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Chapter – 17 Stars and the Solar System
Celestial Bodies: All natural bodies visible inthe sky, outside the Earth's atmosphere, constitute the celestial bodies, e.g. stars, planets, their moons, comets, asteroids, meteors, etc.The Moon is thecelestial body closest to us.
Stars are celestial bodies that emitlight of theirown. Our sunis also a star.
It is convenient to express distances of stars inlight years.A light year is thedistance covered bylight in oneyear.
Stars appear to move fromeast to west.
The pole starappears to be stationary fromthe Earth, because it is situated close to thedirection of theaxis of rotation of the Earth.
Solar system: The Sun and the celestial bodies that revolve around it form the solar system. It comprises large number of bodies like planets(8 known till date), their moons, comets, asteroids, meteoroids, meteors andmeteorites. These objects are held together in the solar system due toSun's gravitational pull.
Sun: It is thesource of almost all energy onEarth. It continuously emits huge amounts of heat andlight.
Planets: 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 thesun is called its period ofrevolution. The timetaken by a planet torotate a full360 degrees onits axis iscalled its period of rotation. Timetaken by a planet to complete one revolution increases as the distance from the sunincreases. There are eight planets in oursolar system: Mercury, Venus, Earth, Mars,Jupiter, Saturn, Uranus, Neptune. Venus is thebrightest planet inthe night sky. Jupiter is thelargest planet ofthe solar system
Other members in our solarsystem are:
(i) Asteroids: These are rockyplanetary bits orbiting around sun. asteroid belt lies between
Mars andJupiter.
(ii) Comets: These are celestial bodies that revolve around the sun. Itappears generally asa bright headwith a longtail.The tail of a cometis always directed away from theSun. A
comet is madeup of rock, dust, water, iceand frozen gases.
(iii) Meteors and Meteorites: Meteoroids are chunks of rock or particle of debris in our solar system. They are smaller than comets. When meteoroids enter the Earth's atmosphere they are called meteors. Most meteors burn up in the atmosphere, but if they survive the frictional heating and strike thesurface of theEarth they arecalled meteorites. Asa meteor
glows brightly whenit falls tothe ground itis called ashooting star.
(iv) Satellites
A body revolving around another body iscalled a satellite. Moon is the natural satellite of the Earth. Some planets also have natural satellites. Artificial Satellites: Man-made objects sent into space to orbit the earth. Example: IRS, EDUSAT, INSAR. The artificial satellites revolve around theEarth. They aremuch closer than the moon. Artificial satellites areused for weather forecasting, long distance communication and remote sensing.
Stars are luminous bodies which appear aspoints of lightin the night sky. Our Sunis also astar. Many of the stars that we seein the skyare much bigger than the sun.But as theyare quite far awayfrom us, theyappear very small.
Constellations: A group of stars forming some kind ofrecongnisable figures orpatterns are known asconstellations. Constellations appear tomove from eastto west asEarth rotates fromwest to east. Orion- the Hunter, Ursa Major- theGreat Bear orSaptarishi, Cassiopeia aresome constellations.
The Moon is the celestial body closest to us. It is the only natural satellite of the Earth. It is a non-luminous bodyand it reflects the sunlight incident on it. Dueto its revolution around the Earth, when it isat different positions in its path, the apparent discof the Moonchanges, which gives riseto its phases.
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