STARS AND OUR SOLAR SYSTEM

Important points to remember : 1. STARS AND OUR SOLAR SYSTEM

1. Stars and constellations : On a clear moonless night, we can

see thousands of stars in the sky. Some of them form beautiful

patterns. The groups of stars forming such patterns are called

constellations. The International Astronomical Committee has

so far recognised 88 constellations. Of these constellations,

37 belong to the northern hemisphere sky and 51 belong to the

southern hemisphere sky. Ancient Indian astronomers had

imagined 27 constellations (called nakshatras). Mriga (Orion),

Krittika and Saptarshi (Great Bear or Ursa Major) are some

examples of the nakshatras.

2. Solar system : The sun, eight planets, moons (satellites) of

planets, numerous asteroids, meteoroids, dwarf planets and

comets all together form the solar system.

3. Sun : The sun is a medium sized star. It is in the centre of our

solar system. Its surface temperature is about 6000 °C. It is big

enough to hold 13 lakh earths within it. Due to its gravitational

force of attraction, the planets, comets and other celestial objects

revolve around the sun.

4. Planets : Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus

and Neptune are the eight planets revolving around the sun in

nearly circular orbits which are well separated from each other.

The time taken by a planet to go once around the sun is

called the period of revolution of the planet. It depends on the

distance between the planet and the sun. If the distance is more,

the period of revolution is more. Every planet rotates about its

axis. The time taken by a planet to complete one rotation about

its axis is called its period of rotation. It is different for different

planets.

5. Satellites or moons : A small heavenly body revolving around

a planet is called its satellite or moon. The earth’s natural

satellite, the moon, is at about 384400 km from the earth.

Mercury and Venus do not have moons. Mars has two

satellites. Jupiter has sixty-three satellites and Saturn has more

than sixty satellites.

5

6. Asteroids : Small remnants of heavenly bodies revolving

around the sun between the orbits of Mars and Jupiter are

called asteroids.

7. Comets : Many comets revolve around the sun in highly

elliptical orbits. When a comet approaches the sun, the matter

in the outer layer of the comet starts vaporising and forms a tail

extending millions of kilometres. Halley’s comet completes one

revolution around the sun in about 76 years. It was last seen in

1986 as it approached the sun. Some comets approach the sun

only once and then move far away from the sun, never to

return.

8. Meteors : Small pieces of matter, called meteoroids, move at

random in the solar system. If any of these pieces approaches

the earth, it is accelerated towards it due to the gravitational

force of attraction. As it enters the earth’s atmosphere, a large

amount of heat is generated due to friction with air. Hence, its

temperature rises so much that it starts burning. It is called a

meteor. A burning meteor looks like a shooting star. Most of the

meteors burn completely before reaching the earth. But some

large meteors do not burn completely before reaching the earth.

The meteors which reach the earth are called meteorites.

9. Artificial satellites : India, as well as some other nations,

have launched many satellites into orbits around the earth.

They are called artificial satellites. They have many uses such

as making contact with objects in space, communication,

weather forecasting, telecommunication, broadcasting radio

and TV programmes, space research, implementing

educational programmes and making accurate maps.

10. Radio telescope : The Tata Institute of Fundamental Research

(TIFR) has set up a radio telescope (GMRT : Giant Metrewave

Radio Telescope) close to the Pune-Nashik Highway at Khodad

near Narayangaon. It is used in space research.

Name the heavenly body nearest to the earth.

Ans. The moon, the natural satellite of the earth, is the

heavenly body nearest to the earth.

Suppose that on a certain day, the moon rises at

8 p.m. At what time will it rise the next day?

Ans. If the moon rises at 8 p.m. on a certain day, it will rise at

8.50 p.m. the next day.

 What is meant by the revolution of a planet ?

Ans. The motion of a planet around the sun is called the

revolution of the planet.

 What is meant by the period of revolution of a planet ?

OR

Define : Period of revolution of a planet.

Ans. The time taken by a planet to go once around the sun is

called the period of revolution of the planet.

*  What is meant by the rotation of a planet ?

Ans. The motion of a planet about its axis is called the rotation

of the planet.

What is meant by the period of rotation of a planet ?

OR

Define : Period of rotation of a planet.

Ans. The time taken by a planet to complete one rotation about

its axis is called the period of rotation of the planet.

What are the phases of the moon? Why do we see

them?

Ans. If we observe the moon everyday, we find that it appears

round on Full Moon Day. Thereafter, it wanes, i.e., it appears

smaller and smaller till we do not see it (New Moon Day).

Thereafter, it waxes, i.e. it appears bigger and bigger till Full Moon

Day. These apparent changes in the moon’s shapes and sizes (on

waning and waxing) are called the phases of the moon.

Fig. 1.1 : Some phases of the moon (Diagram for reference)

A part of sunlight falling on the moon is reflected by the moon’s

surface towards the earth. We see only that part of the moon. The

earth and the moon have rotational motion. Also, the earth revolves

around the sun and the moon revolves around the earth. As a result, Astronomical Committee? How many of

these belong to (i) the northern sky (ii) the southern sky ?

Ans. The International Astronomical Committee has so far

recognised 88 constellations. Of these constellations, 37 belong to

the northern hemisphere sky and 51 belong to the southern

hemisphere sky.

* (What is the importance of the Pole Star ?

Ans. As observed from the northern hemisphere, the Pole

Star is nearest to the north celestial pole of the sky and appears

practically stationary from the earth. It lies almost along the

axis of rotation of the earth. It can be used to find the direction

while navigating.

* If the earth moves from the west to the east, why

do the sun and stars move from the east towards the west ?

Ans. When we travel by a car, the trees and buildings by

the side of the road appear to move in the opposite direction

(in the direction opposite to the direction of motion of the car).

Similarly, as the earth moves from the west to the east, the sun

and stars which are very far from the earth appear to move

from the east towards the west.

[Note : Motion is relative, i.e., it depends on the observer.]

(Which celestial objects constitute the solar system?

Ans. The sun, eight planets revolving around the sun, the

moons of planets, asteroids, dwarf planets, comets and meteoroids

constitute the solar system.

(Up to the year 2006, Pluto was considered a planet of

the solar system. But not any more. Why?

Ans. The revolution of Pluto around the sun does not satisfy

the criteria used by the International Astronomical Society to

define a planet’s orbit. Hence, Pluto is no more considered a planet.

[Note : It is classified as a dwarf planet.]

Answers to the questions given on page 7 of the textbook

* Why should we not cut lanes when driving ?

Ans. To avoid road accidents, we should not cut lanes

when driving.

*  The earth revolves around the sun. Can it be

called a satellite of the sun ?

Ans. A satellite refers to a body revolving around a

planet, not around a star. The earth revolves around the sun

(star), not around a planet. Hence, it cannot be called a

satellite (of the sun).

 Each planet has different period of revolution. Why?

Ans. The period of revolution of a planet depends on the

distance between the planet and the sun. As the distance increases,

so does the period. The radii of orbits of planets differ widely.

Hence, each planet has different period of revolution.

State two characteristics of Mercury.

Ans. Characteristics of Mercury : (1) It is the planet nearest to

the sun. (2) It is the smallest planet in the solar system.

Why is it difficult to see Mercury?

Ans. Mercury is the smallest planet in the solar system and

closest to the sun. Due to bright sunlight, it is difficult to see

Mercury.

 When can Mercury be seen ?

Ans. Mercury can be seen at sunrise or sunset on the horizon.

Answer to the question given on page 9 of the textbook

* Why is it difficult for us to see Mercury because of its

being on the horizon?

Ans. Mercury being very close to the horizon at sunrise

or sunset, it is often hidden behind buildings, mountains, etc.,

and hence cannot be seen. But we can see it in open places, i.e.

where there are no obstacles in the line of sight.

*  What is the special feature of Venus?

Ans. All planets rotate about their axes from the west to the

east while Venus rotates about its axis from the east to the west.

Also, Venus has phases like the phases of the moon.

Why is Venus called (i) the morning star (ii) the

evening star ?

Ans. (i) On some days, Venus is seen very bright just before

sunrise. Hence, it is called the morning star.

(ii) On some days, Venus is seen very bright in the evening

(after sunset). Hence, it is called the evening star.

 Of the eight planets in the solar system, there is life

only on the earth. Why?

Ans. The factors necessary for life such as the right

temperature, water, air, atmosphere, protection from the ozone

layer, etc., are present only on the earth. Hence, of the eight planets

in the solar system, there is life only on the earth.

The earth has seasons like summer and winter. Why?

Ans. The axis of rotation of the earth is not perpendicular to

the plane of revolution of the earth around the sun, but slightly

inclined. Hence, the earth has seasons like summer and winter.

What is an inferior planet ? Name the inferior planets.

Ans. A planet whose orbit lies inside the earth’s orbit is called

an inferior planet. Mercury and Venus are inferior planets.

What is a superior planet ? Namethe superior planets.

Ans. A planet whose orbit lies outside the earth’s orbit is

called a superior planet. Mars, Jupiter, Saturn, Uranus and

Neptune are superior planets.

Name the first two superior planets.

Ans. Mars and Jupiter are the first two superior planets.

Name the last two superior planets.

Ans. Uranus and Neptune are the last two superior planets.

Why is Mars called the red planet ?

Ans. Mars appears red because its soil is rich in iron. Hence,

it is called the red planet.

Name the largest planet in the solar system.

Ans. Jupiter is the largest planet in the solar system.

(Write one special feature of Jupiter.

Ans. Jupiter rotates very fast, completing one rotation only in

10 hours.

How many satellites (moons) does Jupiter have?

Ans. Jupiter has 63 satellites (moons)

State two special features of Saturn.

Ans. (1) Saturn has rings around it. (2) Its density is less than

that of water.

(30) What is the special feature of Uranus?

Ans. The planet Uranus rotates about its axis from the east to

the west. (Just like Venus.)

What are asteroids ?

Ans. Small remnants of heavenly bodies revolving around the

sun between the orbits of Mars and Jupiter are called asteroids.

Write two special features of a comet.

Ans. (1) A comet revolves around the sun in an elliptical orbit

with a very large variation in its distance from the sun. (2) When a

comet approaches the sun, it forms a large tail on the side opposite

to the sun.

What is often referred to as a falling star ?

Ans. When a meteor burns due to the friction with air, it traces

a bright line in the sky before it falls on the earth. It is often referred

to as a falling star.

* Does a falling star cause damage?

Ans. When a meteor falls (falling star), most of it burns

due to friction with air before reaching the earth. Hence,

(usually) it does not cause any damage.

*  What would happen if the earth were to pass

through the tail of a comet?

Ans. When the earth passes through the tail of a comet,

meteor showers are seen. This is a very attractive sight.

Innumerable meteors are seen in the sky.

(What is an artificial satellite ?

Ans. For various purposes, man has launched many satellites

in the sky, orbiting around the earth. These are called manmade or

artificial satellites.

Name some satellites launched by India.

Ans. Some satellites launched by India are Aryabhatta,

INSAT, IRS, Kalpana – 1, Edusat, Bhaskar.

State uses of artificial satellites.

Ans. Uses of artificial satellites : (1) Making contact with

objects in space (2) Weather forecasting (3) Telecommunication,

broadcasting radio and TV programmes (4) Space research

(5) Implementing educational programmes (6) Preparing accurate

maps.

Q. 2 Write notes on the following : (1) Constellations

(2) Comets.

Ans. (1) Constellations : On a clear moonless night, we can

see thousands of stars in the sky. Some groups of stars appear to

form some kind of a pattern. Such groups of stars are known as

constellations. The International Astronomical Committee has so

far recognised 88 constellations. Of these, 37 constellations belong to

the northern hemisphere sky while 51 constellations belong to the

southern hemisphere sky. The constellations are given the names of

gods, goddesses, animals and other familiar objects.

(2) Comets : A comet is a massive celestial object composed of

different gases and dust, and moving around the sun in a highly

elliptical or parabolic path. As a comet approaches the sun, it

begins to absorb solar energy. Hence, the matter in the comet starts

vaporising and spreads behind the comet forming a tail on the side

opposite to the sun. It extends to millions of kilometres. The

luminous end of the comet pointing towards the sun is called the

head of the comet. Halley’s comet revolves around the sun with a

period of 76 years. It was last seen in 1986 when near the sun. Some

comets approach the sun only once and move far away from the sun,

not to return

Q. 3 Give scientific reasons :

* (1) We only see one side of the moon.

Ans. (1) The moon, while revolving around the earth, also

rotates about its axis. (2) The period of rotation and the period of

revolution of the moon are equal. Hence, we only see one side of

the moon.

* (2) Planets do not collide into one another.

Ans. Planets move around the sun in nearly circular orbits.

The radii of these orbits differ widely. Hence, planets do not collide

into one another.

* (3) Saturn is said to have unique characteristics.

Ans. Saturn has rings. The density of Saturn is less than that

of water. Therefore, Saturn is said to have unique characteristics.

* (4) We see the planets and stars moving from the east to

the west.

Ans. The earth rotates about its axis from the west to the east.

Hence, we see the planets and stars moving from the east to the

west.

* (5) Even as an India-England cricket match is played at

Lords, we can watch it live at home.

Ans. (1) Many of the satellites launched by ISRO are still

functioning. (2) They can transmit information to us about events

taking place at different locations on the earth’s surface. Hence,

even as an India-England cricket match is played at Lords, we can

watch it live at home.

(6) When a comet approaches the sun, it develops a long

tail.

Ans. (1) When a comet approaches the sun, it absorbs solar

energy at an increasing rate. Therefore, the matter in its outer

layers starts vaporising. (2) Therefore, the particles of gas and dust

in the comet spread behind the comet forming a long tail.

(7) When a meteoroid enters the earth’s atmosphere, it

starts burning.

Ans. When a meteoroid enters the earth’s atmosphere, there is

friction with air. This produces a lot of heat and high temperature.

Hence, the meteoroid starts burning.

Q. 4 Answer the following questions in one sentence

each :

(1) How many planets are there in the solar system?

Ans. There are eight planets in the solar system.

(2) How big is the sun relative to the earth ?

Ans. The sun is so big that it can accommodate 13 lakh earths

in it.

(3) Name the planet farthest from the sun.

Ans. The planet Neptune is the farthest planet from the sun.

(4) Name the two planets between whose orbits asteroids

are found.

Ans. Asteroids are found between the orbits of Mars and

Jupiter.

(5) State two uses of artificial satellites.

Ans. Artificial satellites are used for communication and

space research.

Q. 5 Find the odd man out and give the reason :

(1) Mercury, Venus, Mars, Sirius.

(2) Mercury, Mars, Jupiter, Saturn.

(3) Sun, Sirius, Pole Star, Venus.

Ans. (1) Sirius. It is a star. Others are planets. (2) Mercury.

It is an inferior planet. Others are superior planets. (3) Venus.

It is a planet. Others are stars.

Q. 6 State whether the following statements are True or

False. If a statement is false, correct it and rewrite :

(1) On any day, the moon rises about 50 minutes earlier than

on the previous day.

(2) The Scorpius can be seen in the sky of the southern

hemisphere just below the equator.

(3) Halley’s comet does not belong to the solar system.

(4) Mercury has two moons.

(5) Venus does not have any satellite.

(6) The density of Saturn is less than that of water.

(7) Like the moon, Venus also has the phases.

(8) Astronauts have their food directly from closed packets.

Ans. (1) False. On any day, the moon rises about 50 minutes

later than on the previous day. (2) True. (3) False. Halley’s comet

belongs to the solar system. (4) False. Mercury does not have any

moon. (5) True. (6) True. (7) True. (8) True.

Q. 7 Fill in the blanks :

1. * (1) There are altogether ……… constellations. Of these

37 belong to the northern hemisphere sky and ……… to the

southern hemisphere sky.

* (2) Jupiter has ……… moons.

* (3) ……… was India’s first artificial satellite.

* (4) It is because of ……… that we can forecast the weather.

(5) The period from one New Moon Day to the next is of ………

days.

Ans. (1) 88, 51 (2) 63 (3) Aryabhatta (4) artificial satellites

(5) 29.5.

Choose the correct alternative and complete the

following statements :

(1) ……… does not have a satellite.

(a) Mars (b) Mercury (c) Jupiter (d) Saturn

(2) Mars has ……… satellites.

(a) 4 (b) 3 (c) 2 (d) 8

(3) Saturn has ……… satellites.

(a) 53 (b) 8 (c) more than 60 (d) 50

(4) Asteroids revolve around the sun between the orbits of

……… .

(a) Mars and Jupiter (b) Jupiter and Saturn

(c) the earth and Mars (d) Venus and the earth

 (5) ……… are superior planets.

(a) Mercury and Venus (b) Jupiter and Saturn

(c) Mercury and Mars (d) Venus and Uranus

(6) ……… are inferior planets.

(a) Mercury and Venus (b) Venus and Saturn

(c) Uranus and Mercury (d) Mars and Jupiter

Ans. (1) Mercury (2) 2 (3) more than 60 (4) Mars and Jupiter

(5) Jupiter and Saturn (6) Mercury and Venus.

Q. 9 Match the following :

* [1] ‘A’ ‘B’

(1) Morning star

(2) The Great Bear

(3) IRS

(4) Asteroids

(5) Halley’s comet

(6) Rotation of the moon

(a) 27.3 days

(b) Between Mars and Jupiter

(c) Venus

(d) Constellation

(e) Artificial satellite

(f) 76 years

Fig. 1.3 : The

Great Bear

Ans. (1) Morning star – Venus

(2) The Great Bear – Constellation

(3) IRS – Artificial satellite

(4) Asteroids – Between Mars and Jupiter

(5) Halley’s comet – 76 years

(6) Rotation of the moon– 27.3 days.

[2] ‘A’ ‘B’

(1) Sun

(2) Mercury

(3) Saturn

(4) Comet

(5) Meteor

(6) Artificial satellite

(a) Communication

(b) Superior planet

(c) Bright line

(d) Star

(e) Long tail

(f) Inferior planet

Ans. (1) Sun – Star (2) Mercury– Inferior planet (3) Saturn –

Superior planet (4) Comet– Long tail (5) Meteor – Bright line

(6) Artificial satellite – Communication.