Brief introduction about the solar system & earth.

The solar system is the sun and all the objects that travel around it. The sun is the center of the solar system. Its mass is more then 750 times as great as that of all the planets combined. The huge mass of the sun creates the gravitation that keeps the other objects travelling around the sun in an orderly manner.

The sun continuously gives off energy in several forms-visible light; invisible infrared, ultraviolet, X, and gamma rays; flow of plasma, which becomes inter planetary plasma and drifts throughout the solar system is called the solar wind.

Astronomers do not have enough information to describe the formation of the solar system completely. Many ideas have been suggested, but parts of all of them been proved wrong.

Unitl the mid-1900s, theories of how the solar system was formed could be based on only five observations, which mentioned below;

1. The sun and most other parts of the solar system spin in the same direction of their axes (imaginary line drawn through their centers).

2. Most parts of the solar system travel around the sun in the same direction that sun spins.

3. Most satelites travel around their planets in the same direction that the planets travel around the sun.

4. Going outward from the sun, the distance between the orbits of the planets increases.

5. The solar system has a circular shape.

The theories that have been suggested to explain the above observations can be divided into two general groups-monistic theories and dualisitc theories. Most astrnomers believe that some form of monistic theory will someday be proved correct.

Monistic theories are based on belief that the solar system was formed from a single flat cloud of gas. According to some monistic theories, all parts of the solar system were formed from the gas at the same time. Other monistic theories suggest that the sun is formed first, and the planets and other objects came later from the remaining gas. The first monistic theory proposed in the early 1600's by French scientist and philospher Rene Decartes. In the 1700's Pierre Simon de Laplace, also of France, suggested the monistic theory called the nebular hypothesis.

Dualistic theories are based on the belief that the solar system was formed when some huge object passed near the sun. According to these theories, the force of gravity of the passing object was a large as a star. In the early 1900s, Thomas Chamerlin and Forest Moulton, both of the University of Chicago, offered a dualisitic theory called the planetesimal hypothesis.

The earth is considered smallest planet in the solar system. Geologists have tried to build up the history of the earth-covering the last 500,000,000 years of its life. The years represent the last quarter when life had separated on it; but the earlier 3/4 of its life, nothing can be guessed. Rocks that make the crust of the earth their formation, arrangements and distribution give us a useful clue to the time during which they were first formed. These rocks contain fossils which have been classified according to the times to which they belong. Each of the classes thus formed provides a landmark in the history of the earth, and gives us some idea about the age of the earth.

The most abundant elements in the earth's crust are:

1. Oxygen 49.85%

2. Silicon 26.03%

3. Alminium 7.23%

4. Iron 4.12%

5. Calcium 3.18%

6. Sodium 2.33%

If were possible to cut the earth into halves, you would see that the earth is imposed of a number of quite distinct layers. There layers are;

a) The crust, or lithosphere

b) The mantle, or shell

C) The outer core

d) The inner core.

The earth is an oblate spheroid and not a true sphere, because it is fkattened at the poles and bulges out at the equator. The earth is roughly round in shape, but it is not perfectly round as represented by the globes. It is slightly flattened at the North Pole and the South Pole, and bulges out slightly at the Equator. We called this is a geoid.

The diameter of the earth at the Equator is 12,746 km (7926 miles). Its diameter from the North Pole to the South Pole is slightly less: 12,704-5 km (97900 miles). The earth's circumference at the Equator is 40232-5 km (nearly 25000 miles). Knowing these distances, find out the distance of each degree of latitude, and the depth of the centre of the earth.

The shape of the earth may be proved by the following ways;

If we stand on the seashore, the first sign of a ship appearing over the horizon is the mast and then the hull.

The rising sun is first seen at places in the east and later at places farther west. If the earth was flat, the whole world would experience sun rise and sunset at the same time.

During an eclipse of the moon , the shadow cast by the earth on the moon has a circular outline.

Finally, photographs taken of the earth from outer space by man-made satelites show the earth's curved surface. These provides perhaps the most conclusive evidence of the earth's shape.

The crust is a cool and solid covering, forming the outermost layer of the earth. It is very thin compared with the earth's diameter, nearly 12,750 km. The thickness of the crust varies from place to place. Under the oceans, its average average thickness is about 9-7 km (6 miles), but under the continets the crust is about 19-3 km (12 miles) thick, reaching a maximum of 40-2 to 48-3 km (25-30 miles) under some high mountains.

The crust consists of two main kinds of rock. The base of the crust is formed by very dense basaltic rocks. The continents however, made of lighter granitic rocks, which seem to float above the denser basalts.

When we take the temperature in a very deep hole, for instance, in an artesian well or the shaft of a mine, we will find temperature increases as the hole gets deeper. The temperature rises by about 1 centigrade for every 33 meters. (1.8 fahrenheit for every 108 feet) of descent. At this rate of increases, the centre of the earth should be about 193,000 centigrade (347,400 fahrenheit). But many scientists do not think that it is so hot, probably only between 1944 centigrade (3500 fahrenheit) and 6093 centigrade (11000 fahrenheit). Still the rocks far beneath the crust are so hot that they should melt or turn to gas, but the great weight and pressure of the outer layers prevent the core from becoming a liquid or gas. The rocks there are believed to be mostly solid though slightly 'waxy' or plastic.

The mantle lies just below the earth's crust. It is about 2900 kilometers (1800 miles) deep, and is believed to consist mainly of solid rocks by which resemble thick liquids. Most of the rocks are rich in olivine basalt, which is part of the lavas from volcanoes in the deep ocean basins.

The outer layer of the core is believed to be mainly liquid, consisting of molten iron nearly 2250 kilometres (1400 miles) thick. The inner core however is believed to be a solid mass of iron and nickel, about 2400 kilometres (1500 miles) in diameter.