What Is the Earth as a Planet?

Earth-The space ship: Artificial spaceships are man-made satellites, spacestations and sky-labs which are launched into the space, where they whirl around the earth. But the earth itself is the natural spaceship like the countless number of celestial bodies, which rotate about in the limitless space. There are more than three thousand artificial satellites orbiting the earth in the outer-space. So also there are about millions / billions of heavenly bodies like galaxies, stars, planets with their natural satellites etc., which revolve in definite orbits in the boundless space- the universe. The earth is one of them, which along with its satellite i.e. our moon and the co-planets orbit the sun since eternity guided by Newtonian universal law of gravitation. While the artificial space crafts, (the man made satellites) sail through the vast ocean of space around the earth in remote controlled orbits, the natural spaceship, the Earth does so in natural and definite orbit around the sun. Hence, the earth can be called a natural spaceship.

Earth’s place in space: The lovely planet, we inhabit, is more or less a huge spherical body of revolution, which is a tiny speck in the vast universe. The following analogy may be cited to give an idea about the size of the earth in relation to the universe. If all the seashores of this globe represent the universe, any sea-shore singularly will be a galaxy, a handful of sands of a seashore will show the size of the solar system and the earth in it will be represented by a tiny

The Universe (The Cosmos) and its birth: Universe means the unimaginably vast space which is unbounded with its dots of bodies around. By definition, the universe includes everything that abounds in space. This space is filled with matter which varies from the tiniest and invisible cosmic particle to the gigantic galaxies. The universe is known to be composed of many types of comparatively small and large dense bodies e.g., the galaxies, the constellations (star groups), the stars, the planets and other heavenly bodies. The totality of the heavenly bodies from grand galaxies down to the smallest cosmic dust filling the endless space describes the Universe whose dimension is beyond our comprehension. Astrophysicists are of the opinion that the universe is ever expanding and hence growing rapidly. There are two divergent theories on the mode of origin of the universe. These are the Big Bang and the Steady State theories. According to ‘Big Bang’ theory the universe has a finite beginning, through a tremendous explosive expansion of superdense and very condensed matter. The ‘steady state’ theory states that the universe expands through a process of continuous creation of matter which (the process) remains unchanged through eternity.

.Galaxy: A galaxy is a rotating stellar system (star system) consisting of a swarm of stars, which are held together by mutual gravitational attraction. In other words, a galaxy is an enormously large cluster of stars. Our galaxy to which the sun along with its planet families belong is an aggregate of about one hundred billion stars. Ours is called “Milky Way” galaxy which looks like a discus (a discshaped metal used in discus throw) when viewed through powerful optical telescope. It is a pancake shaped whirling body, which is gigantic in size. A part of the “Milky way” is clearly seen with naked eye as a faint luminous broad band

across the sky during a clear autumn night. It takes about 100,000 years for the light to go from one end of our galaxy to the other end. There are billions of galaxies in the universe. The nearest galaxy comparable to our Milky Way in size is the Andromeda galaxy.

Stars and Planets: A star is a self-luminous and incandescent globe of hot gases. Stars produce their own light and are seen twinkling in the sky. The stars vary in size, temperature and constituent materials. Many stars occur in pairs (twins). They may even occur in triplets or quadruplets. Most of the stars have their own systems of planets and satellites revolving round them. In such cases, a star constitutes a star system. Our sun is a star having its own planetary and satellite system called the “solar system”. Planets and satellites are non-luminous heavenly bodies, which shine only because of reflection of star (sun) light from their surface. The planets whirl round their stars in definite orbits.

The earth in the solar system

(i) The Solar system: Our sun is a star and hence has its star system known as the solar system. Everything within the sun’s gravitational control constitutes what is called our solar system. This sun-centered system (Fig. 1.2) comprises of the sun, eight planets (our Earth included), dwarf planets (Pluto), sixty one natural satellites called the Moons, about thousands of asteroids (also referred to as planetoids), scores of comets and uncounted millions of meteors, all of which revolve round the sun.

(ii) The Sun: The sun is just a member among the 136 billion stars of the Milky Way galaxy. Our sun is a relatively small star. Epsilon Aurigie, the largest star known so far in our galaxy is about 27 × 107 times as big as the Sun. Our sun is a huge sphere of hot gaseous mass. The elements present are in form of burning gases and vapours. The major elements, which make up the bulk

of the Sun are hydrogen and helium. It emits radiant energy in the form of heat and light and this energy is generated within it by conversion of hydrogen to helium through nuclear reaction. The temperatures at the surface and at the centre of the sun are 6000° C and 20,000,000° C (20 million degree Celsius) respectively. Its diameter is about 190 times greater than the earth’s diameter.

(iii) The Planets: The name planet evolved from the Greek word “Planets” meaning wandering. Planets are, thus, heavenly bodies, which appear to wander or move about the sky, whereas the stars are seemingly fixed in their relative positions in the sky. The sun has eight planets, which revolve round it in elliptical orbits and also rotate about their own axes. The eight planets and the dwarf planet Pluto as arranged in the order of their increasing distances from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto (Fig. 1.2). The nine planets fall into the following two groups, which vary so much in

(i) The inner group of planets which include Mercury, Venus, Earth and Mars. (ii) The outer group of planets which include Jupiter, Saturn, Uranus, Neptune and Pluto.

The inner planets are small in size, fairly dense (higher average density), hotter in comparison with the outer planets, which are larger in size (except Pluto which is a tiny dwarf planet), lighter with low average density and cooler. The outer group contains much of light gases of hydrogen and helium, whereas the planets of the inner groups are almost devoid of light gases. The inner planets, being hotter and of weak gravitational field, lack lighter gases. In general, the planetary motions show systematic uniformity. The orbits of all planets, except Pluto and Mercury lie in one plane. All planets revolve round the sun in elliptical orbits and in the same direction from west to east except, Uranus which rotates east to west. Mercury is the tiniest planet and Jupiter is the largest. In respect of size, the Earth ranks fifth. The order of the planets based on their increasing sizes would be Pluto, Mercury, Mars, Venus, Earth, Uranus, Neptune, Saturn and Jupiter (Fig. 1.2). The Earth is the densest planet of the solar system and Saturn is the lightest planet having the lowest density.

Three of the inner planets i.e. Earth, Venus and Mars are known to have definite atmosphere in some form. The astrophysicists are of the opinion that life may possibly exist on Mars that shows markings of dried river channels etc.

Among the outer planets, Jupiter and Saturn are known to have cold and dense atmosphere, which is rich in methane and ammonia gases. The two planets commonly seen in the sky in naked eyes are Venus and Mars. Venus is the brightest of all planets and clearly visible during sunrise and sunset. Mars looks reddish in colour. The numerical data of the planets of solar system are given in Table 1.1.

The Sun is 1.6 million km in diameter, 13 times larger than Jupiter, the largest planet.

(iv) Asteroids: Also known as planetoids, these are a group of smaller bodies of the system found in between the inner and the outer planets (between Mars and Jupiter). These are thought to be remnants of a planet, which broke into pieces by collision with another bigger heavenly body. The asteroids vary in size (diameter) from 2 km to over 800 km. There are about 1500 asteroid bodies.

(v) Comets: Comets are wandering members of the solar-system. They move around the sun in highly eccentric orbits. Comet derives its name from the Greek word ‘Kometes’ meaning long-haired. Comets are objects that go around the sun with regular periodicity. Some comets are known to reappear to our vision at predictable intervals. The famous “Halley’s comet” appears after every 76 years. It appeared last in 1986 and shall reappear again in 2062.

(vi) Meteors: Meteors are heavenly objects, which move about in space within the solar system in definite courses. They greatly vary in size, shape and composition. During their flight in space, the meteors may come within the earth’s gravitational field. As they fall through the earth’s atmosphere, they get burnt up by frictional heat produced and emit flash of light. Many of them may burn away in ionosphere (an electrically charged layer of the atmosphere) to yield ashes and dusts, while some of them may survive total burning and fall on the earth’s surface in the form of solid bodies called “Meteorites”. The world’s largest meteorite that fell on southwest Africa weighs 70 tonnes. Meteorites vary in composition.

It is surmised that the meteorites represent either broken parts of comets or pieces of asteroids. Meteorites provide us with information about the composition of the earth’s interior.

(vii) Satellites: Satellites (otherwise called the moons) are heavenly bodies, which revolve round the planets. Like the planets, the satellites do not themselves

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