INTRODUCTION
- The Earth, the Planet on which we live, is the fifth largest planet and the third nearest to the sun.
- It is spherical in shape.This shape is called as a geoid.
- Earth is known as the “watery planet” or the “blue planet” due to presence of huge amount of water.
EARTH DATA
(a) Age:
At least 4.5 billion years.
(b) Motion:
- Rotation (spinning motion around an imaginary line connecting the
- North and South Poles) once every 23 hours, 56 minutes, 4.09 seconds.
- Revolution (motion around the sun)-once every 365 days, 6 hours, 9 minutes, 9.54 seconds.
(c) Size:
- Polar diameter (distance through the earth from North Pole to South Pole) - 12.713.54 kilometres.
- Equatorial diameter (distance through the earth at the equator )- 12,756.32 kilometres.
- Polar circumference (distance around the earth through the poles)- 40.008.00 kilometres.
- Equatorial circumference (distance around the earth along the equator)- 40,075.16 kilometres.
(d) Area:
- Total surface area - 509,700,000 square kilometres.
- Land area- approximately 148,400,000 square kilometres, about 29 percent of total surface area.
- Water area- approximately 361,300,000 square kilometres, about 71 percent of the total surface area.
(e) Surface features:
- Highest land - Mount Everest, 8,848 metres above sea level.
- Lowest land- Shore of Dead Sea, about 399 metres below sea level.
(f) Ocean depths:
- Deepest part of ocean- area of the Mariana
- Trench in Pacific Ocean southwest of Guam, 11,033 metres below surface.
- Average ocean depth - 3,730 metres.
(g) Temperature:
- Highest- 58°C at Al Aziziyah, Libya.
- Lowest - –89.60°C at Vostok Station in Anatarctica.
- Average surface temperature - 14°C.
(h) Atmosphere:
- More than 99 percent of the atmosphere is less than 80 kilometres above the earth’s surface
- , but particles of the atmosphere are 1,600 kilometres above the surface.
- (i) Chemical make- up of the earth’s crust (in percent of the crust’s weight):
- Oxygen 46.6; silicon 27.7; aluminium 8.1; iron 5.0; calcium 3.6;
- sodium 2.8; potassium 2.6; magnesium 2.0; and other elements totalling 1.6.
LOCATION
- Location is the geographic situation, or a point or position is space where objects,
- organisms and fields of force may be found or event occur.
- It is also the fact or condition of occupying a particular place.
- The mathematical system commonly used to describe location on
- the earth’s system is based on a series of imaginary lines drawn on a globe according to an agreed method.
- The geographic grid is a network of intersecting lines on the globe,
- based on two natural points- the North Pole and the South Pole.
Great Cicle :-
- The great circle is any hypothetical circle on earth’s surface,
- the plane of which passes through earth’s centre, cutting it into two equal halves, i.e. hemispheres.
- The Equator is a great circle. It divides the earth into the northern hemisphere and the southern hemisphere.
Small Circle:-
- A small circle is a hypothetical circle made by a plane passing through
- the globe anywhere except through the centre. It cuts the globe into unequal parts.
- The Tropic of Cancer at 23°30’N and the Tropic of Capricorn at 23°30’ S are small circles.
- The Arctic Circle is 66°32’N (often taken as 661/2°N) and the
- Antarctic Circle is 66°32’S (often taken as 66 1/2 °S).
- All parallels of latitude except the equator are small circles.
Meridians and Parallels:-
- The set of north- south lines connecting the poles are called meridians
- . All meridians are halves of great circles,
- containing 180° of arc, spaced farest at the equator and converging to common points at the poles.
- The set of lines running east-west are called the parallels. Parallels are equidistant circular lines
- , running parallel to the equator and to each other, intersecting the meridians at right angles.
- A meridian exists for any point selected on the globe as an infinite number of meridians may be drawn on a globe.
- Every point on the globe, except the North Pole and the South Pole,
- lies on a parallel, an infinite number of parallels may be drawn on a globe.
(a) Latitude:-
- Latitude is the angular distance of a place north or south of the equator.
- There are 180 parallels of latitude.
- Each parallel of latitude are not of equal length. The circle become smaller towards the people.
- The distance between any two parallels of latitude is always equal.
- The North Pole and the South Pole are fixed poins and serve as basic points of reference.
(b) Longitude:-
- Longitude is the angular distance of a place east or west of the prime meridian.
- There is are 360 meridian is a longitude of 0°C.
- The prime meridian is a longitude is a semi- circle.
- All meridians are of equal length.
- The distance between any two meridians is not equal. They get closer (converge) from the Equator to the poles.
LATITUDE AND TEMPERATURE ZONE
- As the earth is spherical, different parts of the earth tend to get heated to different degrees.
- The sun’s rays are concentrated more directly in the region round the equator
- . So the temperature is higher in these regions.
- So we move from the equator, due to the curvature of the earth,
- the sun’s rays strike the earth’s surface at an angle and are also spread over a larger area.
- So these regions do not heat to the same extent as do not equatorial regions.
- The temperature thus decreases from the equator to the poles.
- The three main heat ‘zones are shown in the diagram below.
DISTANCE
- It is useful to remember that the length of 1° of longitude is reduced by about one- half at the 60th parallels.
- The length of a degree of latitude is almost the same as the length of 1° of longitude at the equator, i.e., about 111km.
MOTION OF THE EARTH AND THEIR EFFECTS
The earth is not stationary. It rotates on its axis, revolves round the sun and moves,
along with the solar system around the galaxy, even as the galaxy in its entirety moves through the universe.
(a) Rotation:-
- It is the movement of the earth on its axis from the west to east, so that the stars,
- the sun and the moon appear to rise in the east and set in the west.
- Each rotation takes 23 hours 56 minutes and 4.09 seconds. Earth’s rotation results in
(A) causation of day and night;
(B) Deflection of ocean current and winds;
(C) Rise and fall of tides every day.
(b) Revolution:-
- It is earth’s motion in its elliptical orbit around the sun. Earth’s average orbital velocity is 29.79 km/sec.
- One revolution is completed in 365¼ days, resulting in one extra day every fourth year.
- Revolution of the earth results in:
(A) the change of seasons;
(B) variation in the length of day and night at different times of the year;
(C) differences in altitudes of the sun at noon, at different times of the year.
(d) Seasons:-
- Seasons are periods into which the year can be divided as a result of the climatic conditions,
- largely due to changes in the duration and intensity of solar radiation.
- Seasonal changes are caused by the inclination of earth’s axis to the ecliptic plane
- and because the axis constantly points towards the Pole Star.
- During the four seasons-summer, autumn, winter and spring- the position
- of earth vis-a-vis sun changes as it revolves.
(e) Solastice:-
Solastice is one of the two dates in the year on which the sun reaches its greatest altitude north or
South of the equator and is directly overhead along one of the lines of the tropics.
(i) Summer Solastice:-
- On June 21 or 22, the sun is overhead on the Tropic of Cancer (23½°S).
- The northern hemisphere is tipped towards the sun, having the longest day,
- while the southern hemisphere is tipped away from the sun, having the shortest day.
(ii) Winter Solstice:-
On December 21 or 22 the sun is overhead on the Tropic of Capricorn (23½°S),
resulting in the shortest day in the northern hemisphere.
(f) Equinoxes:-
- Two days in a year when day and night are equal throughout the world are equinoxes.
- The vernal equinox occurs on March 20 or 21,
- also experiences 12 hours daylight and 12 hours hemisphere, while the autumnal equinox occurs on September 22 or 23.
- On these two days, every place on the globe experiences 12 hours daylight and 12 hours darkness.
- The sun rises due east and sets due west, and is seen directly overhead on the equator.
(g) Midnight Sun:-
- It is a phenomenon, observable in latitudes 66½° North and South (or the Arctic and Antarctic circles respectively)
- where the sun does not sink below the horizon during summer.
- This results due to the tilt of the earth’s axis,
- each hemisphere being inclined towards the sun during its summer.
- The duration of the phenomenon increases towards the poles,
- where it may be observed for six months of each year.
TIME
To avoid confusion and make the study of time relation simple,
it is necessary to think of the earth as stationary and of the sun as completing one circuit about the earth every 24 hours.
(a) Time and Longitude:-
- One hour of time is equivalent of 15° of longitude.
- The earth completes one rotation on its axis in 24 hours,
- so in each hour, it covers 15° of longitude to cover 360° of longitude in total.
- This also means that 1° of longitude is covered every 4 minutes during the daily rotation of the earth.
- This equality forms the basis of all calculations concerning time belts of the globe.
(b) Local Time and Standard Time:-
- Local time is the time of the day at a place as indicated by the position of the sun, i.e., it is the mean solar time based on the local meridian.
- The standard time is the mean time of a particular meridian (generally a central one),
- adopted as a system of time for the entire country, and all clocks within this belt are set to a single time.
(c) Time Zones:-
- A time zone is an area in the world recording the same time.
- The Greenwich meridian at 0° longitude passes through
- Greenland Sea and Norwegian Sea, and the countries of the United Kingdom,
- France, Spain, Algeria, Mali, Burkina Faso and Ghana, and then through the South Atlantic Ocean.
THE MOON:EARTH'S ONLY SATELLITE
Salient features of the moon, the earth’s only satellite, are as follows:
- The moon is earth’s only satellite.
- The mean distance between the earth and the moon, is about 3,85000 km.
- Moon has a diameter of about 3,480 km and a mass of about that of the earth.
- The orbit of the moon is elliptical.
- The time taken by the moon to complete one revolution around the earth is 27 days, 7 hours,
- 43 minutes and 11½ seconds, or about 27days. (This period is called sidereal month).
- The period of moon’s revolution of the sun is 29.53 days on an average, and is called synodic month.
- When the moon is between the earth and the sun, the position is called the New Moon.
(a) Moon and Tides:-
- Tides are defined as slight oscillations of sea level
- that occur approximately twice a day and attain exaggerated proportions in marginal seas, straits and estuaries.
- The major cause of the tides is the gravitational pull of the moon and the sun.
- The highest level the water reaches is called a high tide and the lowest level is called a low tide.
(b) Eclipses:-
- The total or partial obscuration of light from a celestial body as it passes through the shadow of another body is known as eclipse.
- When both sun and moon are on the same side of the earth (i.e. in conjunction) so that, all three bodies
- lie approximately on a straight line, the possibility exists for an eclipse of the sun, or solar eclipse.
- When the moon and sun are on the opposite sides of the earth (i.e., in opposition),
- the possibility exist for an eclipse of the moon, or lunar eclipse.
- In this case the earth’s shadow falls on the moon partly or completely covering it for a short while.
- A solar eclipse occurs between sunrise and sunset at new moon; a lunar eclipse occurs at full moon.
