Gist of NCERT - Geography - Solar System



The sun, the moon and all those objects shining in the night sky are called celestial bodies.

Some celestial bodies are very big and hot. They are made up of gases. They have their own heat and light, which they emit in large amounts. These celestial bodies are called stars. 


The sun is a star.

While watching the night sky, you may notice various patterns formed by different groups of stars. These are called constellations.


One of the most easily recognisable constellation is the small bear or Saptarishi (Sapta-seven, rishi-sages). It is a group of seven stars. 

In ancient times, people used to determine directions during the night with the help of stars. The North star indicates the north direction. It is also called the Pole Star. It always remains in the same position in the sky.

Some celestial bodies do not have their own heat and light. They are lit by the light of the stars. Such bodies are called planets. The word ‘planet’ comes from the Greek word “Planetai” which means ‘wanderers’.

Jupiter, Saturn and Uranus have rings around them. These are belts of small debris. 

The sun, eight planets, satellites and some other celestial bodies known as asteroids and meteoroids form the solar system.

‘Sol’ in Roman mythology is the ‘Sungod’. ‘Solar’ means ‘related to the sun’. The family of the sun is, therefore, called the solar system.

There are eight planets in our solar system. In order of their distance from the sun, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
 
All the eight planets of the solar system move around the sun in fixed paths. These paths are elongated. They are called orbits. 

Mercury is nearest to the sun. It takes only about 88 days to complete one round along its orbit. 

Venus is considered as ‘Earth’s-twin’ because of its size.

Till recently (August 2006), Pluto was also considered a planet. However, in a meeting of the International Astronomical Union, a decision was taken that Pluto like other celestial objects (Ceres, 2003 UB313) discovered in recent past may be called "dwarf planets."

The earth is the third nearest planet to the sun. In size, it is the fifth largest planet. It is slightly flattened at the poles. That is why, its shape is described as a Geoid. Geoid means an earth-like shape.

From the outer space, the earth appears blue because its two-thirds surface is covered by water. It is, therefore, called a blue planet.

Our earth has only one satellite, that is, the moon. Its diametre is only one-quarter that of the earth. It appears so big because it is nearer to our planet than other celestial bodies. It is about 3,84,400 km away from us.

The moon moves around the earth in about 27 days.

Apart from the stars, planets and satellites, there are numerous tiny bodies which also move around the sun. These bodies are called asteroids. They are found between the orbits of Mars and Jupiter.

The small pieces of rocks which move around the sun are called meteoroids. Sometimes these meteoroids come near the earth and tend to drop upon it. During this process due to friction with the air they get heated up and burn. It causes a flash of light. Sometimes, a meteor without being completely burnt, falls on the earth and creates a hollow.

A galaxy is a huge system of billions of stars, and clouds of dust and gases. There are millions of such galaxies that make the Universe.  Our solar system is a part of the Milky Way galaxy which is part of the universe. 

Light travels at the speed of about 300,000 km per second. Yet, even with this speed, the light of the sun takes about eight minutes to reach the earth.

Neil Armstrong was the first man to step on the surface of the moon on 21 July 1969.

A Satellite is a celestial body that moves around the planets in the same way as the planets move around the sun.

A Human-made Satellite is an artificial body.
It is designed by scientists to gather information about the universe or for communication. It is carried by a rocket and placed in the orbit around the earth. Some of the Indian satellites in space are INSAT, IRS, EDUSAT, etc.


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