Sky Vision: Full Chapter Notes with Important Questions (CG Board Class 8)

1 SKY VISION

 

The planet on which we live is called Earth. During daytime, we see the sun in the sky. Come; let us see if there is anything else other than the sun in the sky.

1.1 WHAT IS THERE IN THE SKY?

Look at the dark sky at night when there is no cloud. You will see a large number of stars. Some of which are very bright and some are less. Some are big and some are small. The interesting point is this, that these small stars are actually quite bigger than our earth and some of them are even many times bigger than the sun. Now the question is -why do these stars appear so small to us? Let us see.

ACTIVITY -1

Take two footballs of the same size. Go to the playground and place these balls at least 50 - 80 metres apart. Now move away to about 30 metres and look at the balls in such a way that the two balls and you are in a straight line. Now note the size of the balls. Are the size of the two balls same? When the things are far away from us, they appear smaller to us. Now you must have understood why the stars which are big in size appear so small to us.

If you observe minutely, you will see some objects other than the stars. They do not twinkle like the stars. These are the planets, which like our earth, revolve round the sun. Some times for a moment a line of bright light can be seen in the sky. Actually, they are burning meteors. In this way our earth and the other planets, the sun and the moon, meteors etc. together form the Milky Way. Let us know some more about them.

1.2 Stars and the Star World

Stars are the most attractive objects in the sky. They are celestial bodies that emit their own light and energy continuously. The Sun is the nearest star to the Earth. Because of the bright light of the Sun, we cannot see other stars during the day.

Most stars are very far away from the Earth. Their light takes many years to reach us. The distance of stars is measured in light-years.

A light-year is the distance travelled by light in one year. Light travels at a speed of about 3,00,000 kilometres per second. So, one light-year is equal to:
9.46 × 10¹² kilometres.

The Sun is about 1.5 × 10⁸ kilometres away from the Earth. Sunlight takes about 8.3 minutes to reach the Earth. So, we can say that the Sun is about 8.3 light-minutes away.

After the Sun, the nearest star to the Earth is Alpha Centauri, which is about 4.3 light-years away. The brightest star, Sirius, is about 8.7 light-years away.

If we look carefully at the sky, we can see a band of stars stretching across it. This is called the Milky Way. It is a galaxy that contains our Solar System. There are many such galaxies in the universe.

All stars, including the Sun, are moving in space. However, because they are very far away, we cannot easily notice their movement.

Pole Star

The Earth rotates from west to east. But we see stars moving from east to west. There is one star in the north that appears to remain fixed. This is called the Pole Star (Dhruv Tara).

The Pole Star has been used for a long time to find directions, especially north.

Constellations

Some stars form patterns or shapes in the sky. These groups of stars are called constellations. The stars in a constellation always remain in the same pattern.

Some important constellations are:

• Big Saptarishi (Ursa Major)
• Small Saptarishi (Ursa Minor)
• Orion (Hunter or Kalpurush)

The Big Saptarishi is easy to recognize. It looks like a saucepan or a question mark. It has seven bright stars. Three stars form the handle and four form the bowl.

Two stars at the top are called pointer stars. The line joining them points towards the Pole Star.

The Orion constellation is also very bright. It looks like a hunter, so it is also called Kalpurush.

Now Answer These

1. Some stars which are bigger than the Sun appear smaller to us because they are very far away from the Earth.
2. If a star is 3.4 light-years away, it means that light from that star takes 3.4 years to reach the Earth.
3. The star that appears in the north can be found by locating the Pole Star. It can be identified using the pointer stars of the Big Saptarishi constellation.

1.3 There are Only Eight Planets, Not Nine

Planets are heavenly bodies that revolve around the Sun. They do not have their own light like stars. They shine because they reflect the sunlight falling on them.

Ancient astronomers discovered Mercury, Venus, Earth, Mars, Jupiter and Saturn because they could be seen with the naked eye. Later, with the invention of the telescope, Uranus, Neptune and Pluto were discovered.

Earlier, it was believed that there are nine planets. But in 2006, Pluto was not considered a planet anymore. So now, there are only eight planets in the Solar System.

Some planets have satellites. Satellites are bodies that move around planets. The Moon is the natural satellite of the Earth.

1.3.1 Mercury

Mercury is the nearest planet to the Sun. It is difficult to see because of the bright sunlight. It is very hot. Its size and mass are similar to the Moon. It has no atmosphere and is rocky and mountainous. Mercury has no satellite.

1.3.2 Venus

Venus is the second planet from the Sun. It is the brightest planet. Its brightness is due to thick clouds that reflect most of the sunlight.

It can be seen before sunrise and after sunset, so it is called the Morning Star or Evening Star. Its size is almost similar to Earth. Venus has no satellite.

1.3.3 Earth

Earth is the third planet from the Sun. It is the only known planet where life exists.

It takes about 365 days to revolve around the Sun and 24 hours to rotate on its axis. This rotation causes day and night. Its tilted axis causes seasons.

Earth has one natural satellite, the Moon.

1.3.4 Mars

Mars is the fourth planet from the Sun. It appears red in colour, so it is called the Red Planet.

Its size is smaller than Earth. There is no clear evidence of life, but scientists are studying it. Mars has two satellites.

1.3.5 Jupiter

Jupiter is the biggest planet in the Solar System. Its size is larger than all other planets combined.

It is very far from the Sun, so it receives less light and heat. However, it appears bright because of its thick atmosphere. Jupiter has many satellites.

1.3.6 Saturn

Saturn is the sixth planet from the Sun. It is famous for its beautiful rings. These rings can be seen with a telescope.

It is similar to Jupiter but colder. Saturn has many satellites.

1.3.7 Uranus

Uranus is the first planet discovered using a telescope. It is very far from the Sun and has many satellites.

1.3.8 Neptune

Neptune is the eighth planet from the Sun. It is the farthest planet and the coldest one. It also has several satellites.

Pluto: Why is it not a Planet?

Pluto is very far from the Sun. It is about 39.5 times farther than the Earth from the Sun. It is also the smallest among such objects. Sunlight takes about 32 hours to reach Pluto.

For many years, there was confusion about whether Pluto is a planet. On 24th August 2006, in a meeting of the International Astronomical Union, the definition of a planet was decided.

According to this definition, a planet is an object which:

1. Revolves around the Sun.
2. Has enough mass to become round (spherical) due to gravity.
3. Has a clear orbit.
4. Has no other objects sharing its orbit.

Why is Pluto not a Planet?

Pluto revolves around the Sun, but:

• It is very small and not perfectly round.
• Its orbit is not clear and it crosses the orbit of Neptune.
• All planets move in the same plane, but Pluto’s orbit is tilted at an angle of about 17°.

Because of these reasons, Pluto is not considered a planet. It is called a dwarf planet (pseudo planet).

Now Answer These

Recognize the planets:

a) The biggest planet – Jupiter

b) The planet which has life – Earth

c) The brightest planet – Venus

d) The red planet – Mars

e) The planet with eight satellites – Jupiter

f) The first planet discovered with a telescope – Uranus

g) The planet with three rings – Saturn

You know that there are some other heavenly bodies in the Solar System apart from the Sun and the planets. Let us learn about them.

1.4 Asteroids

Asteroids are small bodies that move around the Sun. They are found in the space between Mars and Jupiter. Asteroids are pieces of matter that could not form into a planet due to some reasons. They are irregular in shape and are present in large numbers.

1.5 Comets (Dhumketu)

Comets are small heavenly bodies that revolve around the Sun. We can see them only when they come very close to the Sun. A comet has a bright head and a long shining tail. The tail of a comet always points away from the Sun.

Comets usually appear after a fixed interval of time. For example, Halley’s Comet appears after about 76 years. It was last seen in 1986.

1.6 Meteors and Meteorites

Sometimes, on dark nights, we see bright streaks of light in the sky. These are called “shooting stars” or “falling stars,” but they are not real stars. They are called meteors.

Meteors are small bodies that move around the Sun. When they enter the Earth’s atmosphere at very high speed, they burn due to friction and appear as bright light in the sky.

Some meteors do not burn completely and a part of them falls on the Earth. These pieces are called meteorites.

All the planets and their satellites move in their own paths around the Sun in a disciplined manner without disturbing each other.

We should also learn from this and try to live peacefully and maintain harmony in our society.

We Have Learnt

1. Stars are celestial bodies that emit their own light and energy.
2. The distance between celestial bodies is measured in light-years.
3. Constellations are groups of stars that form recognizable shapes or patterns in the sky.
4. Planets are celestial bodies that revolve around the Sun.
5. Satellites are celestial bodies that revolve around planets.
6. The Solar System consists of the Sun, planets, their satellites, asteroids, comets, meteors, and meteorites.
7. The planets revolving around the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. 

Fill in the blanks:

(a) A group of stars forming a particular shape is called a constellation.

(b) The heavenly body which revolves around a planet is called a satellite.

(c) Neptune is the coldest planet.

(d) The star that appears to be fixed in the north direction is the Pole Star.

(e) The constellation which looks like a hunter is Orion.

🌌 Multiple Choice Questions (MCQs)

1. The constellation which looks like a hunter is:
   (a) Ursa Major
   (b) Orion ✅
   (c) Cassiopeia
   (d) Pole Star

2. The planet closest to the Sun is:
   (a) Venus
   (b) Mercury ✅
   (c) Jupiter
   (d) Earth

3. Asteroids are found between:
   (a) Mars and Jupiter ✅
   (b) Saturn and Jupiter
   (c) Venus and Jupiter
   (d) Earth and Mars

4. This planet does not have any natural satellite:
   (a) Earth
   (b) Mars
   (c) Mercury ✅
   (d) Jupiter

✍️ Short Answer Questions

1. Why do we find the Sun to be the biggest and brightest star?

We see the Sun as the biggest and brightest star because it is the closest star to Earth. Other stars are very far away, so they appear smaller and dimmer.

2. Why does the Pole Star appear to be stationary?

The Pole Star appears stationary because it is located almost directly above the North Pole, along the Earth's axis. As Earth rotates, other stars seem to move, but the Pole Star stays in the same position.

3. Why is Venus the brightest planet even though it is not the closest to the Sun?

Venus appears very bright because:

• It is closer to Earth than many other planets
• It has a thick atmosphere that reflects sunlight effectively

4. How will you recognise the Pole Star?

You can find the Pole Star by using the Ursa Major (Saptarishi) constellation.
Draw an imaginary line through the two outer stars of the “Big Dipper” and extend it. This line points to the Pole Star.