THE PLANETS

THE PLANETS

The bodies revolving around the sun (at the same time rotating on their imaginary axis) are called planets. They have no light of their own but shine by radiating the fight they receive from the sun. They all revolve around the sun in elliptical orbits. Until about 200 years ago only six planets were known. Three more planets were discovered later, the latest being Pluto (discovered in 1930). Nine planets can now be identified. Mercury Mercury is the planet nearest to the sun. It rotates on its own axis in 56.65 earth days. It takes 88 days to complete one revolution round the sun. Thus it is the fastest planet in our solar system. Venus Also known as the evening star and morning star, is the brightest object in the sky after the sun and the moon. It is slightly smaller than the earth and is the planet closest to the earth. It is also the hottest planet in our solar system and has a weak magnetic belt. Mars Mars is the fourth planet from the sun and is the next planet after the earth. Being favorably situated, it is brighter than most of the stars and, is therefore, known as the Red Planet. It has two small satellites called Phobos (Fear) and Deimos (Terror). Jupiter Jupiter is the largest planet in our solar system. It is about eleven times larger than the earth. Its volume is one and half times the volume of all the planets combined together. The most conspicuous aspect about Jupiter is its Great Red Spot. It is also known as the giant planet because of its huge size. Saturn Saturn is an outer planet visible to the naked eye. Second in size to Jupiter, it is the least dense of all the planets. The most spectacular feature of Saturn is its system of rings. The ring system is made up of a variety of separate particles which move independently in circular orbits. It has 46 satellites. Titan is its biggest. Uranus Uranus is the seventh planet from the sun and is not visible to the naked eye. It was identified as a planet in 1781 by William Herchel. It has completed only two revolutions round the sun since its discovery, and takes about 84 terrestrial years to circle round the sun. It has 27 satellites. Neptune Neptune is not visible to the naked eye but can be seen through a small telescope as a greenish star. It is eighth in position from the sun. This planet was discovered by J.G. Galle of Berlin in 1846. Till 1930, it was believed to be the farthest planet from the sun and the outermost in our solar system. It has eight satellites, and Triton and Nereid are the most con­spicuous of them. Pluto Pluto is the youngest planet to be discovered in our solar system. It was discovered photographically by C.W. Tombaugh (USA) in 1930. It is the smallest planet in our solar system; slightly smaller than Mercury and visible only through a tele­scope. The duration of its revolution round the sun is the longest and it is, therefore, the slowest planet in our solar system.

SATELLITES

Satellite are bodies which revolve around the planets. All planets have one or more satellites, except Mercury and Venus. The moon is the earth's natural satellite. There are approximately 62 satel­lites in our solar system. In August 1989, the US Space probes Voyager-1 and Voyager-2 revealed six new satellites around Neptune which was earlier believed to have only two satellites. The Moon The moon is the earth's natural satellite and is its nearest neighbour in space. It revolves around the earth while rotating on its own axis. Only 59% of its surface is directly visible from the earth. Of all satellites in the solar system, the moon is the largest in proportion to its primary body, that is, the earth. All other satellites have sizes below 1/8 the size of the mother planet. The moon is about 1/4 the size of its mother planet, the earth. It takes about 1.3 seconds for moonlight to reach the earth, whereas sunlight takes about 8 minutes and 16.6 seconds to reach the earth. The moon takes 27 days 7 hours 43 minutes and 11.47 seconds to complete one revolution of the earth. It rotates on its axis in exactly the same time. Hence, we see only one side of the moon.

THE EARTH

Modern theories on the formation of the Earth and other planets are of course based on the Copernican theory. The age of the Earth was a matter of speculation till very recent times. It was only about 200 years ago, that scientific enquiries were started by geologists. According to their deductions, based on the study of rocks, the age of the Earth is 4.6 billion years. Our knowledge of the internal structure of the Earth is derived from studies of earthquakes. The shock waves sent out by an earthquake indicate the physical nature of the regions through which they pass. These studies show that the centre of the Earth is a solid core–the Inner Core. The density of this core is about 13 g to the cubic centimeter. The Inner Core is about 1,370 km thick and is surrounded by an Outer Core of around 2,080 km. The Outer Core appears to be molten. The Outer Core is surrounded by the Mantle which has a thickness of around 2,900 km. The Mantle is topped by the crust of the Earth, which varies widely in thickness–from 12 to 60 km. At the centre or the Inner Core, that is at a depth of some 6,370 km, temperature goes upto some 4,000°C and pressure reaches nearly 4 million at mospheres. The mantle is important in many ways. It accounts for nearly half the radius of the Earth (2,900 km), 83% of its volume and 67% of its mass. The dynamic processes which determine the movements of the crust plates are powered by the mantle. Starting at an average depth of from 45 to 56 km below the top surface of the Earth, the mantle continues to a depth of 2,900 km where it joins the outer core. The mantle is a shell of red hot rock and separates the Earth's metallic and partly melted core (both the inner and the outer cores) from the cooler rocks of the Earth's crust. It is composed of sllicate minerals rich in magnesium and Iron. The density of the mantle in­creases with depth from about 3.5 gram per cubic centimetre to around 5.5 gram, near the outer core. The outer surface of the Earth is divided into 4 spheres: Lithosphere means the entire top crust of the Earth and includes not only the land surface but also the ocean floor. Hydrosphere is the water sur­face which includes the oceans, lakes and rivers. Atmosphere is the blanket of air that envelops the Earth. It covers both the land surface and the water surface. Biosphere is this sphere of life which spreads over all the three other spheres. Earth's Movements The earth has two types of move­ments, viz. rotation or daily motion and revolution or annual motion. The earth spins on its own im­aginary axis from west to east once in 24 h (in precisely 23 h 56 min and 40.91 s). It is also called diurnal or daily motion. The axis is an imaginary line which runs form north to south and passes through the centre of the earth. It always remains in­clined at an angle of 66½° to the plane of the earth's orbit. Effects of Rotation: (i) Occurrence of day and night. (ii) The position of a place on earth can be fixed. (iii) Change in the direction of wind and ocean currents.

ECLIPSES

When the light of the sun or the moon is obscured by another body the sun or moon is said to be in eclipse. Lunar Eclipse: The moon is said to be in eclipse when the earth comes between the moon and the sun, and this is called Lunar eclipse. The shadow cast by the earth on the moon is called an eclipse. Lunar eclipse occurs only on a full moon day. However, it does not occur on every full moon day because the moon is not in the same position in relation to the earth and the sun on every full moon day. Solar Eclipse: The sun is said- to be in eclipse when the moon comes between the sun and the earth. This is called Solar eclipse. There is either a partial or total obstruction of the sun's light when viewed from the earth. A solar eclipse occurs on a new moon day when the moon is in line with the sun. However, due to the inclination of the moon's orbit, a solar eclipse does not occur on every new moon day.

ATMOSPHERE

The atmosphere is a gaseous en­velope that surrounds a celestial body. The terrestrial atmosphere, by nature of its composition, control of temperature and shielding effect against solar radiation, makes life possible on earth. It covers both the land and the water surface. It is bound to the earth by the gravitational pull of the earth. The composition of the atmosphere changes as we go higher from the earth's surface. Upto about a height of 50 km from the earth, the atmosphere is composed of: Nitrogen 78.09% Oxygen 20.95% Argon 0.93% Minor gases (Carbon dioxide, hydrogen, neon, helium, methane, xenon, krypton, etc.) 0.03% After a height of 50 km above the earth's surface the atmosphere is made up of atomic oxygen (O2), ozone (O3), helium and hydrogen. Atmospheric Layers These are the layers of air that lie above the earth's surface. The atmosphere of the earth is arranged into layers as accrued below, viz. Troposphere: The troposphere is the layer nearest to the earth's surface and extends from sea-level to a height of about 15 km. This region is the densest of all the atmospheric layers and contains water vapour, moisture and dust. In this region the temperature decreases as the height increases from the earth. Tropopause: Tropopause is the layer which separates the troposphere (lowest layer) from the stratosphere (upper layer). Stratosphere: This is the region of uniform temperature extending from an altitude of about 15km above the earth to a height of about 50 kill. It is free from water vapour, clouds and dust. Mesosphere: This is a very cold region and lies above the ozone-rich layer of the stratosphere. It extends from 50 or 80 km above the earth's surface. Menopause: The Menopause separates the mesosphere from the next layer called the ionosphere. Ionosphere: The ionosphere lies immediately above the mesosphere and extends from 60 to 400 km above the earth's surface. This layer contains ionised (or electrically charged) air which protects the earth from the falling meteorites (shooting stars) as most of them burn out in this region. It also protects the earth from the harmful radiations of the sun. The ionosphere consists of ‘D’, ‘E’ and ‘F’ layers and includes the thermosphere and exosphere. Thermosphere: This is the middle layer of the ionosphere. It is the region of the atmosphere where the temperature is above 100°C. Exosphere: The exosphere is the uppermost region of the ionosphere and makes up the outer limits of the atmosphere. Here the gravity of the earth is exceedingly weak. The magnetic belt of the earth which is known as Magnetosphere, extends to about 64,000 km above the earth's surface. The exosphere is now considered as part of the magnetosphere. The outer boundary of the magnetosphere or the final boundary between the earth and outer space is known its magnetopause. The land surface of the earth is made up of immense land masses divided into seven continents and a great number of islands. Together, they cover about one quarter of the earth's surface. It is believed that originally there was only one land mass called Pangaea. This large land mass split into a northern mass Laurasia and a southern one called Gondwana Land. From these two land masses, the continents gradually drifted to where they are now located and the process is still continuing.