NCERT Textbook: Stars & the Solar System | General Science(Prelims) by IRS Divey Sethi - UPSC PDF Download

 Page 1


STARS AND THE
SOLAR SYSTEM
STARS AND THE
SOLAR SYSTEM
P
aheli and Boojho visited their
grandparents’ village during
the summer break. After dinner,
they went on to the roof of the house. It
was a clear cloudless night. They were
surprised to see a large number of  bright
stars in the sky. They had never seen such
a beautiful scene in their city (Fig. 17.1).
associated with them. Night sky
watching can be a fascinating experience
at a place where there are no bright lights
and the atmosphere is clear.
Look at the sky on a dark, clear night.
You see the entire sky dotted with
countless stars, some bright and some
not so bright. Observe them carefully.
Fig. 17.1: Fig. 17.1: Fig. 17.1: Fig. 17.1: Fig. 17.1: Night sky
Paheli wondered, why the village sky
was so different from the night sky in
big cities? Her grandfather explained that
due to bright light, smoke and dust,  the
sky in big cities is rarely clear.
He also identified some objects seen
in the night sky and told stories
Do all of them appear to twinkle? Do
you find any star-like object which does
not twinkle? The objects which do not
twinkle are planets.
The moon is the brightest object in
the night sky. The stars, the planets,
the moon and many other objects in the
sky are called celestial objects.
2022-23
Page 2


STARS AND THE
SOLAR SYSTEM
STARS AND THE
SOLAR SYSTEM
P
aheli and Boojho visited their
grandparents’ village during
the summer break. After dinner,
they went on to the roof of the house. It
was a clear cloudless night. They were
surprised to see a large number of  bright
stars in the sky. They had never seen such
a beautiful scene in their city (Fig. 17.1).
associated with them. Night sky
watching can be a fascinating experience
at a place where there are no bright lights
and the atmosphere is clear.
Look at the sky on a dark, clear night.
You see the entire sky dotted with
countless stars, some bright and some
not so bright. Observe them carefully.
Fig. 17.1: Fig. 17.1: Fig. 17.1: Fig. 17.1: Fig. 17.1: Night sky
Paheli wondered, why the village sky
was so different from the night sky in
big cities? Her grandfather explained that
due to bright light, smoke and dust,  the
sky in big cities is rarely clear.
He also identified some objects seen
in the night sky and told stories
Do all of them appear to twinkle? Do
you find any star-like object which does
not twinkle? The objects which do not
twinkle are planets.
The moon is the brightest object in
the night sky. The stars, the planets,
the moon and many other objects in the
sky are called celestial objects.
2022-23
SCIENCE 216
The study of celestial objects and
associated phenomena is called
astronomy. In acient India our
ancestors performed methodical
observations of sky. Their knowledge of
astronomy was highly advanced for
their time. Passage of the Sun, stars,
moon and planets in the sky helped
them to devise calenders and almanacs.
These were often used by people in their
day to day conduct and a better
understanding of climate and rainfall
patterns for timely sowing and choice
of crops, fixing the dates of seasons and
festivals.
Let us also make some observations
of celestial objects and learn about them.
17.1 The Moon
Activity 17.1
Observe the moon continuously for
several nights, preferably from one
full moon to the next. Make a sketch
of the moon every night in your note
book and note the day from the day
of the full moon. Also note everyday
the part of the sky (east or west) in
which the moon is seen.
Is there a change in the shape of the
moon everyday? Are there days when
the shape of the moon appears to be
perfectly round? Are there days when
the moon cannot be seen at all even if
the sky is clear?
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 and thinner.
On the fifteenth day the moon is not
visible. This day is known as the ‘new
moon day’. 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. On the fifteenth day once again we
get a full view of the moon.
The various shapes of the bright part
of the moon as seen during a month
are called phases of the moon (Fig. 17.2).
Phases of the moon play an important
role in our social life. Almost all festivals
in India are celebrated according to the
phases of the moon. For example, Diwali
is celebrated on the new moon day;
Budh Poornima and Guru Nanak’s
birthday are celebrated on full moon
day; Maha Shivratri is celebrated on
thirteenth night of waning moon; Eid-
ul-Fitr is observed on the day following
the sighting of crescent moon.
Fig.17.2 : Phases of the moon
The time period between one full moon
to the next full moon is slightly longer
than 29 days. In many calendars this
period is called a month.
Why does the moon change
its shape every day?
2022-23
Page 3


STARS AND THE
SOLAR SYSTEM
STARS AND THE
SOLAR SYSTEM
P
aheli and Boojho visited their
grandparents’ village during
the summer break. After dinner,
they went on to the roof of the house. It
was a clear cloudless night. They were
surprised to see a large number of  bright
stars in the sky. They had never seen such
a beautiful scene in their city (Fig. 17.1).
associated with them. Night sky
watching can be a fascinating experience
at a place where there are no bright lights
and the atmosphere is clear.
Look at the sky on a dark, clear night.
You see the entire sky dotted with
countless stars, some bright and some
not so bright. Observe them carefully.
Fig. 17.1: Fig. 17.1: Fig. 17.1: Fig. 17.1: Fig. 17.1: Night sky
Paheli wondered, why the village sky
was so different from the night sky in
big cities? Her grandfather explained that
due to bright light, smoke and dust,  the
sky in big cities is rarely clear.
He also identified some objects seen
in the night sky and told stories
Do all of them appear to twinkle? Do
you find any star-like object which does
not twinkle? The objects which do not
twinkle are planets.
The moon is the brightest object in
the night sky. The stars, the planets,
the moon and many other objects in the
sky are called celestial objects.
2022-23
SCIENCE 216
The study of celestial objects and
associated phenomena is called
astronomy. In acient India our
ancestors performed methodical
observations of sky. Their knowledge of
astronomy was highly advanced for
their time. Passage of the Sun, stars,
moon and planets in the sky helped
them to devise calenders and almanacs.
These were often used by people in their
day to day conduct and a better
understanding of climate and rainfall
patterns for timely sowing and choice
of crops, fixing the dates of seasons and
festivals.
Let us also make some observations
of celestial objects and learn about them.
17.1 The Moon
Activity 17.1
Observe the moon continuously for
several nights, preferably from one
full moon to the next. Make a sketch
of the moon every night in your note
book and note the day from the day
of the full moon. Also note everyday
the part of the sky (east or west) in
which the moon is seen.
Is there a change in the shape of the
moon everyday? Are there days when
the shape of the moon appears to be
perfectly round? Are there days when
the moon cannot be seen at all even if
the sky is clear?
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 and thinner.
On the fifteenth day the moon is not
visible. This day is known as the ‘new
moon day’. 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. On the fifteenth day once again we
get a full view of the moon.
The various shapes of the bright part
of the moon as seen during a month
are called phases of the moon (Fig. 17.2).
Phases of the moon play an important
role in our social life. Almost all festivals
in India are celebrated according to the
phases of the moon. For example, Diwali
is celebrated on the new moon day;
Budh Poornima and Guru Nanak’s
birthday are celebrated on full moon
day; Maha Shivratri is celebrated on
thirteenth night of waning moon; Eid-
ul-Fitr is observed on the day following
the sighting of crescent moon.
Fig.17.2 : Phases of the moon
The time period between one full moon
to the next full moon is slightly longer
than 29 days. In many calendars this
period is called a month.
Why does the moon change
its shape every day?
2022-23
STARS AND THE SOLAR SYSTEM 217
Fig. 17.4: The moon appears different at different positions in its orbit
Let us try to understand why phases
of the moon occur. You have studied in
Chapter 16 that the moon does not
produce its own light, whereas the Sun
and other stars do. We see the moon
because the sunlight falling on it gets
reflected towards us (Fig. 17.3). We,
therefore, see only that part of the moon,
from which the light of the Sun is
reflected towards us.
Activity 17.2
Take a big ball or a pitcher. Paint
half of it white and half black.
Go out into the playground with
two of your friends. Draw a circle of
radius of about 2 m on the ground.
Divide the circle into eight equal
parts as shown in Fig. 17.4.
Stand at the centre of the circle.
Ask a friend to hold the ball at
different points of the circle. Ask her
to keep the white portion of the ball
always towards the Sun. If you are
performing this activity in the
morning then the white portion of
the ball should be kept towards the
east. If the activity is being
performed in the afternoon then the
white portion of the ball should be
kept towards the west. In each case
the line dividing the white and
black portions is kept vertical.
Standing at the centre of the
circle observe the visible white
portion of the ball while your friend
stands at the points on the circle
marked earlier. Draw the shape of
the white portion as you see it.
Compare your drawings with the
different phases of the moon as
shown in Fig. 17.5.
Fig. 17.3 : Moon is visible due to reflected
sunlight
2022-23
Page 4


STARS AND THE
SOLAR SYSTEM
STARS AND THE
SOLAR SYSTEM
P
aheli and Boojho visited their
grandparents’ village during
the summer break. After dinner,
they went on to the roof of the house. It
was a clear cloudless night. They were
surprised to see a large number of  bright
stars in the sky. They had never seen such
a beautiful scene in their city (Fig. 17.1).
associated with them. Night sky
watching can be a fascinating experience
at a place where there are no bright lights
and the atmosphere is clear.
Look at the sky on a dark, clear night.
You see the entire sky dotted with
countless stars, some bright and some
not so bright. Observe them carefully.
Fig. 17.1: Fig. 17.1: Fig. 17.1: Fig. 17.1: Fig. 17.1: Night sky
Paheli wondered, why the village sky
was so different from the night sky in
big cities? Her grandfather explained that
due to bright light, smoke and dust,  the
sky in big cities is rarely clear.
He also identified some objects seen
in the night sky and told stories
Do all of them appear to twinkle? Do
you find any star-like object which does
not twinkle? The objects which do not
twinkle are planets.
The moon is the brightest object in
the night sky. The stars, the planets,
the moon and many other objects in the
sky are called celestial objects.
2022-23
SCIENCE 216
The study of celestial objects and
associated phenomena is called
astronomy. In acient India our
ancestors performed methodical
observations of sky. Their knowledge of
astronomy was highly advanced for
their time. Passage of the Sun, stars,
moon and planets in the sky helped
them to devise calenders and almanacs.
These were often used by people in their
day to day conduct and a better
understanding of climate and rainfall
patterns for timely sowing and choice
of crops, fixing the dates of seasons and
festivals.
Let us also make some observations
of celestial objects and learn about them.
17.1 The Moon
Activity 17.1
Observe the moon continuously for
several nights, preferably from one
full moon to the next. Make a sketch
of the moon every night in your note
book and note the day from the day
of the full moon. Also note everyday
the part of the sky (east or west) in
which the moon is seen.
Is there a change in the shape of the
moon everyday? Are there days when
the shape of the moon appears to be
perfectly round? Are there days when
the moon cannot be seen at all even if
the sky is clear?
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 and thinner.
On the fifteenth day the moon is not
visible. This day is known as the ‘new
moon day’. 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. On the fifteenth day once again we
get a full view of the moon.
The various shapes of the bright part
of the moon as seen during a month
are called phases of the moon (Fig. 17.2).
Phases of the moon play an important
role in our social life. Almost all festivals
in India are celebrated according to the
phases of the moon. For example, Diwali
is celebrated on the new moon day;
Budh Poornima and Guru Nanak’s
birthday are celebrated on full moon
day; Maha Shivratri is celebrated on
thirteenth night of waning moon; Eid-
ul-Fitr is observed on the day following
the sighting of crescent moon.
Fig.17.2 : Phases of the moon
The time period between one full moon
to the next full moon is slightly longer
than 29 days. In many calendars this
period is called a month.
Why does the moon change
its shape every day?
2022-23
STARS AND THE SOLAR SYSTEM 217
Fig. 17.4: The moon appears different at different positions in its orbit
Let us try to understand why phases
of the moon occur. You have studied in
Chapter 16 that the moon does not
produce its own light, whereas the Sun
and other stars do. We see the moon
because the sunlight falling on it gets
reflected towards us (Fig. 17.3). We,
therefore, see only that part of the moon,
from which the light of the Sun is
reflected towards us.
Activity 17.2
Take a big ball or a pitcher. Paint
half of it white and half black.
Go out into the playground with
two of your friends. Draw a circle of
radius of about 2 m on the ground.
Divide the circle into eight equal
parts as shown in Fig. 17.4.
Stand at the centre of the circle.
Ask a friend to hold the ball at
different points of the circle. Ask her
to keep the white portion of the ball
always towards the Sun. If you are
performing this activity in the
morning then the white portion of
the ball should be kept towards the
east. If the activity is being
performed in the afternoon then the
white portion of the ball should be
kept towards the west. In each case
the line dividing the white and
black portions is kept vertical.
Standing at the centre of the
circle observe the visible white
portion of the ball while your friend
stands at the points on the circle
marked earlier. Draw the shape of
the white portion as you see it.
Compare your drawings with the
different phases of the moon as
shown in Fig. 17.5.
Fig. 17.3 : Moon is visible due to reflected
sunlight
2022-23
SCIENCE 218
Fig. 17.5: Positions of the moon in its orbit and its corresponding phases
Fig. 17.6 : Earth accompanied by moon
revolving around the Sun
I have heard that we
never see the back side of
the moon from the Earth.
Is it true?
Remember that the moon revolves
around the Earth. The Earth along
with the moon, revolves around the
Sun (Fig. 17.6).
these positions in your notebook. In
which part of the sky would you look
for the full moon?
The size of the illuminated part of
the moon visible from 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 every day.
Can you now guess the relative
positions of the Sun, moon and the
Earth on the day of the full moon and
on the day of the new moon? Sketch
Activity 17.3
Draw a circle of about 1m diameter
on the ground. Ask one of your
friends to stand at the centre of this
circle. You revolve around your
2022-23
Page 5


STARS AND THE
SOLAR SYSTEM
STARS AND THE
SOLAR SYSTEM
P
aheli and Boojho visited their
grandparents’ village during
the summer break. After dinner,
they went on to the roof of the house. It
was a clear cloudless night. They were
surprised to see a large number of  bright
stars in the sky. They had never seen such
a beautiful scene in their city (Fig. 17.1).
associated with them. Night sky
watching can be a fascinating experience
at a place where there are no bright lights
and the atmosphere is clear.
Look at the sky on a dark, clear night.
You see the entire sky dotted with
countless stars, some bright and some
not so bright. Observe them carefully.
Fig. 17.1: Fig. 17.1: Fig. 17.1: Fig. 17.1: Fig. 17.1: Night sky
Paheli wondered, why the village sky
was so different from the night sky in
big cities? Her grandfather explained that
due to bright light, smoke and dust,  the
sky in big cities is rarely clear.
He also identified some objects seen
in the night sky and told stories
Do all of them appear to twinkle? Do
you find any star-like object which does
not twinkle? The objects which do not
twinkle are planets.
The moon is the brightest object in
the night sky. The stars, the planets,
the moon and many other objects in the
sky are called celestial objects.
2022-23
SCIENCE 216
The study of celestial objects and
associated phenomena is called
astronomy. In acient India our
ancestors performed methodical
observations of sky. Their knowledge of
astronomy was highly advanced for
their time. Passage of the Sun, stars,
moon and planets in the sky helped
them to devise calenders and almanacs.
These were often used by people in their
day to day conduct and a better
understanding of climate and rainfall
patterns for timely sowing and choice
of crops, fixing the dates of seasons and
festivals.
Let us also make some observations
of celestial objects and learn about them.
17.1 The Moon
Activity 17.1
Observe the moon continuously for
several nights, preferably from one
full moon to the next. Make a sketch
of the moon every night in your note
book and note the day from the day
of the full moon. Also note everyday
the part of the sky (east or west) in
which the moon is seen.
Is there a change in the shape of the
moon everyday? Are there days when
the shape of the moon appears to be
perfectly round? Are there days when
the moon cannot be seen at all even if
the sky is clear?
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 and thinner.
On the fifteenth day the moon is not
visible. This day is known as the ‘new
moon day’. 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. On the fifteenth day once again we
get a full view of the moon.
The various shapes of the bright part
of the moon as seen during a month
are called phases of the moon (Fig. 17.2).
Phases of the moon play an important
role in our social life. Almost all festivals
in India are celebrated according to the
phases of the moon. For example, Diwali
is celebrated on the new moon day;
Budh Poornima and Guru Nanak’s
birthday are celebrated on full moon
day; Maha Shivratri is celebrated on
thirteenth night of waning moon; Eid-
ul-Fitr is observed on the day following
the sighting of crescent moon.
Fig.17.2 : Phases of the moon
The time period between one full moon
to the next full moon is slightly longer
than 29 days. In many calendars this
period is called a month.
Why does the moon change
its shape every day?
2022-23
STARS AND THE SOLAR SYSTEM 217
Fig. 17.4: The moon appears different at different positions in its orbit
Let us try to understand why phases
of the moon occur. You have studied in
Chapter 16 that the moon does not
produce its own light, whereas the Sun
and other stars do. We see the moon
because the sunlight falling on it gets
reflected towards us (Fig. 17.3). We,
therefore, see only that part of the moon,
from which the light of the Sun is
reflected towards us.
Activity 17.2
Take a big ball or a pitcher. Paint
half of it white and half black.
Go out into the playground with
two of your friends. Draw a circle of
radius of about 2 m on the ground.
Divide the circle into eight equal
parts as shown in Fig. 17.4.
Stand at the centre of the circle.
Ask a friend to hold the ball at
different points of the circle. Ask her
to keep the white portion of the ball
always towards the Sun. If you are
performing this activity in the
morning then the white portion of
the ball should be kept towards the
east. If the activity is being
performed in the afternoon then the
white portion of the ball should be
kept towards the west. In each case
the line dividing the white and
black portions is kept vertical.
Standing at the centre of the
circle observe the visible white
portion of the ball while your friend
stands at the points on the circle
marked earlier. Draw the shape of
the white portion as you see it.
Compare your drawings with the
different phases of the moon as
shown in Fig. 17.5.
Fig. 17.3 : Moon is visible due to reflected
sunlight
2022-23
SCIENCE 218
Fig. 17.5: Positions of the moon in its orbit and its corresponding phases
Fig. 17.6 : Earth accompanied by moon
revolving around the Sun
I have heard that we
never see the back side of
the moon from the Earth.
Is it true?
Remember that the moon revolves
around the Earth. The Earth along
with the moon, revolves around the
Sun (Fig. 17.6).
these positions in your notebook. In
which part of the sky would you look
for the full moon?
The size of the illuminated part of
the moon visible from 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 every day.
Can you now guess the relative
positions of the Sun, moon and the
Earth on the day of the full moon and
on the day of the new moon? Sketch
Activity 17.3
Draw a circle of about 1m diameter
on the ground. Ask one of your
friends to stand at the centre of this
circle. You revolve around your
2022-23
STARS AND THE SOLAR SYSTEM 219
friend in such a manner that your
face always remains towards him.
Can your friend see your back? How
many rotations did you complete in
one revolution? The moon revolves
around the Earth in a similar
manner.
The moon completes one rotation on
its axis as it completes one revolution
around the Earth.
The Moon’s Surface
The moon is a fascinating object for
poets and story-tellers. But when
astronauts landed on the moon, they
found that the moon’s surface is dusty
and barren. There are many craters of
different sizes. It also has a large number
of steep and high mountains (Fig. 17.7).
Some of these are as high as the highest
mountains on the Earth.
Can we hear any sound
on the moon?
We learnt in Chapter 13
that sound cannot
travel when there is no
medium. Then, how can
we hear any sound on
the moon?
Fig. 17.7 : Surface of the moon
The moon has no atmosphere. It has
no water. Can any life exist on the
moon?
Did you know?
On July 21, 1969 (Indian time) the
American astronaut, Neil Armstrong,
landed on the moon for the first time.
He was followed by Edwin Aldrin.
17.2 The Stars
What other objects do you see in the
night sky? There is a large number of
stars in the sky. Observe carefully on a
dark night and from a place away from
a big city. Are all the stars equally
NASA
NASA
Fig. 17.8 : An astronaut on the moon
2022-23