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17. Stars and Solar System

This chapter talks about our solar system and other celestial bodies.

The stars, the planets, the moon & the other objects that we see in the sky are called Celestial Objects.

17.1     The Moon

The various shapes of the bright part of the moon as seen during a month are called phases of the moon.

17.1.1             Full Moon Day

The day on which the whole disc of the moon is visible is known as the full moon day. Thereafter, every night the size of the bright part of the moon appears to become thinner & thinner.

17.1.2             New Moon Day

On the fifteenth day, the moon is not visible. This day is known as the ‘new moon day’.

17.1.3             Crescent Moon

The next day, only a small portion of the moon appears in the sky. This is known as the Crescent Moon. Then again the moon grows larger every day & again on the fifteenth day, we see a full view of the moon.

17.1.4             Why do we see parts of the moon on different days?

This is because moon does not produce its own light. We see only that part of the moon from which the light of the sun is reflected towards us.

The size of the illuminated part of the moon visible form the Earth increases each day after the new moon day. After the full moon day, the sunlit part of the moon visible from the Earth decreases in size everyday.

17.2                 Moon’s Rotation & Revolution

The moon completes one rotation on its axis as it completes one revolution around the Earth.

17.2.1             The Moon’s Surface

Moon’s surface is dusty & barren. There are many craters of different sizes. It has a large number of steep & high mountains, some of them as high as the highest mountains on the Earth.

17.3                 The first Astronaut

The American astronaut, Neil Armstrong, landed on the moon for the first time on July 21, 1969. He was followed by Edwin Aldrin.

17.4                 The Stars

When we see in the dark sky in the night, we see so many stars, some shining brighter than the others. Stars emit their own light. Sun is also a star. Sun is the nearest star, that is why it appears to be much bigger than the others.

Sun is nearly 150 million km away from the Earth.

17.5                 Light Year

Large distances are expressed in a unit known as light year. It is the distance travelled by light in one year. Light year is the distance travelled by light in one year. (Speed of light is 300,000 km per second). Thus, the distance of the sun from the Earth is said to be about & light minutes.

  • The stars are present in the sky during the day time also, but they are not visible then because of the bright sunlight.
  • Stars appear to move from east to west. Same way for Sun, it being a star, appears to rise in the east & set in the west.

17.6                 Pole Star

Pole star is actually situated in the direction of the earth’s axis, so it does not appear to move.

17.7                 Constellations

The stars forming a group that has a recognizable shape is called a Constellation. In ancient times, constellations were devised to be able to recognize stars in the sky.

17.7.1             Ursa Major

One of the most famous constellations that can be seen in the summer sky during night is Ursa Major. It is also known as the Big Dipper, the Great Bear or the Saptarshi. Ursa Major has seven prominent stars. It appears like a big ladle or a question mark. There are three stars in the handle of the ladle & four in its bowl.

17.7.2             Orion

Another well-known constellation can be seen during winter in the late evenings. It is on e of the magnificent constellations in the sky. It has seven or eight bright stars. Orion is also called as Hunter as the three, middle stars represent the belt of the hunter. The four bright stars appear to be arranged in the form of a quadrilateral.

17.7.3             Cassiopeia

Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the early part of the night. It looks like a distorted letter ‘W’ or ‘M’.

17.8                 The Solar System

The Sun & the celestial bodies which revolve around it, form the Solar System.

Solar System consists of large number of bodies such as planets, comets, asteroids, meteors. The gravitational attraction between the Sun & these objects keeps them revolving around it.

The Earth revolves around the Sun. It is a planet and is a part of the solar system. There are eight planets in the solar system that revolve around the Sun. The eight planets in their order of distance from the Sun are – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune.

17.8.1             The Sun

The Sun is the nearest star from us. It emits huge amounts of the heat & light. The sun is the source of almost all energy on the Earth. In fact, the sun is the main source of heat & light for all the planets.

17.8.2             The Planets

The planets look like stars, but they do not have a light of their own. They just reflect the sunlight that falls on them.

The simplest method of identifying planets from stars is that stars twinkle, whereas planets do not. Also, planets keep changing their positions with respect to the stars.

17.8.3             Orbit

A planet has a definite path in which it revolves around the Sun. This path is called as Orbit.

17.8.4             Period of Revolution

The time taken by a planet to complete one revolution is called its period of revolution. The period of revolution increases as the distance of the planet increases from the sun.

17.8.5             Period of Rotation

Besides revolving around the sun, a planet also rotates on its own axis like a top. The time taken by a planet to complete one rotation is called its period of rotation.

17.8.6             Satellite

Any celestial body revolving around another celestial body is called its Satellite. We use this term ‘satellite’ for the bodies revolving around planets. Moon is a satellite of the Earth.

17.8.7             Artificial Satellite

The man-made satellites revolving around the Earth are called the Artificial Satellites.

17.9                 Facts and Figures about Planets

17.9.1             MERCURY (BUDH)

  • Nearest planet to Sun.
  • The smallest planet.
  • Has no satellite of its own.
  • Its size is that of the moon.
  • Takes 88 days to complete one revolution around the sun
  • Has weak gravity like moon.
  • Also covered with craters.

17.9.2             VENUS (SHUKRA)

  • Earth’s nearest planetary neighbour.
  • Brightest planet.
  • It is also called a morning or an evening star as it appears in the western sky just after sunset &  in the eastern sky before sunrise.
  • Has no satellite of its own.
  • Rotates from East to west
  • Doesn’t contain O2 but contains CO2

17.9.3             EARTH (Prithvi)

  • Only planet on which life exists.
  • Also known as blue planet as ¾ th surface is covered with water.
  • Earth has only one moon.

17.9.4             MARS (Mangal)

  • The first planet outside the orbit of Earth.
  • Is called a red planet.
  • Has two small natural satellites.

17.9.5             JUPITER (BRIHASPATI)

  • Largest planet of solar system.
  • Approx 1300 earths can be placed on this giant planet.
  • Rotates very fast on its axis
  • Has large no. of satellites
  • It has faint rings around it
  • Mass of Jupiter is 328 times that of earth.

17.9.6             SATURN (SHANI)

  • Yellow Planet
  • Is unique due to its beautiful rings.
  • Has large no. of satellites.
  • Least density

17.9.7             URANUS

  • Rotates from East to West
  • Most remarkable feature of Uranus is that it has highly tilted rotational axis.
  • Atmosphere mainly consists of Hydrogen
  • Has ring system

17.9.8             NEPTUNE

  • 8th Planet
  • Has 8 moons around it.
  • Surface is made up of rocks & its atmosphere made up of gases.

17.10            The Inner focus & the Outer focus

The first four planets, Mercury, Venus, Earth & Mars are much nearer the Sun than the other four planets. They are called the inner planets. The inner planets have very few moons.

The planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus & Neptune are much farther off than the inner planets. They are called the outer planets. They have a ring system around them. The outer planets have large no. of moons.

17.11            Some other members of the Solar System

There are other members of the solar system as well. They are

  • Asteroids
  • Comets
  • Meteors & Meteorites

17.11.1         Asteroids

There is a large gap in between the orbits of Mars & Jupiter. This gap is occupied by a large number of small objects that revolve around the Sun. These are called asteroids. These can be seen through large telescopes.

17.11.2         Comets

Comets are also members of our solar system. They revolve around the sun in highly elliptical orbits. However, their period of revolution round the Sun is usually very long.

A comet appears as a bright head with a long tail. The length of the tail grows in size as it approaches the Sun. The tail of the comet is always directed away from the Sun.

Comets keep appearing from time to time. One such comet is Hailey’s Comet  which appears after nearly every 76 years. It was last seen in 1986.

17.11.3         Meteors & Meteorites

The bright streaks of light seen sometimes, when the sky is clear & the moon is not there are called as Meteors. They are commonly called as shooting stars.

It is actually a small object that occasionally enters the earth’s atmosphere. At that time, it has a very high speed. The friction of atmosphere heats it up, so it glows & evaporates quickly. So, the bright streaks last for a very short time.

17.11.3.1           Meteorites

Some meteors are large & so they can reach the Earth before they evaporate completely. The body that reaches the Earth is called a Meteorite.

17.11.3.2           Meteor Shower

When the earth crosses the tail of a comet, swarms of meteors are seen. These are known as meteor shower which keep occurring at regular intervals each year.

17.12            Artificial Satellites

Artificial satellites are man-made & are launched from the Earth. They revolve around the Earth much closer than earth’s natural satellite, the moon. Artificial satellites are used for weather forecasting, long distance communication & remote sensing.

17.13            QUICK TO REMEMBER

  1. Largest Planet – Jupiter
  2. Smallest Planet – Mercury
  3. Planets moving from East to west – Venus & Uranus
  4. Nearest Planet – Mercury
  5. Brightest Planet – Venus
  6. Blue Planet – Earth
  7. Planet called as morning or Evening Star – Venus
  8. Red Planet – Mars
  9. Planets having rings around them – Jupiter, Saturn, Uranus, Neptune
  10. Yellow Planet – Saturn