(GK) THE WORLD : The Earth and the Solar System

GK : THE WORLD : The Earth and the Solar System

The Earth is a member of the Solar System. It is one of nine major planets revolving round the Sun. Of these, Mercury and Venus are nearer, and Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are farther away from the Sun than the Earth. The planets radiate no light of their own, but shine with that reflected from the Sun. The Sun has a diameter of 864,000 miles (1,390,000 Kms) and it is 93,000,000 miles (150,000,000 Kms) from the Earth. 

The Sun is a star. The stars, unlike the planets, are self-luminous bodies. The other stars appear small because they are so far away; the nearest star, Proxima Centauri, is 175,000 times more distant from us than the Sun. The Moon is a dead planet. It is about 240,000 miles (386,000 Kms) from the Earth. The Moon revolves round the Earth taking approximately 29 days to complete one round. 

The phases of the Moon are the result of its position in relation to the Earth and the Sun. The Moon’s orbit is ecliptical and inclined at an angle of 5o to the plane of the Earth’s orbit. This explains why we do not have a total eclipse of the Sun every time there is a new moon. 

The Earth:
The Earth is a sphere but it is not a perfect sphere. It is slightly flattened at the poles and bulges at the equator. The circumference of the earth is approximately 25,000 miles (40,000 Kms). It rotates on its axis once in every 24 hours, spinning from west to east. Besides spinning on its axis, it also moves round the Sun, called the revolution. Its orbit round the Sun is oval or ecliptical. 

The time taken to complete one revolution is approximately 365¼ days or one year. For convenience, one year is taken as 365 days and the shortfall of ¼ day each year is made good in the Leap Year which consists of 366 days. The Earth’s axis inclined to the plane of its orbit at an angle of 66½^O.

The seasons are due to the change of the Earth’s position in the course of its revolution about the Sun, and to the inclination of its axis. The Equator is an imaginary line drawn round the Earth midway between the Poles. There are two other lines, namely, Tropic of Cancer (23½O N) and the Tropic of Capricon (23½O S). The word tropic means, ‘turning place’. 

The inclination of the Earth’s axis together with its revolution round the Sun is the cause of the varying length of day and night in different parts of the world. On March 21 (Vernal Equinox) and September 23 (Autumnal Equinox) the Sun is overhead at the Equator. On these dates, except at the Poles, (a) days and nights are equal all over the world; and (b) the Sun rises exactly due east and set exactly due west at all places on the Earth’s surface. At the Equator itself days and nights are equal throughout the year. Between March 21 and September 23, when the North Pole is tilted towards the Sun, the days are longer than the nights throughout the Northern Hemisphere and there is continuous daylight at the North Pole. Similar conditions are experienced in the Southern Hemisphere and the South Pole between September 25 and March 21. 

Latitude and Longitude:
Latitude is distance, measured in degrees, north or south of the Equator. Longitude is distance, measured in degrees, east or west of any fixed meridian. The meridian passing through Greenwich is numbered 0O. On a globe the meridians are numbered from 0O to 180O E (East) or W (West). At the equator the degrees are 69 to 70 miles apart (25000÷360). Since earth completes one rotation on its axis in 24 hours, 360 meridians pass under the Sun in that time. Therefore, 1 degree passes under the Sun every 4 minutes.

International Date Line:
If we travelled westward to a place X on longitude 180^o W, the time there would be 12 hours behind Greenwich time (180 x 4 minutes = 720 minutes = 12 hours). If we journeyed eastward to a place Y on longitude 180o E, the time there would be 12 hours ahead of Greenwich time. Thus X and Y both on 180o have the same time but differ in date by a day (12 hours + 12 hours = 24 hours). To overcome the confusion that would otherwise arise, the International Date Line has been established. 

It runs along 180o E or W. Westward-bound vessels crossing the Date Line drop a day from the calendar, while those going eastward add a day by giving the same date to two consecutive days. Instead of changing the time exactly according to change in degrees at the rate of 4 minutes per degree, certain time zones have been established. All places in the same area or time zone or time belt, use what is called Standard Time. Thus we have the Greenwich Mean Time (G.M.T) and the Indian Standard Time (I.S.T). There are five time-belts in Canada and four in United States.

The Lithosphere:
The mass of the Earth is generally divided into three layers, namely, Crust, Mantle and Core. The Lithosphere is the name given to the outer Crust which is not more than 10 miles thick. It is made up of a great variety of rocks, soils, etc.

Rocks:
1. Sedimentary Rocks:These rocks are made up of deposits laid down on the floor of river beds, lakes and seas. Examples:Sand and sandstone, clay, lime stone, chalk and carbonaceous rocks, such as lignite, coal and anthracite.

2. Igneous Rocks:These are primary rocks which are formed by cooling and solidification of molten lava. When such rocks are poured out on the surface they are known as Volcanic rocks, e.g. basalt. When the molten material solidifies at considerable depth, plutonic rocks are formed, e.g. granite. 

3. Metamorphic Rocks:These rocks are formed as a result of alteration by extreme heat and or pressure of igneous or sedimentary rocks. Example, slate, gneiss, schist etc.

Soil:
The upper layers of rocks weather to form the soil. There are three distinct layers of soil. The uppermost layer forms the top soil. The second layer is called the subsoil. The third layer is made up of decomposing and much-broken rock, known as mantle- rock. The type of soil depends on a number of factors, namely, climatic conditions, the nature of the parent rock, relief, vegetation and the period over which it has been worked by man. Soils may by broadly classified as (a) Forest, (b) Grassland and (c) Desert types.

Mountains:
In past geological ages disturbances in the Earth’s interior have caused crumpling and cracking of the crust. This has resulted in great upholds forming Fold- Mountains which are mainly made up of folded strata of sedimentary rocks, e.g. the Alps, the Rockies, the Andes, and the Himalayas. The mountain structures worn down by prolonged denudation are known as Residual Mountains, e.g. Highlands of Scotland and Scandinavia.

Denudation:
The process known as denudation or the wearing away of the land is continually going on. The chief causes of such erosion are (a) changes in temperature; (b) frost; (c) winds; (d) water, including rivers; (e) ice; and (f) the ction of the sea. Steps to combat soil erosion include (i)  terracing; (ii) contour ploughing; (iii) strip cropping (iv) planting shelter belts of trees; and (v) plugging the gullies by building small dams etc.

The Atmosphere:
The air is composed mainly of nitrogen (78%) and oxygen (21%) with small proportions of carbon dioxide, water vapour and rarer gases like argon and neon. Atmosphere is 175 miles thick, but nine-tenths of the air composing it is found within 12 miles, and half within 3½ miles of the earth’s surface. We are concerned mainly with the lower layer of troposphere. The upper layers in the ascending order are Stratosphere, Mesosphere and Ionosphere. Troposphere extends to a distance of about ten kilometres. 

Stratosphere is a region extending from an altitude of about 11 Km to 50 Km above the earth. The upper part of stratosphere has plenty of ozone which protects us from the fatal effects of Sun’s ultraviolet radiation. Mesosphere is the next layer extending from 50 o 80 Kms above the earth. It is a very cold region.  Ionosphere extends from about 60 Kms upwards. It includes Thermosphere and Exosphere which marks the outer limits of the earth’s atmosphere.

Wind is air in motion:
The chief cause of wind is difference in atmosphere pressure. One of the main reasons for differences in pressure is unequal heating of the air. From the high pressure belts the air flows outwards to the regions of low pressure. Owing to the rotation of the earth, the winds do not blow due north and south, but are deflected. In this deflection they obey Ferrel’s Law which states, “Any moving body on the earth surface including a current of air, tends to be deflected, the deflection being to the right in the northern hemisphere and to the left in southern hemisphere.” Land and seabreezed are local winds caused by the unequal heating of land and water. During the day the land becomes very much hotter than the sea, with the result that there is marked low pressure over the land. Thus the air over the sea flows rapidly loses heat, but the sea remains warm for a longer time. 

Thus at night, heavy cool air blows from the land to take the place of warm air rising over the sea. The monsoon or seasonal winds may be regarded as land and sea breezes on a large scale, in which the time-frame is a year instead of a day. This phenomenon is to be found in south-east Asia, but is especially marked in the subcontinent of India. A cyclone is a portion of the atmosphere in which the pressure is lowest in the centre. The winds blow inwards in anticlockwise direction in the Northern Hemisphere. In the Southern Hemisphere cyclonic winds blow in a clockwise direction in accordance with Ferrel’s Law. An anticyclone is a portion of the atmosphere in which the pressure is highest in the centre. The winds blow outwards in a clockwise direction in the Northern Hemisphere and in an anti-clockwise direction in the Southern Hemisphere.

The Oceans:
It is estimated that 72% of the surface of the globe is covered with water. The Pacific, which is the greatest of all oceans, covers a third of the earth’s surface, its total area being greater than that of all the dry land. Atlantic is slightly less than half the size of the Pacific, yet so many great rivers flow into it that it receives half the drainage of the world. The other oceans are Indian, Mediterranean, Antarctic and Arctic. The average depth of the ocean is 12,500 feet, compared with the average height of the land which is about 2,500 feet. The greatest known depth is that of the Marianas Trench in the Pacific, where a depth of 35,800 feet has been recorded.

Tides:
Tides are caused by the gravitational attraction the ‘pull’ of the moon on the earth, sometimes assisted by and sometimes hindered by the Sun.At new and full moon, when the Sun, Moon and Earth are practically in a straight line the attractive force of the Sun increases that of the Moon. Such high tides are called spring tides.

Weather and Climates:
Weather may be defined as the condition of the atmosphere at any place at a particular time. The average weather conditions determine the climate. Humidity, temperature, elevation, distance from the sea, ocean currents, atmosphere pressure and prevailing winds combine to affect the climate of a region.

Humidity:
Humidity, the term used to express the dampness of the atmosphere, is due to the pressure of water vapour. Evaporation is the mean by which water is drawn off as invisible water vapour from oceans, lakes, rivers, etc., when the air contains as much water vapour as it can hold is said to be saturated. When saturated air is cooled, condensation takes place and some of the water which form the clouds grow larger, they precipitate i.e., fall to the earth as rain. The proportion of water vapour in the air, compared with the maximum it can hold at the same temperature, is known as Relative Humidity. Dew is caused by the condensation of water vapour on the cold ground during the night. 

The temperature at which such condensation takes place is called the Dew Point. When condensation takes place near the surface (and not on the ground) the result is usually mist or fog. When condensation takes place at some distance from the ground, clouds are formed. When water vapour is condensed at a temperature below freezing point it forms snow. There are various kinds of clouds. The layer like clouds often seen on the horizon at sunrise and sunset are called stratus clouds. The light wispy clouds formed high in the sky are called cirrus clouds. The heaped up clouds, looking rather like masses of cotton wool are called cumulus clouds. The black rain clouds are known as nimbus.

Temperature:
Of all climatic factors, temperature is the most important. It affects man’s food, crops, dress and the type of dwelling he builds. Temperature decreases 1^oF for every 300 feet above the sea-level. This decrease is largely due to the fact that the rarified air, found in elevated regions, absorbs less heat than the denser air at lower levels.