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

?
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 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.
Fig. 11.1: Shading
{ Is the size of bright portion of the Moon increasing or the dark portion
decreasing from the previous day. of the Moon
{ 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.

* I t is best to begin this activity a month or more before this chapter

is scheduled to be learnt.

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 Table 11.1: Documenting changes in the Moon’s appearance

Size of the Moon and Sun

Appearance
Moon seen bright portion separation in the
Day Date of the Moon
at compared to the sky compared to
in the sky
previous day the previous day

1. Sunrise/Sunset — —

Increased/
2. Sunrise/Sunset Closer/Farther
Decreased

Increased/
3. Sunrise/Sunset Closer/Farther
Decreased

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

k You may have observed that the
ee bright portion of the Moon decreases
Ab

w
ou

1
ut from a full circle to a half circle
t1

o
Ab
w

Gibbous Crescent in about a week, as shown in

ee

Waxing
k

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
New Moon
the Moon. Different names are given
Full Moon
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
Ab Gibbous Waning Crescent Purnima), and the day when it is not
ou k
t1 ee visible is called the new Moon day
w
w 1
ee out (or Amavasya).
k
Ab After the new Moon, its bright side
grows to a half circle in about a week
Fig. 11.2: Waxing (Shukla Paksha) and waning period and to a full circle (full Moon) in another
(Krishna Paksha) of the Moon as viewed from the Earth week. The period when the bright part
of the Moon increases is called the

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 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.

7 April 2025 8 April 2025

14:23 15:17

The time and position of moonrise changes from one day to the next.

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 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
Sunrays

Illuminated Non-
reflects sunlight that falls on it. The half of the Moon that
illuminated
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
Fig. 11.3: Sunlight falling on half of the Moon always faces the Earth. However, the
the Moon 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.
Why does the illuminated
Let us do an activity to understand how the
portion of the Moon seen
from the Earth decrease illuminated portion of the Moon, as seen by us,
when it appears closer changes when its position changes with respect
to the Sun? 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
Fig. 11.4: (a) Ball with stick 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.
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 Chapter 11 — Keeping Time with the Skies 175

H G F

A E
B C D

(b)
A B C D E F G H A

(c)
Fig. 11.4: (b) A student using a ball and stick to understand phases of
Moon; (c) The ball as seen by the student at different positions
When the ball is held opposite to the direction of the lamp
(at A), you are facing the entire illuminated portion of the ball,
just like the full Moon day. On the other hand, when the ball
is held towards the direction of the lamp (at E), you are facing
the non-illuminated portion of the ball, and cannot see the
illuminated portion of the ball at all. This is similar to the new
Moon day. Notice how in other cases, the line separating the
illuminated and non-illuminated portions of the ball appears
curved (Fig. 11.4c), similar to the shape of the illuminated portion
of the Moon viewed from the Earth on other days.
Using our observations of Activity 11.2, let us now try to
understand the phases of Moon. Fig. 11.5a shows the positions of
the Moon corresponding to the different positions of the ball in
Fig. 11.4b. Also shown are the Earth and the sunrays. As shown
in Fig. 11.5a, the Moon revolves once around the Earth from
position A to H and back to position A in about one month. The
side of the Moon that faces the Sun is illuminated.

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 The portion of the Moon that faces the Earth is marked by the
orange dashed lines and arrows. The illuminated portion of only
this part of the Moon can be seen from the Earth. At positions
B and H, more than half of the illuminated portion, called the
gibbous phase, can be seen. At positions D and F, less than half
of the illuminated portion, called the crescent phase, can be
seen. The change in the fraction of the illuminated portion of the
Moon seen from Earth causes phases of the Moon.
The phases that will be seen from the Earth at different
positions of the Moon are shown in Fig. 11.5b. From A to C to E,
we see the waning phase, and from E to G and back to A, we
see the waxing phase. Since the rotation period of the Earth of
one day is much smaller compared to the revolution period of
the Moon which is nearly a month, on a given day, people on
different parts of the Earth see nearly the same phase.
As can be seen in Fig. 11.5a, on the New Moon day, the Moon
appears closest to the Sun and it appears farthest on the Full
Moon day. Is this not what we also observed in Activity 11.1?
Half of the illuminated
portion of Moon can be seen

G Less than half of the

More than half of the
illuminated portion Day 22 illuminated portion
H F of Moon can be seen
of Moon can be seen Day 26 Day 18
(Gibbous phase) (Crescent phase)

Illuminated
Entire illuminated Day 1 Day 15 portion of
SUNRAYS
portion of Moon A E
Moon cannot
can be seen
be seen (a)
(Full Moon)
(New Moon)
Day 4 Day 12

Day 8
More than half of the B Less than half of the
D
illuminated portion illuminated portion
of Moon can be seen of Moon can be seen
(Gibbous phase) (Crescent phase)
C
Half of the illuminated
portion of Moon can be seen
A B C D E F G H A

(b)

Fig. 11.5: (a) The Moon at different positions in its orbit around the Earth; (b) The corresponding
phases of the Moon as seen from the Earth. (The sizes and the distances are not to scale in this figure.)

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 Chapter 11 — Keeping Time with the Skies 177

In Activity 11.1, we (b) In the same time of

also observed that the one day, the Moon would Moon’s orbit around
revolve a bit further in its the Earth
position of the Moon orbit around the Earth.
at sunrise (or sunset)
appeared to be shifted
on successive days.
This happens because,
(c) So I will find the (a) Today the Moon is
as shown in Fig. 11.6, overhead. The Earth
Moon overhead about 50
the Moon moves ahead minutes later than today, will rotate once in one
in its orbit while the when the Earth has day, and I will be back
Earth completes one rotated some more. at this same position.
rotation about its axis
in 24 hours. Earth
needs to rotate some
more for the Moon to
appear in nearly the Fig. 11.6: The Moon takes about 50 minutes longer to
same spot in the sky. come back to nearly the same position in the sky.

A step further
The Moon phases do not happen due to Earth’s shadow. It is an incorrect
explanation for the Moon’s phases that Earth’s shadow falls on it. As we
have learnt, the phases of the Moon occur due to the relative change in
orientation of the Sun, Moon, and Earth as the Moon revolves around
the Earth. The Earth’s shadow on the Moon causes a lunar eclipse, not
the Moon’s phases as we learnt earlier (in chapter ‘Earth, Moon and the
Sun’ in the Curiosity, Grade 7).
Lunar eclipses can only happen on a full Moon day and solar eclipses
can happen only on a new Moon day. But they do not occur every month
because of the small tilt of the Moon’s orbit with respect to the Earth’s
orbit around the Sun. Position
Earth’s orbit favourable for
around Sun eclipses Lunar eclipse at
Moon’s slightly this position No eclipses
tilted orbit
on these full
around Earth
and new
Moon days
Solar eclipse
Sun at these
No eclipses positions
on these full
and new
Moon days
Position
favourable for
eclipses Lunar eclipse at
this position

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 So, changing phases of the
Moon is a natural periodic Yes, along with the natural periodic
event, with a cycle of almost events of day and night and the
a month, which can also be changing seasons about which we
used for time keeping. learnt earlier. But how are these
periodic events used for keeping time?

11.2 How Did Calendars Come into Existence?

We have learnt earlier that when viewed from the Earth, the Sun
appears to rise in the eastward direction, set in the westward
direction every day, and rise again the next day. This apparent
periodic motion of the Sun seen by us is primarily due to the
rotation of the Earth around its own axis. This natural cycle of
the Sun due to the rotation of the Earth, is the foundation of the
day, a unit to measure time.
The average time that the Sun takes to go from its highest
position in the sky on one day to the highest position in the
sky the next day, is 24 hours, and is called the mean solar
day. The highest position of the Sun in the sky can be found by
measuring the length of the shadows cast by an object during
the day. The shadow is shortest when the Sun is at the highest
point in the sky.

Activity 11.3: Let us measure a day!

z Find a small flat area in a ground which receives sunlight
during the day. Fix a 1 m stick vertically in it as shown in
Fig. 11.7.
z Start observing at 11:00 a.m. Every
minute, mark a dot on the ground
at the tip of the stick’s shadow. Keep
marking dots until around 1:10 p.m.
z Identify when the shadow was
shortest and find out its time by
counting the number of dots. Record
this time in Table 11.2. Repeat this
exercise for the next few days.
z Find the duration of the solar day
by finding a difference in time on
two consecutive days as shown in
Fig. 11.7: Observing the length of the shadow
Table 11.1.
at different times during the day.

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 Chapter 11 — Keeping Time with the Skies 179

Table 11.2: Finding the duration of a solar day

Time of shortest shadow

Date Duration of day (hh:mm)
(hh:mm)

22 March 2025 12:20 ——

23 March 2025 12:20 24:00

24 March 2025 12:19 23:59

Find the average duration of the day. Is it nearly equal to

24 hours?
The phases of the Moon give us
another natural cycle with a duration Cycle of Moon’s phases
that is longer than a day. The Moon Cycle of Earth’s seasons due to its revolution
due to its revolution around the Earth
takes about 29.5 days (nearly a month) around the Sun define defines a month
to cycle through all its phases. The a year
cycle of the phases of the Moon is the
basis for a month, another unit to
measure time (Fig. 11.8).
The next larger unit to measure
time is related to the natural cycle of
seasons. Do you remember learning Cycle of Sun returning
to its highest position
earlier that the Earth revolves in the sky due to Earth’s
around the Sun and takes nearly 365 rotation defines a day
and a quarter days to complete one
Fig. 11.8: The Earth goes around the Sun, and
revolution around the Sun? The Earth the Moon revolves around the Earth in regular
undergoes one cycle of seasons during intervals of time.
this time, which can be used to define
a solar year (Fig. 11.8).

11.2.1 Lunar calendars

In ancient times, people had noticed that during one cycle of
seasons, one can fit nearly 12 cycles of the phases of the moon,
that is, 12 lunar months. This is how lunar calendars came
into being, with the day as the shortest unit, a month of nearly
29.5 days, and a lunar year consisting of 12 lunar months. The
phases of the Moon thus gave an easy and a perfectly sound way
to track the passage of time.
However, in a lunar calendar the seasons do not remain
synchronised to the same lunar months in successive lunar years.
The reason is that the seasons repeat in approximately 365 days
while the lunar year is 354 days long.

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 11.2.2 Solar calendars
It was important to know the arrival of seasons for agricultural
purposes. This need for a year to synchronise with seasons led to
the creation of solar calendars. The Gregorian calendar, widely
used today, is a solar calendar. The months in solar calendars are
adjusted to add up to 365 days. That is why in Gregorian calendars,
some months have 30 days, others 31, and February has only
28 days.
On top of the 365 days, the Earth takes nearly an extra quarter
of a day to complete one revolution around the Sun. These extra
hours add up to approximately one day every four years. To
adjust for this, solar calendars add an extra day every four years
using the concept of a leap year. In the Gregorian calendar, if a
year is divisible by four, then an extra leap day is added. So in a
leap year, February has 29 days, which keeps the calendar well
synchronised with the seasons.

A step further
The Earth takes slightly less time than 365 and a quarter day to
go from one spring equinox to the next spring equinox. Adding a
day every four years helps to synchronise with the seasons, but
it actually adds a little too much over time. To fix this, leap years
are skipped every 100 years—like in 1700, 1800, and 1900. But
skipping all of them would make the calendar lag slightly behind.
So every 400 years, a leap year is again added back—like in 1600
and 2000. These careful corrections keep the calendar closely
matched with the seasons over long periods of time!

A step further

As we learnt earlier, seasons are caused by the Earth’s revolution

around the Sun and its movement from the spring equinox to
winter equinox and back. The time between successive spring
equinoxes is called the tropical year. Gregorian calendar is based
upon tropical year.
We have also learnt earlier that the stars that rise at sunset
change throughout the year due to the Earth’s revolution around
the Sun. The time duration required for the same stars to rise
again at sunset is called the sidereal year, and it can also be used to
define a solar calendar. The sidereal year is longer than the tropical
year by a mere 20 minutes, and so it takes a long time before
the differences between the two calendars become noticeable.
In modern times, astronomers use the sidereal year to keep track of
the Earth’s position in its orbit around the Sun.

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 Chapter 11 — Keeping Time with the Skies 181

Our scientific heritage

For thousands of years, people — including those in India — have been
observing the sky and developing calendars. People in ancient times did not
know that Earth revolves around the Sun, and lacked modern instruments.
Yet through years of careful sky observations, they noticed patterns and cycles
in natural events. Hence, they could Sun rises exactly in the Path of Sun on
determine that the length of the year East and sets exactly in June solstice
the west on equinoxes
was approximately 365 days allowing
Path of Sun
them to create calendars.
on December
For example, careful observation solstice
reveals that the Sun does not always
rise exactly in the East. In summer,
West
it rises a little northward of East
South North
and in winter a little southward of
East. These extremes happen on East
the solstices, around June 21 and
December 21 each year. The Sun’s Uttarayan from December to June
apparent northward movement from December to June is called Uttarayan,
and its apparent southward movement from June to December is Dakshinayan.
This cycle repeats every year and is closely linked to the changing seasons. The
Taittirīya Saṁhitā records it in the verse 6.5.3:
तस्‍‍माादाादि�त्‍‍य: षण्माासाा दक्षि�णेेनैैति� षडुुत्तरेेण ।
“Thus the Sun moves southwards for six months and northwards for six
months.”
In the past, the equinoxes and solstices were also tracked by identifying the
stars that rose at sunset. Ancient Indian texts like the Surya Siddhanta noted
that the pattern of stars, Capricorn (called Makar in India), would be in the
background of the Sun around the winter solstice during those ancient times.
भाानोोर्ममकरसंस् ं ‍ङ्
् ् क्राान्तेःः� षण्माासाा उत्तराायणम्् ।
कर्क्‍का��ा�देेस्तुु तथैैव स्याात्् षण्माासाा दक्षि�णाायनम्् ।।9।।
Translation: From the moment of the Sun’s entrance into the constellation
of Capricorn, six months make up its northward progress (Uttarayana), so
likewise from the moment of entrance into the constellation of Cancer, six
months are its southward progress (Dakshinayana).
Over the years, different types of calendars have evolved based on specific
needs. A number of these calendars are used in different parts of India to track
time and celebrating festivals.

11.2.3 Luni-solar calendars

There is another kind of calendar which primarily uses the Moon’s
phases for counting days and months but also make adjustments
to stay in sync with the cycle of seasons.
The 12 lunar months add up to 354 days and thus fall short by
nearly 11 days compared to the solar year. Thus every 2–3 years,

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 the accumulated difference becomes close to a full month.
Therefore, every few years, an extra month (called Adhika Maasa
or intercalary month) is added to the year in some calendars. This
keeps the solar year and the lunar cycle in step. Such calendars
are called luni-solar calendars. They combine elements from
both the solar and the lunar calendars and are used in many
parts of India.
Ever heard of ...
You may have heard of the names (or similar sounding names)
of the months in various Indian luni-solar calendars — Chaitra,
Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika,
Margashirsha (or Agrahayan), Pausha, Magha, and Phalguna. In some
communities, the new month starts on the first day after the new
Moon and ends on the day of the new Moon. Such calendars are
called Amant. In others, the start of the new month corresponds to
the day after the full Moon, and the month ends on the full Moon.
Such calendars are called Purnimant.

11.2.4 The Indian National Calendar

A national calendar by the Government of India is used along
with Gregorian calendar for multiple official purposes.

Shaka Era 1947

Chaitra Vaisakha Jyestha
(March 22 - April 20, 2025) (April 21 - May 21, 2025) (May 22 - June 21, 2025)
SUN MON TUE WED THU FRI SAT SUN MON TUE WED THU FRI SAT SUN MON TUE WED THU FRI SAT

1 22
MAR

1 21 2 22 3 23 4 24 5 25 6 26 1 22 223 3 24
MAY
APR

2 23 3 24 4 25 526 6 27 7 28 8 29 7 27 8 28 929 10 30 11 1 12 2 13 3 4 25 526 6 27 728 8 29 930 10 31

MAY

9 30 10 31 11 1 12 2 13 3 14 4 15 5 11 1
APR

14 4 122 13 3 144 15 5 166 17 7

JUN

15 5 16 6 177 18 8 19 9 20 10
16 6 17 8 18 7 19 9 20 10 21 11 22 12 21 11 22 12 2313 2414 25 15 26 16 27 17 18 8 199 20 10 2111 22 12 2313 24 14
23 13 24 15 25 14 26 16 27 17 28 18 29 19 28 18 29 19 30 20 3121 25 15 2616 27 17 2818 29 19 3020 31 21
30 20

Fig. 11.9: Indian National Calendar

It is a solar calendar (Fig. 11.9) consisting of 365 days in a year.
The year begins on 22 March, which is the day after the spring
equinox. Unlike the Gregorian calendar, months in the Indian
National Calendar have either 30 or 31 days. The names of these
months were taken from traditional Indian calendars. In a regular
year, the second to sixth months have 31 days and the rest have
30 days. The leap years are matched to the Gregorian calendar by
adding a day to Chaitra, the first month of the year. In such years,
the new year begins on 21 March of the Gregorian calendar.

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 Chapter 11 — Keeping Time with the Skies 183

Ever heard of ...

In 1952, the Government of India set up a Calendar Reform

Committee (CRC) to examine all existing calendars which were
being followed in the country at that time and to recommend an
accurate and uniform calendar for the whole of India. The CRC
recommended ‘Unified National Calendar’ was adopted for use
with effect from 21 March 1956 CE, that is, 1 Chaitra 1878 Saka.
The Indian National Calendar follows the general principles as that
of the Surya Siddhanta.

Be a scientist
Meghnad Saha (1893 –1956)
Meghnad Saha was a pioneering astrophysicist of
India who studied stars and their temperatures and
developed a mathematical equation, famously known
as the Saha equation. The Saha Institute of Nuclear
Physics, in Kolkata, is named after him. He was also
the chairperson of the Calendar Reform Committee.

11.3 Are Festivals Related to Astronomical

Phenomena?
Many Indian festivals are tied to the phases
of the Moon and hence are based on either Why do most Indian
lunar or luni-solar calendars. For instance, festivals fall on different
Diwali falls on the new Moon of the month of dates every year?
Kartika, Holi on the full Moon of Phalguna,
Buddha Purnima on the full Moon of Vaisakha, Eid-ul-Fitr is
celebrated after sighting the crescent Moon at the end of the
month of Ramazan, while Dussehra is celebrated on the tenth
day in the month of Ashwina. Hence, they occur on different dates
in the Gregorian calendar in successive years.
For festivals based on luni-solar calendars, the Gregorian
calendar dates can shift, but this shift is typically less than
a month. This is because the luni-solar calendars add the
intercalary month every few years which correct for the
difference between the lunar and the solar year. In contrast,
purely lunar calendars do not account for this difference.
Any festival celebrated according to the phases of the Moon,
such as Eid-ul-fitr, therefore can occur in different months of
the Gregorian calendar year after year.

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 A step further
A few festivals in India, like Makar Sankranti, Pongal, Bihu, Vaisakhi,
Poila Baisakh, and Puthandu, follow a solar sidereal calendar.
These festivals happen on almost the same date every year in the
Gregorian calendar which is based on the tropical year.
A long time ago, these festivals were tied to either a solstice or
an equinox. Due to the small difference in the sidereal and tropical
years, the dates of these festivals slowly shift away from the
solstices/equinoxes. This shift is due to slow wobble of the Earth’s
axis, similar to the movement of the axis of a wobbling top.
This causes the dates of festivals based on the sidereal calendar
to move ahead in tropical calendar. For example, Makar Sankranti
moves ahead by one day every 71 years.

Ever heard of ...

The dates of many Indian festivals are based on the exact lunar
phase at sunrise. As sunrise occurs earlier in Eastern India and
later in Western India, these dates can also shift by a day between
these regions even in the same year. To maintain uniformity
throughout the country, the Positional Astronomy Center of the
Government of India annually publishes the Rashtriya panchang,
a detailed calculation of the positions of celestial objects, such as
the Moon and the Sun for a central location in India. Based on
these calculations, it provides an advance intimation on dates of
festivals to Government of India for holiday declaration.

Ever heard of ...

The Moon and moonlight have inspired ragas in Indian classical
music. Chandrakauns, Chandranandan, and Shubhapantuvarali
(which also means “auspicious moon”) are a few ragas that display
the moon’s imagery in their names and melodic expressions.
Similarly, mudras (hand gestures), for example, Chandrakala, and
Ardhachandran relating to the Moon can be found in Indian classical
dance Bharatanatyam.
The same is true for other dance forms — Kathak, Odissi, and
Kuchipudi. Even the
traditional painting styles:
Madhubani, Warli, and other
forms of art, such as sculpture
and pottery among Saura,
Gond and other tribes invoke
depictions of the Moon and
the Sun prominently implying
Warli painting Dhokra Brass
their significance in daily life.
sculpture

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 Chapter 11 — Keeping Time with the Skies 185

11.4 Why Do We Launch Artificial Satellites

in Space?
The Moon is Earth’s natural satellite, orbiting
When I look at the night
our planet. Besides the Moon, man-made sky in early evening, I
satellites sent by various countries also orbit see some moving stars.
the Earth. These artificial satellites appear as What are they? Is their
tiny specks moving in the night sky. Most orbit motion also periodic?
about 800 km above Earth’s surface and take
roughly 100 minutes to complete one orbit.
These satellites help us in many ways like communication,
navigation, weather monitoring, disaster management, and
scientific research. The Indian Space Research Organisation (ISRO)
has launched many satellites that support these activities.

Our scientific heritage

The Cartosat series of satellites, launched by ISRO, capture high-quality

images of the Earth to improve maps, plan cities, and handle natural
disasters in India. One such mapping platform, Bhuvan, uses these
images to show terrain, soil, land use, vegetation, and more.
AstroSat, another ISRO mission, makes scientific observations of
stars and other celestial objects. India’s other space missions include
Chandrayaan 1, 2, and 3 to the Moon; Aditya L1 to study the Sun; and
Mangalyaan to Mars. ISRO also lets Indian students build and launch
small satellites, such as AzaadiSat, InspireSat-1, and Jugnu.

Cartosat AstroSat

Activity 11.4: Let us identify

z Spotting an artificial satellite is a night sky watching activity
like we have done previously. Just before sunrise or after
sunset, go to a location, accompanied by an adult, that has
a clear view of the sky, without any obstruction of trees or
tall buildings.

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 z To identify satellites in the sky, look for any moving object
in the sky that appears as a point of light with steady or
flickering brightness and is moving very fast across the sky.
You can see them with the naked eye or with binoculars.
z You may use mobile apps or websites that provide details
of satellites visible in your location and when they will be
passing above you in the sky.

A step further
A lot of artificial satellites are being sent up in space by many
countries. After their useful life, many of them and their rocket
parts become space junk or space debris. This debris crowds space,
and could collide with working satellites. While small debris burns
up in the atmosphere when it falls towards the Earth, the larger
pieces can crash on ground. Countries are now working together to
remove this dangerous debris.

Be a scientist

Vikram Ambalal Sarabhai (1919 –1971)

Vikram Sarabhai, a researcher in space science and
nuclear physics is known as the Father of the Indian
Space programme. He pioneered the effort to launch
the first artificial satellites. The Vikram Sarabhai
Space Centre (VSSC), located in Thiruvananthapuram,
the ISRO centre that develops rockets and launch
vehicle technology, is named after him.

Snapshots
‹ The illuminated part of the Moon changes its shape from day to
day through phases, like the new Moon, crescent, and full Moon.
‹ The phases of the Moon happen because we see different parts of
the illuminated portion of the Moon as it moves around the Earth.
‹ A full cycle of phases of the Moon takes about a month.
‹ The various cycles observed in nature resulted in the creation of
calendars.
‹ Lunar calendars follow the Moon’s cycle, while solar calendars
follow the cycle of seasons, which depend upon the position of
the Earth in its orbit around the Sun. Luni-solar calendars adapt
to both the cycles.
‹ Artificial satellites are human-made which are launched from the
Earth. They provide important information for our well-being
and space-science studies.

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 Chapter 11 — Keeping Time with the Skies 187

Keep the curiosity alive

1. State whether the following statements are True or False.

(i) We can only see that part of the Moon which reflects
sunlight towards us.
(ii) The shadow of Earth blocks sunlight from reaching the
Moon causing phases.
(iii) Calendars are based on various astronomical cycles
which repeat in a predictable manner.
(iv) The Moon can only be seen at night.
2. Amol was born on 6th of May on a full Moon day. Does his
birthday fall on the full Moon day every year? Explain your
answer.
3. Name two things that are incorrect in Fig. 11.10.

Fig. 11.10
4. Look at the pictures of the Moon in Fig. 11.11, and answer
the following questions.
A B C D E F

Fig. 11.11
(i) Write the correct panel number corresponding to the
phases of the Moon shown in the pictures above.
Picture label (e.g. A, B, C, etc.) Phase of Moon
Three days after New Moon
Full Moon
Three days after Full Moon
A week after Full Moon
Day of New Moon

(ii) List the picture labels of the phases of the Moon that
are never seen from Earth. Hint: You can use your
observations from Activity 11.1 or Fig. 11.2 as reference.
5. Malini saw the Moon overhead in the sky at sunset.
(i) Draw the phase of the Moon that Malini saw.
(ii) Is the Moon in the waxing or the waning phase?

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 6. Ravi said, “I saw a crescent Moon, and it was rising in the
East, when the Sun was setting.” Kaushalya said, “Once I saw
the gibbous Moon during the afternoon in the East.” Who out
of the two is telling the truth?
7. Scientific studies show that the Moon is getting farther away
from the Earth and slower in its revolution. Will luni-solar
calendars need an intercalary month more often or less often?
8. A total of 37 full Moons happen during 3 years in a solar
calendar. Show that at least two of the 37 full moons must
happen during the same month of the solar calendar.
9. On a particular night, Vaishali saw the Moon in the sky from
sunset to sunrise. What phase of the Moon would she have
noticed?
10. If we stopped having leap years, in approximately how many
years would the Indian Independence day happen in winter?
11. What is the purpose of launching artificial satellites?
12. On which periodic phenomenon are the following measures
of time based: (i) day (ii) month (iii) year?

Discover, design, and debate

z The Moon’s crescent always faces towards the Sun (Fig. 11.12).
On days when you see the crescent Moon, point your finger
towards the Sun, and slowly move it across the sky towards
the Moon taking as short a path as you can. Note how your
finger always crosses the illuminated part of the Moon first
and clearly shows us that we see sunlight reflected off the
Moon. The line joining the tips of the crescent would
correspond to the diameter of the Moon.
Fig. 11.12 z Most of the dates in the Indian National Calendar always
map to the same dates in the Gregorian calendar. Can you
find out which ones may differ for certain years?
z Different states in India celebrate the New Year according to
their local cultures. Find out the names of the New Year festival
celebrated in any 10 states of India. Also find out whether it is

When?
Prepare some questions based on your
Why? Why not?
Where? learnings so far ...
How long?
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 Chapter 11 — Keeping Time with the Skies 189

based on the lunar calendar or the solar calendar or the luni-

solar calendar.
z Collect Gregorian calendars (the regular calendar you use
every day) for the last five years with the help of your family
members or teachers or the internet. For each year, look for
the dates on which the festivals Eid-ul-Fitr and Diwali were
celebrated and list them year wise in a tabular form. Do you
notice that the date of Eid-ul-Fitr moves earlier each year —
by about 11 days? If you have a corresponding lunar calendar
at home or on the internet, check that the month and the
day for Eid-ul-Fitr according to the lunar calendar remains
the same. Does Diwali follow the same steady pattern, or
are there some sudden jumps? Based on your chart, try to
guess which year might have included an intercalary month
(Adhikamaasa). Obtain a luni-solar calendar and confirm if
there is an intercalary month between Diwali in the previous
year and that year. Society Science

z Every morning on your way to school, notice the direction in

Inter-
which the Sun rises. Decide on a spot and look towards east, disciplinary
with trees, poles, or buildings acting as markers. Sketch the Projects
eastern horizon in your notebook. For the next one year, at
the start of each month, stand at the same spot and mark the
Sun’s position on your sketch. Label it with the name of the
month. At the end of the year analyse your sketch. Do you
find that the positions of sunrise shift in particular direction?
Can you identify it with the Uttarayaan and Dakshinayaan
that our ancestors noticed? (Refer the ‘A step further’ box on
page 181).

A step further
If you visit a place near the sea, you may notice that water levels
rise and fall. The rise and fall of water levels are called tides. Tides
also follow a regular pattern. If there’s a low or a high tide at a
certain time on one day, a similar tide will come about 50 minutes
later the next day. We also learnt that the Moon also rises about
50 minutes later each day. Careful observations show that tide
levels are closely related to the Moon’s position and phase.

I
Reflect on the questions framed by your think ... Shouldn’t it
But we be ...
friends and try to answer ... thought ... Maybe ...
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