), also typical, is responsible for the change of seasons.
Compared with the other eight planets of the solar system, the Earth is relatively small. Although it is the largest of the inner planets, it is considerably smaller than the gas giants of the outer solar system. The Earth has a single satellite, the Moon. The Moon is one of the bigger natural satellites in the solar system and is in fact relatively large compared with the Earth itself. Some people consider the Earth-Moon system a double planet, with some similarities to the Pluto-Charon system.
The Earth’s gravitational field is manifested as the attractive force acting upon a free body at rest, causing it to accelerate in the general direction of the centre of the planet. Departures from the spherical shape and the effect of planetary rotation cause gravity to vary with latitude over the terrestrial surface. The average gravitational field at sea level is about 9.80 m/s2, although values range from about 9.78 m/s2 at the Equator to about 9.83 m/s2 at the poles. So, the gravitational pull is essentially higher at poles than at equator due to proximity to the core.
Gravity typically is not measured at sea level. So corrections must be made for its decrease in value with increasing elevation. Such height-related gravity anomalies may be corrected for by using free-air or Bouguer reductions. In the Bouguer reduction, the effect of the attraction of the additional mass located above sea level is taken into account, while in the free-air reduction this mass effect is ignored. The Bouguer anomaly can be used to indicate variations of density within the Earth by measuring the corresponding variation in gravity.
The Earth’s gravity keeps the Moon in its orbit around the planet and also generates tides in the body of the Moon. Such deformations are manifested in the form of slight bulges at the lunar surface, detectable only by sensitive instruments. The Moon, owing to its relatively large mass, exerts a gravitational force that likewise causes tides on the Earth. These are most readily observable as the daily rises and falls of the ocean water, although tidal deformations occur in the solid Earth as well as in its atmosphere.

Some Important Facts about the Earth
1. Age- 5000 million years
2. Total surface area- 51 x 107 km2
3. Land area- 29.2% of the total surface area
4. Water area- 70.8% of the total surface area
5. Highest land point- 8848 metres (Mt. Everest)
6. Lowest land point- 397 metres below sea level (Dead Sea in Jordan)
7. Mean ocean depth -3554 metres
8. Mean land elevation -3800 metres
9. Average density -5.5 gm/cm3
10. Average temperature -15^oC
11. Lowest ocean point -11033 metres (Mariana Trench)
12. Difference between polar & equatorial diameters- 43 Kms
13. Difference between longitudinal &equatorial circumference -69 Kms
 

CONTINENTS