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Solar System

Solar system comprises the sun, the planets and their satellites, and many other heavenly bodies such as asteroids, comets and meteors. The radius of the solar system is 39.5 AU. The average distance between the sun and Pluto is taken as the radius of the solar system.
Models of Solar System
The early Greek astronomers assumed that each planet revolved around the earth in a small circle, known as epicycle. The centre of the epicycle itself followed a circular path around the unmoving earth.
Geocentric Model In this model, the earth was considered to be fixed at the centre of the universe and all the celestial bodies, including sun, revolved around it. This theory was proposed by Ptolemy.
This model was complex and could not correctly predict the planetary positions.
Heliocentric Model A famous Polish astronomer Nicolaus Copernicus developed a new model of solar system called ‘sun-centre’ or ‘Helio-centric’ model of solar system. According to this model, the sun remained fixed at the centre and all the other celestial bodies, including the earth, revolved around it in circular orbits. This model is simple and correctly predicts the planetary positions.


Sun is a medium sized star and it is the centre of our solar system. The sun is the nearest star to the earth and is 150,000,000 km away from the earth, i.e., 1.5 × 108 km. Light takes 8.3 min to travel from the sun to the earth. Mass of the sun is calculated using Kepler’s law and is found to be 2 × 1030 kg. This is more than 3 × 105 times the mass of the earth. The sun is the source of light which protects the life on the earth (Figure).
Structure of Sun Sun is predominantly made up of light gaseous elements such as hydrogen and helium. The sun’s core, the innermost part of the sun, i.e., the heart of the sun, is the source of all its power. Here, hydrogen nuclei are fused into helium by thermo-nuclear fusion reaction releasing tremendous amounts of energy. So the temperature in this region is about 20 million degree celsius. When this happens, energy is released in the form of electromagnetic radiations, particularly high-energetic gamma rays (γ-radiations) and this is emitted out towards the surface of the sun. The central region where the energy production occurs is called the core (Figure).
Beyond the core, there is a layer zone called radiation zone which has densely packed hydrogen and helium nuclei undergoing bombardment by the gamma rays of the core. Less-energetic electromagnetic radiations such as X-rays and ultra violet rays are generated because of collisions.
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The Sun
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Cross-section of the Sun
Beyond this, there is a turbulent layer called convection zone, which is 1,30,000 km thick. It is churned by the thrust of energy from below. The electromagnetic radiations carry energy from sun’s interior and reach the outer surface of the sun called photosphere. The temperature of photosphere is 6000°C.
The surface of the sun or the visible disc of the sun is the photosphere. There are special telescopes designed to study the sun and these are called solar telescopes.
By using these solar telescopes when photosphere is examined carefully it reveals grain like structures or patterns and these are called granulations. In addition, we also see thread-like structures called spicules.
Occasionally, hot matter streams out of the sun’s surface and bursts out like thunder storms. These are called solar flares. These solar flares sometimes loop back into the photosphere. They are called solar prominences.
Sunspots Sunspots are the dark, irregular patches present on the surface of the sun. Their size varies from 300 to 80,000 km. They look dark because they are cooler regions in the photosphere. The temperature in sunspots is around 4500°C.

Solar Atmosphere

The solar atmosphere consists of two regions, namely, the chromosphere and the corona. The layer just above the photosphere is called chromosphere. The chromosphere is 10,000 to 15,000 km thick. The region beyond this is corona which extends to millions of kilometres from the photosphere. The matter in the corona is in the plasma state and the temperature is of the order of a million degrees.
The planets are the ‘wanderers’ that revolve around the sun in elliptical orbits. The word ‘planet’ derives from the Greek ‘planetai’, meaning wanderers. It refers to the apparently erratic drifting motion of these bodies with respect to stars whose relative positions are fixed. The planets spin about their own axes and also revolve around the sun.
The planets, in the increasing order of distance from the sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto is called the dwarf planet or planetoid. Under the influence of gravitational force of the sun, each planet revolves around the sun in elliptical orbits in anticlockwise direction. Each planet also rotates about an axis in the anticlockwise direction, except for Venus and Uranus, which rotate in the clockwise direction.
The planets are divided into two groups according to their size and density. The first category are the four small solid planets: Mercury, Venus, Earth and Mars. These planets are made up of solid rock and other solid materials. They are called terrestrial planets. As these planets are closer to the sun they are also called as the inner planets.
The second category of four planets, Jupiter, Saturn, Uranus and Neptune, are called the outer planets. These planets are large in size and mostly made of gases and hence they are called gas giants.
Mercury The smallest, innermost planet mercury comes within 47 million kilometres of the sun while it is closest to the sun and goes as far as 70 million kilometres when it is farthest. It goes round the sun in 88 days and rotates about its axis in 59 days. The planet has no atmosphere, and due to the absence of atmosphere the mean surface temperature is 360 K, but it ranges from 100 K to 620 K.
Venus It is the second planet from the sun. It is surrounded by an atmosphere consisting of thick impenetrable yellowish white clouds of carbon dioxide. Because of the nearness to the sun and the high reflectivity of its dense atmosphere, it is the brightest object in the sky, except for the sun and the moon. It is called the morning star or evening star. As the surface temperature is about 750 K, it is the hottest planet. It has retrograde (backward) rotational motion, i.e., it rotates from east to west. The length of Venus day is about 243 earth days.
The Earth It is the third planet from the sun and fifth largest planet in the solar system. It rotates about its axis in 23 hours, 56 minutes and 4 seconds. It is the only planet where life is known to exist. Life originates and flourishes on the earth because of the following reasons:
  1. The presence of fundamental building blocks of life such as carbon, nitrogen, hydrogen, oxygen and their complex organic molecules.
  2. The presence of transparent liquid water for transportation of nutrients in living things and also for external transportation.
  3. Moderate temperature which is convenient for life to exist.
  4. The protective atmosphere around the earth that protects the earth from harmful radiations and electric discharges from the sun.
  5. The convenient distance of the earth from the sun.
  6. The presence of atmosphere consisting of oxygen, nitrogen and carbon dioxide.

Structure of the Earth

The earth has mainly three layers namely, crustmantle and core.
The Crust The crust is the outermost layer of the earth, whose thickness varies about 30–40 km. But, all the life activities in it are limited to about 5 km below the surface. It holds up a lot of water. The crust is the home of all living things. Most plants grow on lands. They need a firm soil into which the roots can penetrate. This gives stability for the plants and trees to stand erect. The soil contains the nutrients needed for the growth and development of plants.
All the metals such as iron, copper, gold, aluminium, etc., that we extract from the minerals are obtained from the crust. The crust not only supports our weight, but also provides a rich variety of substances for our daily use.
Mantle Mantle is the second layer of the earth and is mainly composed of silicates of iron and magnesium. Because of the high temperature and pressure the matter in the mantle is in the hot molten liquid form called magma.
Core It is the innermost layer of the earth where iron is in the molten state. The core has two parts—the outer core and inner core. In the outer core, the iron is in the molten form whereas in the inner core, it is in the solid form due to extremely high pressure (Figure).

Earth’s Atmosphere

The earth’s atmosphere is the thin blanket of gases, dust and water vapour surrounding the earth. This layer extends to about 1000 kms from the surface of the earth.
The atmosphere comprises mainly nitrogen (78%), oxygen (21%), argon (0.934%), carbon dioxide (0.0314%) and traces of neon, helium, methane and ozone.
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Sectional View of the Earth

Layers of the Atmosphere

Troposphere It is the lowest layer of the atmosphere which extends to a height of 18 km at the equator, 13 km at the middle of latitudes and 8 km at the poles. All the weather phenomena such as cloud formation, rain, etc., take place in the troposphere.
Stratosphere It is the second layer of the earth’s atmosphere which extends up to a height of 50 km from the troposphere. This layer contains ozone layer which absorbs ultraviolet rays emitted by the sun and shields the earth from intense harmful radiations.
Mesosphere It is the third layer of the atmosphere which is 50–80 km between stratosphere and the next layer, ionosphere. In this layer, temperature falls with elevation and has very low pressure.
Ionosphere This layer exists above the mesosphere and extends up to 80 km, where the temperature increases with height. The matter here is in the form of ions and hence it is called ionosphere. It is a good conductor of electricity and hence it is a layer which helps in communication.
Exosphere It is the outermost layer of the atmosphere and extends nearly to 900 km from the ionosphere.
Moon It is the natural satellite of the earth. It is a barren dry land with many craters on its surface. The period of revolution of moon is equal to its period of rotation around its own axis and so we always see the same face of the moon. Since the gravitational pull of the moon’s surface is only about one-sixth of that on the earth’s surface, there is no atmosphere on the moon. The temperature on moon’s surface during day is nearly 130°C and during night is 180°C.
Phases of moon Moon is a non-luminous body. It shines by reflecting the sunlight that falls on it.
The moon as we see from the earth appears to change its shape everyday from a thin crescent to the full moon. These different shapes of the moon as seen from the earth are called the different phases of the moon.
  1. When the side of the moon facing us gets no sunlight at all, we cannot see the moon. This phase of the moon is called the new moon day (day 1).
  2. As the moon moves along its orbit, a small portion of the side facing us gets sunlight and we can see the crescent moon (day 3).
  3. In a week’s time, we can see only half of the moon (day 7).
  4. In 10 days time, we can see three quarters of the moon. This is called gibbous moon or waxing (day 10).
  5. In 14 days time, the entire side of the moon facing us gets sunlight, and we can see the full moon (day 14).
After this, the phases are reversed as shown in Figure.
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Phases of the Moon
Mars It is the fourth planet from the sun. It is only 1/10 as massive as the earth. About 2/3 of the planet is covered by deserts in which soil colour ranges from deep red to bright orange. Mars soil composition is rich in iron, covered with rust thus making it red. The atmosphere on Mars is very thin because of its low gravity. It contains moisture and its polar regions are covered with frost. Mars has two tiny satellites, Phobes and Deimos.
Jupiter Jupiter is the largest of all planets in the solar system. It has a diameter of 1,41,824 km which is about 11 times the earth and a mass about 317 times that of the earth. Jupiter is mainly composed of light gases such as hydrogen and helium and it is the largest gaseous giant. It rotates on its own axis in less than 10 hours and it takes 12 earth years to revolve round the sun. Jupiter gives about twice the amount of heat than it receives from the sun. Therefore, it is believed that it generates heat of its own, like a star.
It has bright bands called ‘zones’ and dark bands called the ‘belts’. In the midst of the shifting bands there is the most prominent and stable feature of the planet—the great red spot.
Jupiter has 63 satellites out of which four satellites are big and bright. They are called Galilean satellites because they were first observed by Galileo with his telescope. Ganymede is the largest satellite in the solar system.
Saturn It is the sixth planet in the solar system and is the second largest planet in the solar system. It has internal features similar to Jupiter and its atmosphere has great amount of hydrogen, methane and small amount of ammonia.
The rings of Saturn are the most significant features of the planet, though recently planets Jupiter and Uranus also have been found to have rings. Saturn’s rings are composed of millions of bits of ice or ice-covered rock fragments. The empty space between two Saturn rings is called ‘Cassini Division’. It has 38 satellites out of which Titan is an important satellite. It is the only satellite having atmosphere made up of methane gas.
Uranus It is the seventh planet from the sun and exhibits retrograde motion and it is called rolling planet because it rotates about an axis nearly in the plane of its orbit. This means that, at one end of its orbit, Uranus’s south pole receives all the warmth the distant sun provides and at the other end, 42 years later, the north pole does the same.
The atmosphere of Uranus shows the presence of methane. Uranus has 27 satellites.
Neptune It is the eighth planet and the farthest planet in the solar system. Being the farthest planet, its period of revolution around the sun is 165 years. It is a cold planet and its temperature is about 220°C. It has five satellites of which Triton and Nereid are prominent.
Pluto Pluto is the second largest known dwarf planet in the solar system (after Eris). Originally classified as a planet, Pluto is now considered the largest member of a distinct population called the Kuiper belt.
Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small, approximately a fifth the mass of the earth’s moon and a third its volume. Pluto has three satellites. The atmosphere consists of a thin envelope of nitrogen, methane and carbon monoxide.
Asteroids or Planetoids  In the region between the orbits of Mars and Jupiter, there is a wide gap occupied by a large number of rocky materials called Asteroids. They are mostly huge lumps of rocks of varying sizes. Ceres is the biggest asteroid and has a diameter of 760 km. Vesta, Chiron and Trofan are the other important asteroids.
Comets Comets are the most spectacular and unpredictable bodies in the solar system. A comet has two distinct parts: a nucleus (or head) and a tail (Figure). The head of the comet is made up of frozen gases, cosmic dust and ice. Hence, comets are called dirty snow balls.
When a comet comes close to the sun, its outer layer evaporates. The radiation from the sun blows the gas and dust particles away from the sun. This looks like a bright tail. The comet develops a long tail extending millions of kilometres in space.
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A Comet with its Nucleus, Coma and Tail
Comets that can be viewed at regular intervals are called periodic comets.
Halley’s Comet is the most famous amongst the periodic comets. It visits the earth once in 76 years. Its most recent visit was in 1986.
Meteors Meteors are the broken fragments of minor planets (asteroids) or comets or may even be the debris left at the time of formation of planets. They are small in size and burn as they enter earth’s atmosphere and become luminous. They are popularly called ‘shooting stars or falling stars’. The unburnt part of a meteor that falls on the earth is called a ‘meteorite’.
Constellations Though stars appear to be scattered irregularly all over the sky, often they appear to form regular groups with definite shapes. These groups of stars that can be imaginarily connected to form regular shapes are called constellations.
All visible stars are divided into 88 constellations. The constellations are named so that it is easy to remember them. For example, the group of seven stars in the north is called ‘Saptharishi Mandala’ (ursa major) which is a part of a constellation. Another group in the north is called ‘Kunthi’ or ‘Cassiopeia’. The constellation called ‘Orion’ (Mahavyadha), seen in the evenings of December to March, is one of the brightest constellations.
Uses of Constellations
  1. They serve as direction indicators.
  2. Some special heavenly bodies in the celestial sphere are specified with the name of the constellations so that identification becomes easier.
  3. The members of the solar system such as planets, comets and the asteroids move with reference to stars. The constellations serve the purpose of marking their movement.

Zodiac Constellations

These are the constellations through which the apparent paths of planets, the sun and the moon pass through. They form an imaginary belt that is visible at any time (Figure).
Some Important Facts About Our Neighbourhood
  1. The nearest galaxy to our Milky Way is Andromeda galaxy, which is at a distance of 2.5 × 106 years.
  2. The nearest star to the earth and next to the sun is Proxima Centauri (4.25 light years from the earth).
  3. The luminosity and colour of stars depend on their temperature. Sirius (Dog star) is the brightest star and is 24 times brighter than the sun.


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Zodiac Constellations

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