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Solar System

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Solar System - Lesson Summary

The Sun and the celestial bodies that revolve around it form the solar system. It comprises large number of bodies like planets, their moons, comets, asteroids, meteoroids, meteors and meteorites. These objects are held together in the solar system due to Sun's gravitational pull. The sun is a major source of heat and light for all the planets in the solar system. Planets reflect sunlight that is incident on them. They have no light of their own, so they don’t twinkle like the stars. Planets have definite paths called orbits in which they revolve around the sun. The time taken by a planet to complete one full revolution around the sun is called its period of revolution. The time taken by a planet to rotate a full 360 degrees on its axis is called its period of rotation. A celestial body that revolves around another celestial body is called a satellite..

According to the new definitionadopted by the International Astronomical Union, there are eight planets in the solar system. In the order of their increasing distance from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto, once considered as the farthest planet, has lost its status of being a planet and is now treated as a dwarf planet.

The Sun is the star closest to the Earth, which is at a distance of eight light minutes from the Earth. It is a huge ball of gases, mainly hydrogen. The nuclear fusion reactions that take place in the Sun, converting hydrogen gas into a helium gas, gives out tremendous amount of energy in the form of heat and light, which is responsible for the brightness of the Sun. The Sun is so huge that nearly a million eaths can be enclosed in it!

Mercury is the smallest planet in the solar system and is the closest to the Sun. It takes about 88 days to complete one revolution around the Sun. It has no satellite of its own. 

Venus is the second closest planet to the Sun. It takes about 225 days to complete one revolution around the sun. It has no satellites or moons. Venus rotates from east to west. It is the brightest planet in the night sky. It appears in the eastern sky before sunrise and in the western sky after sunset. For this reason Venus is called a morning star or an evening star, although it is not a star. 

Earth is the only planet on which life is known to exist. It takes 365.25 days for the earth to complete one revolution around the sun. It takes 24 hours to complete one rotation about its axis. The tilt of the Earth's axis is responsible for the change of seasons on the Earth. There is a thin blanket of air surronding the Earth, called atomsphere. The right temperature range, the presence of water and suitable atmosphere and a blanket of ozone support life on Earth. Earth has only one natural satellite, i.e. our Moon. 

The red planet Mars has two moons: Phobos and Deimos. The Martian soil contains large amounts of iron oxide which gives the reddish appearance to the planet. Mars completes one revolution around the sun in about 687 days. 

Jupiter is the largest planet in the solar system. Jupiter has four large moons called Io, Europa, Callisto and Ganymede, and many small moons. It rotates the fastest among all planets.  It has the famous Great Red Spot which is a great hurricane on Jupiter. It has faint rings around it. Jupiter can be seen in the sky easily as it appears quite bright. 

Saturn is the second largest planet in the solar system after Jupiter. It appears yellowish in colour. The rings of Saturn are made of ice particles and dust. Saturn is the only planet that is lighter than water. Saturn has many moons and the largest of them is called Titan.

Uranus is the coldest planet. Like Venus, Uranus also rotates from east to west. It has a highly tilted rotational axis so that, in its orbital motion, it appears to roll on its side. 

Neptune was discovered through mathematical calculation. It has two known satellites. It has a ring system too. 

Mercury, Venus, Earth and Mars are called the inner planets. Jupiter, Saturn, Uranus and Neptune are called the outer planets. The outer planets have several moons and a system of rings. The inner planets are smaller and denser than the outer planets. The outer planets are colder than the inner planets as they are farther away from the Sun.


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