• We can see many celestial bodies in a clear night sky.
    Stars are one of the celestial bodies which emit light of their own.
  • The moon is a natural satellite of the Earth.
  • It revolves around the Earth in its orbit.
  • The different shapes of the bright visible part of the moon as seen from the Earth are called phases of the moon.
  • Sun is also one of the stars which emit light and is a great source of heat.
  • It is the closest star and is the centre of our solar system.
  • The stars are millions of km far from Earth and from each other.
  • Such large distances are expressed in a unit known as light year.
  • It is the distance travelled by light in one year, i.e., 9.46 × 1012 km.
  • Stars are many light years away from the Earth and thus they look very small from Earth. Stars appear to travel from east to west.
  • The Pole star is the most shining star in the night sky. The pole star appears to be stationary. It is situated near the axis of rotation of Earth and is thus helpful in finding direction.
  • Other important parts of the night sky are planets. Planets revolve around the Sun.
  • Our solar system consists of eight planets revolving around the Sun. It also consists of many other celestial bodies like asteroids, comets and meteors.
  • Inner or Terrestrial Planets: First four planets Mercury, Venus, Earth and Mars are much nearer to the Sun and have fewer satellites. They are called the inner planets. These are also called terrestrial planets because their structure is rocky similar to that of Earth.


The planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus and Neptune are called outer planets because they are much farther off than inner planets. They are also known as Jovian planets because their structure is gaseous and are similar to that of Jupiter.



  • It is the closest planet to the Sun. Its distance from Sun is 57 × 106 km.
  • Since it is very close to the Sun, most of the time it is hidden in the glare of the Sun.
  • It can be visible before the Sunrise in the east and after the Sunset in the west.
  • It appears quite bright and correspondingly it is termed as ‘morning star’ and ‘evening star’.
  • It is termed a star because it appears very bright in the sky.
  • It is of the same size as the moon.
  • It revolves around the Sun in 88 days and takes 58 days to complete one rotation on its axis.
  • Life cannot exist on mercury due to lack of atmosphere and extreme temperature [340°C ⇔-150°C) and it has no protective blanket around it to save it from harmful radiations.
  • The surface features of mercury resemble those of the moon more than those of the Earth.
  • It has no moon or satellite of its own.


  • Its distance from the Sun is 108 x 106 km.
  • It completes its orbit around the Sun in 225 days.
  • It has almost the same radius, density and mass as that of Earth. Thus, it is called the twin of Earth.
  • It is the brightest planet and appears as a morning and evening star.
  • The surface temperature of Venus is about 450°C and it is covered by a thick blanket of cloud made up of CO2, H2, O2, N2. NO life is possible on this planet because of high temperature, absence of water and insufficient oxygen.
  • It has no moon or satellite of its own.


  • Its distance from the Sun is 149 x 106 km.
  • It has plenty of water, oxygen in the atmosphere and is neither too cold nor too hot, making life possible on this planet.
  • It takes 365 (1/4)days to complete one revolution around the Sun and 24 hours to complete one rotation on its axis.
  • It has a thick blanket of the ozone layer high up in its atmosphere to save the life from harmful effects of ultraviolet radiations coming from the Sun.
  • It has one satellite called the moon.


  • Its distance is 227 × 106 km from the Sun.
  • It takes 687 days to complete one revolution around the Sim and 24 hours to complete one rotation on its axis. It has a reddish appearance.
  • It has two natural satellites or moons named Phobos and Deibos.
  • Unlike Mercury and Venus, they can be seen in any part of the night sky.
  • The day temperature varies from 5°C to 15°C and there is no evidence as yet of life on Mars.
  • It has no protective blanket to protect it from harmful solar radiations.


  • Its distance from the Sim is 778 × 106 km.
  • It takes 12 years to complete one revolution around the Sun.
  • It is the largest planet and is more massive than the combined mass of other planets of the solar system.
  • It has a dozen satellites or moons. Four of them are quite large and bright and can be seen with a low power telescope.
  • There is a faint ring consisting of extremely small particles around Jupiter.


  • After Jupiter, Saturn is the second biggest planet in the solar system.
  • It looks like a large yellow star to the naked eye.
  • It possesses a well-developed set of rings around it.
  • These rings consist of particles whose sizes vary from specks to rocks measuring a few kilometres in diameter.
  • It is at a distance of 1427 × 106 km from the Sun.
  • It takes about 29.5 years to complete one revolution around the Sun.
    It is said to have 30 satellites or moons of its own. u-U) Uranus (Arun)
  • This is the seventh planet from the Sun and is 2870 x 106 km away from the Sun.
  • It takes 84 years to complete one revolution around the Sun.
  • It has 21 satellites or moons of its own.
  • It rotates about its axis from east to west in contrast to other planets which rotate from west to east.
  • Its atmosphere contains hydrogen and methane.


  • It is the eighth planet in terms of its distance from the Sun.
  • It has 8 satellites revolving around it.
  • Its distance from Sim is 4504 × 106 km.
  • It takes 165 years to complete one revolution around the Sim.


(i) Asteroids: These are rocky planetary bits orbiting around the sun. The asteroid belt lies between Mars and Jupiter. 
(ii) Comets: These are heavenly bodies that revolve around the sun. It appears generally as a bright head with a long tail. 
(iii) Meteors and Meteorites: A meteor is a brief streak of light in the night sky caused by a meteoroid. Smaller meteors melt and burn up creating streaks of light. Few meteoroids which survive as they pass through the Earth’s atmosphere and reach the Earth surface are called meteorites. 
(iv) Artificial Satellites: The artificial satellites revolve around the Earth much closer than the moon. Artificial satellites are used for weather forecasting, long-distance communication and remote sensing. Ex: IRS, EDUSAT, INSAR. 
(v) Constellations: The group of stars which appear to form some recognizable shape or pattern is known as a constellation. These groups of stars or constellations are named after the objects which they seemed to resemble such as an animal, a human being. Constellations appear to move from east to west as Earth rotates from west to east. Orion-(a constellation of 7 or 8 stars which looks like a hunter),Ursa Major- the Great Bear or Saptarishi, Cassiopeia are some constellations. 

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