NCERT Textbook: Keeping Time with the Skies | Science Curiosity Class 8 - New NCERT PDF Download

 Page 1


170
Curiosity — Textbook of Science for Grade 8
Keeping Time 
with the Skies
11
Probe and ponder
 
 z Have you ever seen the Moon during the day? Why do you think 
it is sometimes visible when the Sun is up?
 z Imagine you lived on the Moon instead of Earth. What would 
you mean by a day, a month or a year?
 z What would happen if Earth had two moons instead of one? 
How would that change the night sky?
 z If we didn’t have clocks or calendars, how else could we  
measure time?
 z Share your questions 
  ?
Chapter 11.indd   170 Chapter 11.indd   170 6/28/2025   6:04:22 PM 6/28/2025   6:04:22 PM
Page 2


170
Curiosity — Textbook of Science for Grade 8
Keeping Time 
with the Skies
11
Probe and ponder
 
 z Have you ever seen the Moon during the day? Why do you think 
it is sometimes visible when the Sun is up?
 z Imagine you lived on the Moon instead of Earth. What would 
you mean by a day, a month or a year?
 z What would happen if Earth had two moons instead of one? 
How would that change the night sky?
 z If we didn’t have clocks or calendars, how else could we  
measure time?
 z Share your questions 
  ?
Chapter 11.indd   170 Chapter 11.indd   170 6/28/2025   6:04:22 PM 6/28/2025   6:04:22 PM
Chapter 11?—?Keeping Time with the Skies
171
It was Makar Sankranti, and Meera was in Ahmedabad for the 
Patang Mahotsav, the International Kite Festival. As she looked 
up at the sky filled with colourful kites, she noticed the Moon 
shining during the daytime. She was surprised as she had always 
thought the Moon appeared only at night. Also, the Moon did 
not appear like a full circle, but that didn’t surprise her as much. 
She knew its shape changed every night. Still, it got her thinking. 
She remembered learning that the Moon is spherical and shines 
by reflecting sunlight. Then why isn’t the whole Moon visible 
every night? For a moment, she wondered if it was due to a 
lunar eclipse. But eclipses are rare and brief. So, what causes the 
Moon’s changing shape?
11.1  How Does the Moon’s Appearance 
Change and Why?
Let us carefully watch the Moon to understand how its 
appearance changes over a month. You may have done a similar 
activity earlier, but let us now do it in more detail. Begin this 
activity from the sunrise after a full Moon day, that is when it is 
easiest to spot the Moon in the sky.
Activity 11.1: Let us explore
*
 z Spot the Moon at sunrise in the western direction on the first 
day after the full Moon.
 z Make a table similar to Table 11.1 in your notebook. Document 
the following:
 { Date
 { When you saw the Moon (at sunrise or sunset)?
 { Shade the corresponding Circle with pencil to show the 
bright portion of the Moon as shown in Fig. 11.1.
 z From the second day onwards also document the following.
 { Is the size of bright portion of the Moon increasing or 
decreasing from the previous day.
 { Whether the Moon appears closer to or farther from the 
Sun in the sky than the day before.
 z After about 15 days, you may not be able to see the Moon at 
sunrise or sunset. For the next 15 days, carry out this activity 
at sunset.
Fig. 11.1: Shading 
the dark portion 
of the Moon
*   It is best to begin this activity a month or more before this chapter 
is scheduled to be learnt.
Chapter 11.indd   171 Chapter 11.indd   171 6/28/2025   6:04:29 PM 6/28/2025   6:04:29 PM
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170
Curiosity — Textbook of Science for Grade 8
Keeping Time 
with the Skies
11
Probe and ponder
 
 z Have you ever seen the Moon during the day? Why do you think 
it is sometimes visible when the Sun is up?
 z Imagine you lived on the Moon instead of Earth. What would 
you mean by a day, a month or a year?
 z What would happen if Earth had two moons instead of one? 
How would that change the night sky?
 z If we didn’t have clocks or calendars, how else could we  
measure time?
 z Share your questions 
  ?
Chapter 11.indd   170 Chapter 11.indd   170 6/28/2025   6:04:22 PM 6/28/2025   6:04:22 PM
Chapter 11?—?Keeping Time with the Skies
171
It was Makar Sankranti, and Meera was in Ahmedabad for the 
Patang Mahotsav, the International Kite Festival. As she looked 
up at the sky filled with colourful kites, she noticed the Moon 
shining during the daytime. She was surprised as she had always 
thought the Moon appeared only at night. Also, the Moon did 
not appear like a full circle, but that didn’t surprise her as much. 
She knew its shape changed every night. Still, it got her thinking. 
She remembered learning that the Moon is spherical and shines 
by reflecting sunlight. Then why isn’t the whole Moon visible 
every night? For a moment, she wondered if it was due to a 
lunar eclipse. But eclipses are rare and brief. So, what causes the 
Moon’s changing shape?
11.1  How Does the Moon’s Appearance 
Change and Why?
Let us carefully watch the Moon to understand how its 
appearance changes over a month. You may have done a similar 
activity earlier, but let us now do it in more detail. Begin this 
activity from the sunrise after a full Moon day, that is when it is 
easiest to spot the Moon in the sky.
Activity 11.1: Let us explore
*
 z Spot the Moon at sunrise in the western direction on the first 
day after the full Moon.
 z Make a table similar to Table 11.1 in your notebook. Document 
the following:
 { Date
 { When you saw the Moon (at sunrise or sunset)?
 { Shade the corresponding Circle with pencil to show the 
bright portion of the Moon as shown in Fig. 11.1.
 z From the second day onwards also document the following.
 { Is the size of bright portion of the Moon increasing or 
decreasing from the previous day.
 { Whether the Moon appears closer to or farther from the 
Sun in the sky than the day before.
 z After about 15 days, you may not be able to see the Moon at 
sunrise or sunset. For the next 15 days, carry out this activity 
at sunset.
Fig. 11.1: Shading 
the dark portion 
of the Moon
*   It is best to begin this activity a month or more before this chapter 
is scheduled to be learnt.
Chapter 11.indd   171 Chapter 11.indd   171 6/28/2025   6:04:29 PM 6/28/2025   6:04:29 PM
172
Curiosity — Textbook of Science for Grade 8
Day Date
Moon seen 
at
Appearance 
of the Moon 
in the sky
Size of the 
bright portion 
compared to the 
previous day
Moon and Sun 
separation in the 
sky compared to 
the previous day
1. Sunrise/Sunset — —
2. Sunrise/Sunset
Increased/
Decreased
Closer/Farther
3. Sunrise/Sunset
Increased/
Decreased
Closer/Farther
Table 11.1: Documenting changes in the Moon’s appearance
Analyse the data recorded by you in Table 11.1. Did the Moon 
appear different each day? Was the Moon visible on all days? 
Did the Moon appear at the same position in the sky as on the 
previous day? 
11.1.1 Phases of the Moon
You may have observed that the 
bright portion of the Moon decreases 
from a full circle to a half circle 
in about a week, as shown in  
Fig. 11.2. The bright portion continues 
to shrink for another week until it 
is no longer visible. This two-week 
period is called the waning period of 
the Moon. Different names are given 
to the Moon’s visible shapes during 
this cycle (Fig. 11.2). The day when 
the Moon appears as a full bright 
circle is called the full Moon day (or 
Purnima), and the day when it is not 
visible is called the new Moon day 
(or Amavasya).
After the new Moon, its bright side 
grows to a half circle in about a week 
and to a full circle (full Moon) in another 
week. The period when the bright part 
of the Moon increases is called the 
Fig. 11.2: Waxing (Shukla Paksha) and waning period 
(Krishna Paksha) of the Moon as viewed from the Earth
Waxing
Gibbous Crescent
Crescent
Waning
Full Moon
About 1 week
About 1 week
About 1 week
About 1 week
New Moon
Gibbous
Chapter 11.indd   172 Chapter 11.indd   172 6/28/2025   6:04:37 PM 6/28/2025   6:04:37 PM
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170
Curiosity — Textbook of Science for Grade 8
Keeping Time 
with the Skies
11
Probe and ponder
 
 z Have you ever seen the Moon during the day? Why do you think 
it is sometimes visible when the Sun is up?
 z Imagine you lived on the Moon instead of Earth. What would 
you mean by a day, a month or a year?
 z What would happen if Earth had two moons instead of one? 
How would that change the night sky?
 z If we didn’t have clocks or calendars, how else could we  
measure time?
 z Share your questions 
  ?
Chapter 11.indd   170 Chapter 11.indd   170 6/28/2025   6:04:22 PM 6/28/2025   6:04:22 PM
Chapter 11?—?Keeping Time with the Skies
171
It was Makar Sankranti, and Meera was in Ahmedabad for the 
Patang Mahotsav, the International Kite Festival. As she looked 
up at the sky filled with colourful kites, she noticed the Moon 
shining during the daytime. She was surprised as she had always 
thought the Moon appeared only at night. Also, the Moon did 
not appear like a full circle, but that didn’t surprise her as much. 
She knew its shape changed every night. Still, it got her thinking. 
She remembered learning that the Moon is spherical and shines 
by reflecting sunlight. Then why isn’t the whole Moon visible 
every night? For a moment, she wondered if it was due to a 
lunar eclipse. But eclipses are rare and brief. So, what causes the 
Moon’s changing shape?
11.1  How Does the Moon’s Appearance 
Change and Why?
Let us carefully watch the Moon to understand how its 
appearance changes over a month. You may have done a similar 
activity earlier, but let us now do it in more detail. Begin this 
activity from the sunrise after a full Moon day, that is when it is 
easiest to spot the Moon in the sky.
Activity 11.1: Let us explore
*
 z Spot the Moon at sunrise in the western direction on the first 
day after the full Moon.
 z Make a table similar to Table 11.1 in your notebook. Document 
the following:
 { Date
 { When you saw the Moon (at sunrise or sunset)?
 { Shade the corresponding Circle with pencil to show the 
bright portion of the Moon as shown in Fig. 11.1.
 z From the second day onwards also document the following.
 { Is the size of bright portion of the Moon increasing or 
decreasing from the previous day.
 { Whether the Moon appears closer to or farther from the 
Sun in the sky than the day before.
 z After about 15 days, you may not be able to see the Moon at 
sunrise or sunset. For the next 15 days, carry out this activity 
at sunset.
Fig. 11.1: Shading 
the dark portion 
of the Moon
*   It is best to begin this activity a month or more before this chapter 
is scheduled to be learnt.
Chapter 11.indd   171 Chapter 11.indd   171 6/28/2025   6:04:29 PM 6/28/2025   6:04:29 PM
172
Curiosity — Textbook of Science for Grade 8
Day Date
Moon seen 
at
Appearance 
of the Moon 
in the sky
Size of the 
bright portion 
compared to the 
previous day
Moon and Sun 
separation in the 
sky compared to 
the previous day
1. Sunrise/Sunset — —
2. Sunrise/Sunset
Increased/
Decreased
Closer/Farther
3. Sunrise/Sunset
Increased/
Decreased
Closer/Farther
Table 11.1: Documenting changes in the Moon’s appearance
Analyse the data recorded by you in Table 11.1. Did the Moon 
appear different each day? Was the Moon visible on all days? 
Did the Moon appear at the same position in the sky as on the 
previous day? 
11.1.1 Phases of the Moon
You may have observed that the 
bright portion of the Moon decreases 
from a full circle to a half circle 
in about a week, as shown in  
Fig. 11.2. The bright portion continues 
to shrink for another week until it 
is no longer visible. This two-week 
period is called the waning period of 
the Moon. Different names are given 
to the Moon’s visible shapes during 
this cycle (Fig. 11.2). The day when 
the Moon appears as a full bright 
circle is called the full Moon day (or 
Purnima), and the day when it is not 
visible is called the new Moon day 
(or Amavasya).
After the new Moon, its bright side 
grows to a half circle in about a week 
and to a full circle (full Moon) in another 
week. The period when the bright part 
of the Moon increases is called the 
Fig. 11.2: Waxing (Shukla Paksha) and waning period 
(Krishna Paksha) of the Moon as viewed from the Earth
Waxing
Gibbous Crescent
Crescent
Waning
Full Moon
About 1 week
About 1 week
About 1 week
About 1 week
New Moon
Gibbous
Chapter 11.indd   172 Chapter 11.indd   172 6/28/2025   6:04:37 PM 6/28/2025   6:04:37 PM
Chapter 11?—?Keeping Time with the Skies
173
waxing period. In India, the waning period of the Moon is generally 
called the Krishna Paksha, while the waxing period is called the 
Shukla Paksha. The Moon goes through a waning period followed by 
a waxing period in a cyclical manner as shown in Fig. 11.2. The cycle 
from one full Moon to the next takes about a month.
The changing shapes of the bright portion of the Moon from 
one day to another as seen from the Earth are called the phases 
of the Moon.
11.1.2 Locating the Moon
When you checked the Moon at the same time on successive 
days (for example, at sunrise), did you see it in a different part 
of the sky? On a full Moon day, the Moon is nearly opposite the 
Sun — when the Sun rises in the East, the Moon is almost setting 
in the West. On subsequent mornings at sunrise, as its bright 
part continues to decrease, the Moon appears to move closer in 
the sky to the Sun. When the bright part of the Moon decreases 
to a half circle shape, the Moon is overhead at Sunrise. A few 
days later, the crescent Moon appears even closer to the Sun.  
Knowing the phase of the Moon and whether it is waxing or 
waning can thus help us find out where and when to look for 
the Moon on any given day. A waxing Moon is easiest to spot at 
sunset, and a waning Moon at sunrise. Because of these shifts, 
the Moon always rises and sets at different times than the Sun.
A step further
Many people believe the Moon rises when the Sun sets, but that 
is not always true. Look in a local newspaper or on the Positional 
Astronomy Centre (India Meteorological Department) website to find 
the moonrise time in your area. Check these times for several days 
in a row and you will see that the Moon rises about 50 minutes later 
each day. Sometimes moonrise happens in the afternoon (around 
2:00 – 4:00 p.m.), so you can spot the Moon in the eastern sky during 
daylight. You may need to wait about 30 minutes past the listed 
moonrise time for the Moon to climb high enough for it to be seen.
The time and position of moonrise changes from one day to the next.
7 April 2025
14:23
8 April 2025
15:17
Chapter 11.indd   173 Chapter 11.indd   173 6/28/2025   6:04:43 PM 6/28/2025   6:04:43 PM
Page 5


170
Curiosity — Textbook of Science for Grade 8
Keeping Time 
with the Skies
11
Probe and ponder
 
 z Have you ever seen the Moon during the day? Why do you think 
it is sometimes visible when the Sun is up?
 z Imagine you lived on the Moon instead of Earth. What would 
you mean by a day, a month or a year?
 z What would happen if Earth had two moons instead of one? 
How would that change the night sky?
 z If we didn’t have clocks or calendars, how else could we  
measure time?
 z Share your questions 
  ?
Chapter 11.indd   170 Chapter 11.indd   170 6/28/2025   6:04:22 PM 6/28/2025   6:04:22 PM
Chapter 11?—?Keeping Time with the Skies
171
It was Makar Sankranti, and Meera was in Ahmedabad for the 
Patang Mahotsav, the International Kite Festival. As she looked 
up at the sky filled with colourful kites, she noticed the Moon 
shining during the daytime. She was surprised as she had always 
thought the Moon appeared only at night. Also, the Moon did 
not appear like a full circle, but that didn’t surprise her as much. 
She knew its shape changed every night. Still, it got her thinking. 
She remembered learning that the Moon is spherical and shines 
by reflecting sunlight. Then why isn’t the whole Moon visible 
every night? For a moment, she wondered if it was due to a 
lunar eclipse. But eclipses are rare and brief. So, what causes the 
Moon’s changing shape?
11.1  How Does the Moon’s Appearance 
Change and Why?
Let us carefully watch the Moon to understand how its 
appearance changes over a month. You may have done a similar 
activity earlier, but let us now do it in more detail. Begin this 
activity from the sunrise after a full Moon day, that is when it is 
easiest to spot the Moon in the sky.
Activity 11.1: Let us explore
*
 z Spot the Moon at sunrise in the western direction on the first 
day after the full Moon.
 z Make a table similar to Table 11.1 in your notebook. Document 
the following:
 { Date
 { When you saw the Moon (at sunrise or sunset)?
 { Shade the corresponding Circle with pencil to show the 
bright portion of the Moon as shown in Fig. 11.1.
 z From the second day onwards also document the following.
 { Is the size of bright portion of the Moon increasing or 
decreasing from the previous day.
 { Whether the Moon appears closer to or farther from the 
Sun in the sky than the day before.
 z After about 15 days, you may not be able to see the Moon at 
sunrise or sunset. For the next 15 days, carry out this activity 
at sunset.
Fig. 11.1: Shading 
the dark portion 
of the Moon
*   It is best to begin this activity a month or more before this chapter 
is scheduled to be learnt.
Chapter 11.indd   171 Chapter 11.indd   171 6/28/2025   6:04:29 PM 6/28/2025   6:04:29 PM
172
Curiosity — Textbook of Science for Grade 8
Day Date
Moon seen 
at
Appearance 
of the Moon 
in the sky
Size of the 
bright portion 
compared to the 
previous day
Moon and Sun 
separation in the 
sky compared to 
the previous day
1. Sunrise/Sunset — —
2. Sunrise/Sunset
Increased/
Decreased
Closer/Farther
3. Sunrise/Sunset
Increased/
Decreased
Closer/Farther
Table 11.1: Documenting changes in the Moon’s appearance
Analyse the data recorded by you in Table 11.1. Did the Moon 
appear different each day? Was the Moon visible on all days? 
Did the Moon appear at the same position in the sky as on the 
previous day? 
11.1.1 Phases of the Moon
You may have observed that the 
bright portion of the Moon decreases 
from a full circle to a half circle 
in about a week, as shown in  
Fig. 11.2. The bright portion continues 
to shrink for another week until it 
is no longer visible. This two-week 
period is called the waning period of 
the Moon. Different names are given 
to the Moon’s visible shapes during 
this cycle (Fig. 11.2). The day when 
the Moon appears as a full bright 
circle is called the full Moon day (or 
Purnima), and the day when it is not 
visible is called the new Moon day 
(or Amavasya).
After the new Moon, its bright side 
grows to a half circle in about a week 
and to a full circle (full Moon) in another 
week. The period when the bright part 
of the Moon increases is called the 
Fig. 11.2: Waxing (Shukla Paksha) and waning period 
(Krishna Paksha) of the Moon as viewed from the Earth
Waxing
Gibbous Crescent
Crescent
Waning
Full Moon
About 1 week
About 1 week
About 1 week
About 1 week
New Moon
Gibbous
Chapter 11.indd   172 Chapter 11.indd   172 6/28/2025   6:04:37 PM 6/28/2025   6:04:37 PM
Chapter 11?—?Keeping Time with the Skies
173
waxing period. In India, the waning period of the Moon is generally 
called the Krishna Paksha, while the waxing period is called the 
Shukla Paksha. The Moon goes through a waning period followed by 
a waxing period in a cyclical manner as shown in Fig. 11.2. The cycle 
from one full Moon to the next takes about a month.
The changing shapes of the bright portion of the Moon from 
one day to another as seen from the Earth are called the phases 
of the Moon.
11.1.2 Locating the Moon
When you checked the Moon at the same time on successive 
days (for example, at sunrise), did you see it in a different part 
of the sky? On a full Moon day, the Moon is nearly opposite the 
Sun — when the Sun rises in the East, the Moon is almost setting 
in the West. On subsequent mornings at sunrise, as its bright 
part continues to decrease, the Moon appears to move closer in 
the sky to the Sun. When the bright part of the Moon decreases 
to a half circle shape, the Moon is overhead at Sunrise. A few 
days later, the crescent Moon appears even closer to the Sun.  
Knowing the phase of the Moon and whether it is waxing or 
waning can thus help us find out where and when to look for 
the Moon on any given day. A waxing Moon is easiest to spot at 
sunset, and a waning Moon at sunrise. Because of these shifts, 
the Moon always rises and sets at different times than the Sun.
A step further
Many people believe the Moon rises when the Sun sets, but that 
is not always true. Look in a local newspaper or on the Positional 
Astronomy Centre (India Meteorological Department) website to find 
the moonrise time in your area. Check these times for several days 
in a row and you will see that the Moon rises about 50 minutes later 
each day. Sometimes moonrise happens in the afternoon (around 
2:00 – 4:00 p.m.), so you can spot the Moon in the eastern sky during 
daylight. You may need to wait about 30 minutes past the listed 
moonrise time for the Moon to climb high enough for it to be seen.
The time and position of moonrise changes from one day to the next.
7 April 2025
14:23
8 April 2025
15:17
Chapter 11.indd   173 Chapter 11.indd   173 6/28/2025   6:04:43 PM 6/28/2025   6:04:43 PM
174
Curiosity — Textbook of Science for Grade 8
11.1.3 Making sense of our observations
The shape of the Moon itself does not change, only what 
we see changes. You may recall learning earlier that the 
Moon does not emit light of its own, but shines because it 
reflects sunlight that falls on it. The half of the Moon that 
faces the Sun receives sunlight and becomes illuminated 
(Fig. 11.3). The other half facing away from the Sun does 
not receive sunlight and remains non-illuminated.
The Moon revolves around the Earth and, only one 
half of the Moon always faces the Earth. However, the 
portion of the Moon facing the Earth is not always its 
illuminated part. We can only see the illuminated portion 
of the Moon from Earth. Sometimes, the entire illuminated portion 
of the Moon faces the Earth, and at other times only a part of it. 
At such times the illuminated portion of the Moon that we see is 
not a full circle. On New Moon day, we do not see the illuminated 
portion of the Moon at all, as only the non-illuminated portion of 
the Moon faces the Earth. Therefore, the Moon appears different 
on different days.
Let us do an activity to understand how the 
illuminated portion of the Moon, as seen by us, 
changes when its position changes with respect 
to the Sun.
Activity 11.2: Let us explore
 z Take a small soft ball and insert a stick into it (Fig. 11.4a). 
This represents the Moon.
 z Go to a dark open place (at night), and ask a teacher or 
guardian to shine a torchlight towards you from about 
3 m to represent light coming from the Sun or stand near an 
electric lamp. Your head represents the Earth.
 z Now hold the ball at arm’s length in one hand such that it is 
slightly above your head as shown in Fig. 11.4b. Keep the ball at 
position E towards the direction of the lamp. Does the portion 
of the ball facing you appear to be illuminated or not?
 z Turn around slowly , in the anti-clockwise direction, with your 
arm outstretched as shown in Fig. 11.4b and keep looking at 
the ball. Does the shape of the illuminated portion change? 
Is the line separating the illuminated and non-illuminated 
portions of the ball curved?
 z Was your observation similar to the changing shape of the 
illuminated portion of ball shown in Fig. 11.4c? The shape of 
the illuminated portion of the ball, as seen by you, changes 
depending on where the ball is with respect to the lamp. 
Fig. 11.3: Sunlight falling on 
the Moon
Sunrays
Illuminated
Non-
illuminated
Why does the illuminated 
portion of the Moon seen 
from the Earth decrease 
when it appears closer 
to the Sun?
Fig. 11.4: (a) Ball with stick
Chapter 11.indd   174 Chapter 11.indd   174 6/28/2025   6:04:51 PM 6/28/2025   6:04:51 PM