NCERT Solutions Class 8 Science Chapter 11 Keeping Time with the Skies

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 8 · SCIENCE

NCERT Solutions

Chapter 11: Keeping Time with
the Skies

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

170 – 189 13 37 English

Solutions, notes, sample papers & more at 43 pages

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

CLASS 8 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 11: Keeping Time with the
Skies
The Moon does not change shape — our view of it does. This chapter builds the whole idea of a calendar out
of that one fact: the phases of the Moon come from the changing Sun–Earth–Moon angle, and the three
natural cycles of rotation, revolution of the Moon and revolution of the Earth give us the day, the month
and the year — and with them lunar, solar and luni-solar calendars.

TEXTBOOK BOOK PAGES

Curiosity (Class 8) 170 – 189

SECTIONS QUESTIONS

13 37

MEDIUM

English

Probe and ponder — Page 170
Chapter opener

PROBE AND PONDER

Q1 Have you ever seen the Moon during the day? Why do you think it is sometimes
visible when the Sun is up?

Yes — the Moon is in the daytime sky far more often than most people notice. Meera saw it over
the kites at the Patang Mahotsav on Makar Sankranti. It is visible in daylight because it shines by
reflecting sunlight, and that reflected light is bright enough to show against the blue sky
whenever the Moon is above the horizon and far enough away from the Sun.

Why it happens: The Moon is above the horizon for roughly 12 hours out of every
24, but those hours are not fixed to the night. The Moon rises about 50 minutes later
each day, so its "up time" slides all the way round the clock in one month. For about
half the month, a good part of that time falls in daylight. The book's own example:
on 7 April 2025 moonrise was at 14:23 — the middle of the afternoon.

You will not see it near new Moon, for two reasons at once: the Moon is then in almost the same
direction as the Sun, so it is lost in the glare, and its non-illuminated half is the one turned
towards us.

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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Check it yourself: Look towards the east in the late afternoon during the week after
full Moon, or towards the west in the morning during the week after new Moon. A
pale, washed-out Moon against a blue sky is a common sight once you start looking.

Q2 Imagine you lived on the Moon instead of Earth. What would you mean by a day, a
month or a year?

Every unit of time comes from a natural cycle seen from where you stand, so all three would
change.

A day — one sunrise to the next. The chapter tells us that only one half of the Moon always
faces the Earth. For that to be true the Moon must turn once on its own axis in exactly the
time it takes to go once round the Earth. So a day on the Moon lasts about 29.5 Earth days
— roughly 14¾ Earth days of continuous sunlight followed by 14¾ of darkness.
A month — on Earth a month is one cycle of the phases of our companion in the sky. From
the Moon that companion is the Earth, and the Earth would show phases in the very same
29.5-day rhythm. But that is the same as the lunar day, so on the Moon the month and the
day would be one and the same unit. A Moon-dweller would probably not need a separate
month at all.
A year — the Moon travels round the Sun along with the Earth, so its cycle of seasons is the
Earth's cycle. A year on the Moon would still be about 365¼ days.

Why it happens: A "day" measures rotation, a "month" measures the companion
body's revolution, and a "year" measures the journey round the Sun. On the Moon
the first two cycles have become the same length, which is why its calendar would
look so strange to us.

Q3 What would happen if Earth had two moons instead of one? How would that change
the night sky?

Two moons would almost certainly have different orbits and different periods, and that single
fact changes everything that follows.

Two sets of phases at once. Each moon would go through its own cycle at its own rate. On
most nights one might be a waxing crescent while the other is a waning gibbous, and only
rarely would both be full together.

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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Brighter, more varied nights. Truly dark nights — when both moons are new or both are
below the horizon — would become rare.
The "month" would stop being one clean unit. Our month exists because one cycle of
phases repeats reliably. With two moons a calendar-maker would have to pick one of them,
or invent a longer unit in which both cycles fit — much harder than fitting 12 lunar months
into a year.
More eclipses. Two moons mean two chances every cycle for a moon to pass into Earth's
shadow or across the Sun's face, and the moons could even eclipse each other.
More complicated tides. Tides follow the Moon's position and phase. With two pulls, high
tides would sometimes add together and sometimes cancel, so their timing and height
would be far less regular.

Did you know? Mars really does have two small moons, Phobos and Deimos.
Phobos goes round Mars in only about 7½ hours — faster than Mars turns — so
from the surface it rises in the west and sets in the east.

Q4 If we didn’t have clocks or calendars, how else could we measure time?

By counting natural periodic events — events that repeat with a fixed period. That is exactly how
every calendar in history began, and the chapter uses three of them.

NATURAL CYCLE WHAT IS HOW YOU WOULD USE IT LENGTH
REALLY
MOVING

Sunrise, noon, sunset Earth turning Fix a stick upright and mark the tip of its 24 hours
on its axis shadow. The shadow is shortest at noon; the
gap between two noons is one day (Activity
11.3)

Phases of the Moon Moon Count nights from one full Moon to the next; About 29.5
revolving each night's shape tells you where you are in days
round the the count
Earth

Cycle of seasons; Earth revolving Mark where the Sun rises against a tree or a About 365
sunrise point moving round the Sun hill. It creeps north till about 21 June and days
along the horizon south till about 21 December — Uttarayan
and Dakshinayan

Within a day, people also used devices built on steady processes rather than on the sky — a
sundial, a water clock, a sand glass, or the burning of a marked lamp.

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Class 8 Science Chapter 11 Keeping Time with the Skies
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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

11.1 How Does the Moon’s Appearance Change and Why?

ACTIVITY

Q1 Analyse the data recorded by you in Table 11.1. Did the Moon appear different each
day?

TABLE 11.1: DOCUMENTING CHANGES IN THE MOON’S APPEARANCE

DAY DATE MOON SEEN APPEARANCE SIZE OF MOON AND
AT OF THE THE SUN
MOON IN BRIGHT SEPARATION
THE SKY PORTION IN THE SKY
COMPARED COMPARED
TO THE TO THE
PREVIOUS PREVIOUS
DAY DAY

1. Sunrise/Sunset — —

2. Sunrise/Sunset Increased/ Closer/Farther
Decreased

3. Sunrise/Sunset Increased/ Closer/Farther
Decreased

Table 11.1, page 172 — reproduced as printed. The blank cells are the ones you fill in
from your own observations.

Yes. The shape of the bright part is a little different every single day, and the change goes in one
direction for a whole fortnight before it reverses.
Starting at sunrise the day after full Moon, this is what your Table 11.1 should show:

Page 5 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

ROUGHLY APPEARANCE OF BRIGHT PORTION MOON–SUN
WHICH DAY THE MOON COMPARED TO THE SEPARATION COMPARED
PREVIOUS DAY TO THE PREVIOUS DAY

Day 1 Full bright circle — —

Days 2–7 Gibbous, shrinking Decreased Closer

About Day 8 Half circle Decreased Closer

Days 9–14 Crescent, getting Decreased Closer
thinner

About Day 15 Not visible (new Decreased to nothing Closest to the Sun
Moon)

Days 16–29 Crescent → half → Increased Farther
gibbous → full again

Why it happens: The first fortnight is the waning period (Krishna Paksha) and the
second is the waxing period (Shukla Paksha). Since the whole cycle takes about 29.5
days, the visible shape changes by only about one-fifteenth of the disc from one day
to the next — small, but quite easy to see if you sketch it.

Q2 Was the Moon visible on all days?

No. For a day or two around new Moon (Amavasya) you will not find the Moon at all.

Why it happens: Two things go wrong for the observer at once on that day. First,
only the non-illuminated half of the Moon is turned towards the Earth, so there is
nothing bright to see. Second, the Moon is then in almost the same direction as the
Sun, so it rises and sets with the Sun and is drowned in daylight.

This is also why the activity asks you to change your observing time midway. During the first
fortnight the Moon is waning, so it is easiest to catch at sunrise; during the second fortnight it
is waxing and is easiest to catch at sunset. In between, near new Moon, it is not available at
either time.

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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Q3 Did the Moon appear at the same position in the sky as on the previous day?

No. At the same clock time each day the Moon has clearly shifted — it appears further east, and
steadily closer to the Sun through the waning fortnight.

The Moon completes 360° round the Earth in about 29.5 days

Daily eastward shift = 360° ÷ 29.5 ≈ 12.2° per day

The Earth turns 360° in 24 hours = 15° per hour

Extra turning needed = 12.2° ÷ 15° per hour ≈ 0.81 h = about 49 minutes

Why it happens: While the Earth is completing one rotation, the Moon has not
stayed put — it has run ahead in its own orbit. The Earth must turn a little further
before the Moon comes back over the same spot, and "a little further" works out to
about 50 minutes (Fig. 11.6).

Over the fortnight the pattern at sunrise is very regular: on full Moon day the Moon is setting in
the west; about a week later, when it is a half circle, it is overhead; a few days after that the thin
crescent is low in the east, close to the Sun.

In-text Questions — Page 173
11.1.2 Locating the Moon

Q1 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?

Yes, and the movement is completely predictable, because the phase of the Moon and its
distance from the Sun in the sky are the same piece of information.

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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

DAYS AFTER PHASE YOU ANGLE BETWEEN MOON AND WHERE IT IS AT
FULL MOON SEE SUN IN THE SKY SUNRISE

0 Full circle About 180° Setting on the western
horizon

About 7 Half circle About 90° Overhead

About 11 Crescent About 45° In the eastern sky, well
above the Sun

About 15 Not visible Nearly 0° Rising with the Sun —
lost in daylight

Why it happens: Sunlight lights one half of the Moon. How much of that lit half we
can see depends only on the angle from which we look at it — and that angle is
exactly the Moon–Sun separation in our sky. So as the Moon slides towards the Sun
day by day, the bright part must shrink at the same time. One observation, two
effects.

Tip: Turn this round and it becomes a useful skill. If tonight's Moon is a fat gibbous
and waxing, it is roughly 135° from the Sun, so look in the east a few hours before
sunset. A waning crescent is only about 30° from the Sun — look low in the east just
before sunrise.

A step further — Page 173
11.1.2 Locating the Moon

A STEP FURTHER

Q1 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.

Do this for a week and the delay turns out to be strikingly steady. The chapter's own two
readings show it:

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Class 8 Science Chapter 11 Keeping Time with the Skies
a g l AglaSem · NCERT Solutions

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Moonrise on 7 April 2025 = 14:23
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Moonrise on 8 April 2025 = 15:17
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Different days give slightly different values — usually between about 30 and 70 minutes — but
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Why it happens: In the 24 hours the Earth takes to turn once, the Moon has moved
about 12° further along its orbit (360° ÷ 29.5 days). The Earth then has to turn that
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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

PHASE MOON RISES MOON IS OVERHEAD MOON SETS
ABOUT ABOUT ABOUT

New Moon Sunrise Noon Sunset

Half Moon, Noon Sunset Midnight
waxing

Gibbous, waxing Mid-afternoon (2–4 Late evening Early morning
p.m.)

Full Moon Sunset Midnight Sunrise

Half Moon, Midnight Sunrise Noon
waning

Tip: The listed moonrise time is the moment the Moon's edge touches the horizon.
Wait about 30 minutes for it to climb clear of trees and buildings before you look for
it.

In-text Questions — Page 174
11.1.3 Making sense of our observations

Q1 Why does the illuminated portion of the Moon seen from the Earth decrease when
it appears closer to the Sun?

Because the fraction we can see depends entirely on the angle from which we view the lit half,
and that angle is the Moon–Sun separation in the sky.
Sunlight always lights exactly one half of the Moon — the half turned towards the Sun (Fig.
11.3). That never changes. What changes is where the Earth is standing when it looks at that lit
half.

When the Moon is far from the Sun in the sky (full Moon, about 180°), we are looking at the
Moon from nearly the same direction the sunlight is coming from, so the lit half is turned
almost squarely towards us. We see all of it.
As the Moon comes closer to the Sun in the sky, we begin to see the lit half more and more
edge-on. Less and less of it is turned our way, so the bright shape narrows: gibbous, then
half, then a thin crescent.
At new Moon (about 0°) we are looking at the Moon from almost exactly the opposite side to
the sunlight. The whole lit half faces away and only the dark half faces us.

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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

The lit half (light) always faces the Sun.
Only the angle of our view changes.

New
Earth
0°

Full
180°

Crescent
45°

Half
90° Sunrays

The Sun lights the same half of the Moon at every position. As the Moon moves towards the Sun's
direction in the sky, the Earth sees that lit half more and more edge-on, so the bright shape shrinks
from full to crescent to nothing.

Careful: This has nothing to do with Earth's shadow. Earth's shadow falling on the
Moon is a lunar eclipse, which can happen only on a full Moon day and does not
happen every month, because the Moon's orbit is slightly tilted to the Earth's orbit
round the Sun.

Activity 11.2: Let us explore — Page 174
11.1.3 Making sense of our observations

ACTIVITY

Q1 Does the portion of the ball facing you appear to be illuminated or not?

At position E — the ball held out towards the lamp — the side facing you is not illuminated. You
see a dark ball.

Page 11 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Why it happens: The lamp lights the far half of the ball, the half pointing away from
your eye. The half turned towards you lies in the ball's own shadow. Your head is the
Earth, the ball is the Moon and the lamp is the Sun, so this arrangement — Moon
between Earth and Sun — is exactly new Moon day (Amavasya).

Q2 Does the shape of the illuminated portion change?

G
H F

lamp

A E
C

B D

Fig. 11.4(b), page 175 — redrawn sketch: the ball on a stick is held at arm’s length and
carried once round the eight positions, the lamp staying fixed. Your head is at the
centre of the circle.

Yes, and it changes smoothly and in a fixed order as you turn anticlockwise:

E (no bright part) → F thin crescent → G half → H gibbous → A whole bright disc → B

gibbous → C half → D crescent → back to E

Page 12 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Why it happens: The lamp never changes what it lights — exactly half the ball, all
the time. What changes is your line of sight. Turning your body swings your eye
round to a new angle, so a different fraction of the lit half is turned towards you.
That is the whole mechanism of the Moon's phases, reproduced in a dark room.

Tip: Hold the ball a little above your head. If you hold it exactly level, your own head
gets in the way at position A — which is, in miniature, why we do not get a lunar
eclipse every month.

Q3 Is the line separating the illuminated and non-illuminated portions of the ball
curved?

Yes — at every position except the two half-lit ones (C and G), where it looks perfectly straight.

Why it happens: On the ball itself the boundary between light and dark is a circle
running right round the ball. When you look at a circle from an angle, you do not see
a circle — you see a flattened circle, that is, an ellipse, and half of that ellipse is a
curved line. Only when your eye lies in the plane of that circle, at the half-lit
positions, does the ellipse squash down to a straight line across the middle.

This is why crescent and gibbous Moons have a curved edge on the inside while a half Moon has
a straight one. It is a clue you can use: a straight inner edge means the Moon is exactly 90° from
the Sun in the sky.

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Class 8 Science Chapter 11 Keeping Time with the Skies
a g l AglaSem · NCERT Solutions

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In-text Questions — Page 176 a

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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

11.1.3 Making sense of our observations

Q1 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
More than half of the
illuminated portion
Less than half of the
of Moon can be seen
G illuminated portion
(Gibbous phase)
of Moon can be seen
(Crescent phase)
H F
Day 22
Day 26 Day 18

Entire illuminated
Illuminated portion of

SUNRAYS
portion of Moon Day 1 Day 15
can be seen A E Moon cannot be seen
(Full Moon) (New Moon)

Day 4 Day 12
Day 8
More than half of the B D Less than half of the
illuminated portion illuminated portion
of Moon can be seen C of Moon can be seen
(Gibbous phase) (Crescent phase)

Half of the illuminated
portion of Moon can be seen

Fig. 11.5(a), page 176 — redrawn: the Moon at eight positions in its orbit round the
Earth, with sunrays coming from the right. The orange dashed line on each Moon marks
the half that faces the Earth. Sizes and distances are not to scale.

Yes — it is the very same result, arrived at from two different directions.

In Activity 11.1 you filled in a column called "Moon and Sun separation in the sky compared
to the previous day". Through the waning fortnight you kept writing Closer, and at the same
time the bright portion kept Decreasing; through the waxing fortnight both entries reversed.
In Fig. 11.5a you can see why. At position E the Moon lies in the direction of the Sun, so from
Earth the two appear together in the sky; at position A the Moon lies on the far side, so the
two appear at opposite ends of the sky.

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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Why it happens: The observation and the diagram are two views of one geometry.
The angle between the Moon and the Sun as seen from Earth is precisely what
decides how much of the lit half is turned towards us. So the separation and the
phase can never disagree — a full Moon must be far from the Sun in the sky, and a
thin crescent must be near it.

Check it yourself: On a full Moon evening, look east as the Sun sets in the west. The
Moon will be rising almost exactly opposite the setting Sun.

In-text Questions — Page 178
11.2 How Did Calendars Come into Existence?

Q1 But how are these periodic events used for keeping time?

By counting them. Any event that repeats after a fixed interval can be used as a unit, and
measuring time is simply counting how many of those intervals have gone by. The three cycles
in this chapter give the three units we still use.

NATURAL CYCLE WHAT IS ACTUALLY LENGTH UNIT OF TIME IT
MOVING DEFINES

Sun returns to its highest point Earth rotating on its axis 24 hours The (mean solar) day
in the sky

Moon completes one cycle of Moon revolving round the About 29.5 The month
phases Earth days

Seasons return to where they Earth revolving round the About 365¼ The year
started Sun days

Why it happens: A calendar is nothing but a scheme for packing the smaller counts
neatly inside the larger ones. The trouble — and the reason different kinds of
calendars exist at all — is that the three cycles are not whole multiples of one
another. Twelve lunar months are 354 days, not 365, and the year is not a whole
number of days either. Every calendar in the chapter is one particular way of
handling those two awkward leftovers.

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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Activity 11.3: Let us measure a day! — Page 178
11.2 How Did Calendars Come into Existence?

ACTIVITY

Q1 Find the duration of the solar day by finding a difference in time on two consecutive
days as shown in Table 11.2.

TABLE 11.2: FINDING THE DURATION OF A SOLAR DAY

DATE TIME OF SHORTEST SHADOW DURATION OF DAY
(HH:MM) (HH:MM)

22 March 12:20 ——
2025

23 March 12:20 24:00
2025

24 March 12:19 23:59
2025

Table 11.2, page 179 — reproduced as printed, with the last row left blank for your own
reading.

The shortest shadow of the day marks the moment the Sun is highest in the sky, so the gap
between two such moments on consecutive days is one solar day. Using the readings printed in
Table 11.2:

Page 17 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

22 March 2025 — shortest shadow at 12:20

23 March 2025 — shortest shadow at 12:20

Duration = 12:20 next day − 12:20 = 24 h 00 min

23 March 2025 — 12:20

24 March 2025 — 12:19

Duration = 24 h − 1 min = 23 h 59 min

Why it works: As the Sun climbs, the shadow of a vertical stick swings round and
gets shorter; as the Sun sinks, the shadow lengthens again. The single shortest
shadow of the day is therefore the Sun's highest point — local noon. Marking a dot
every minute from 11:00 a.m. to 1:10 p.m. gives you that moment to within about a
minute without any clock reading of the Sun itself.

Tip: Fix the stick truly vertical and choose flat ground, or the shortest dot will be in
the wrong place. Do not look directly at the Sun to judge its height — read the
shadow instead.

Q2 Find the average duration of the day. Is it nearly equal to 24 hours?

Yes, it is almost exactly 24 hours.

Average = (24 h 00 min + 23 h 59 min) ÷ 2

= (1440 min + 1439 min) ÷ 2

= 2879 ÷ 2 = 1439.5 min

= 23 h 59 min 30 s, that is, nearly 24 hours

Page 18 of 43

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Class 8 Science Chapter 11 Keeping Time with the Skies
a g l AglaSem · NCERT Solutions

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Why it happens: The Sun's apparent daily journey is not perfectly even through the

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In-text Questions — Page 183 a
l a seFestivals Related to Astronomical Phenomena?
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Q1 Why do most Indian festivals fall on different dates every year?

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a

Because most Indian festivals are fixed by the phase of the Moon, while the dates we quote
come from the Gregorian solar calendar — and a lunar year and a solar year are not the same
co m
m .
as e
length.
m l
.co
m lunar months = 12 × 29.5 ≈ 354 days a g
a s e12
agl 1 solar year ≈ 365 days
se m
com g l a
.
Shortfall each year = 365 − 354 = about 11 days
m a
ase
agl
What happens next depends on which kind of calendar the festival follows.

co m
m .
m as e
.co a g l
se m
g l a
a c
m .
m a s e
e m . co agl
g l as
a

co m
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m ase
.co


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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

KIND OF EXAMPLE FESTIVALS HOW THE GREGORIAN DATE BEHAVES
CALENDAR

Purely lunar Eid-ul-Fitr, after the crescent Moon is Moves about 11 days earlier every year and
sighted at the end of Ramazan drifts right through all the seasons — it can fall
in any Gregorian month

Luni-solar Diwali (new Moon of Kartika), Holi Moves about 11 days earlier, but an Adhika
(full Moon of Phalguna), Buddha Maasa every 2–3 years pushes it back by a
Purnima (full Moon of Vaisakha), month. So it wanders within a window of about
Dussehra (tenth day of Ashwina) a month and never leaves its season

Solar (sidereal) Makar Sankranti, Pongal, Bihu, Falls on almost the same Gregorian date every
Vaisakhi, Poila Baisakh, Puthandu year

Each solar year the lunar year falls 11 days short. The gap piles up.

Year 1 354 days — 12 lunar months
11 days behind
Year 2 354 days
22 days behind
Year 3 354 days
33 days behind

+1 month
Fix

After 2–3 years the gap is nearly a whole month, so an Adhika Maasa is
added.
The luni-solar year is back in step with the seasons.

Why a luni-solar festival wobbles but never drifts away: the 11-day annual shortfall is cancelled by an
extra month every two or three years.

Why it happens: A festival tied to the Moon has a fixed lunar date but no fixed solar
date. Since 12 lunar months are 11 days short of a solar year, that lunar date must
arrive 11 days earlier on the Gregorian calendar each time. The intercalary month is
exactly the correction that stops the drift — which is why Diwali always stays in the
autumn while Eid-ul-Fitr can fall in any season.

Page 20 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Did you know? Because these dates are decided by the exact lunar phase at sunrise,
and the Sun rises earlier in eastern India than in western India, a festival can even
fall on different days in different parts of the country in the same year. To keep
things uniform, the Positional Astronomy Centre publishes the Rashtriya panchang
every year.

In-text Questions — Page 185
11.4 Why Do We Launch Artificial Satellites in Space?

Q1 When I look at the night sky in early evening, I see some moving stars. What are
they? Is their motion also periodic?

They are not stars at all — they are artificial satellites orbiting the Earth. (An aeroplane can look
similar, but it shows blinking red and green lights; a satellite does not.)
How to tell a satellite apart:

It is a point of light with a steady or slowly flickering brightness, with no colour and no
blinking.
It moves visibly and fast, crossing a large part of the sky in a few minutes, while the real
stars stay fixed relative to one another.
It is seen just after sunset or just before sunrise. That is the giveaway: it does not glow on
its own but shines by reflecting sunlight, so it must still be up in sunlight while the ground
below you has already gone dark.

Yes, the motion is periodic. Most of these satellites orbit about 800 km above the Earth's
surface and take roughly 100 minutes to complete one orbit.

Orbits in one day = 24 × 60 min ÷ 100 min

= 1440 ÷ 100 = about 14 orbits per day

Why it matters: Because the period is fixed, the passes can be calculated in advance
— which is exactly what the mobile apps and websites in Activity 11.4 do when they
tell you when a satellite will cross your sky. It is the same reason that made the
Moon useful for a calendar: a motion that repeats predictably can be counted, and
anything that can be counted can be used to keep time.

Page 21 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Did you know? The Moon is our natural satellite. India's artificial ones do very
different jobs — Cartosat photographs the Earth for maps, city planning and disaster
management, feeding the Bhuvan platform, while AstroSat looks outward at stars
and other celestial objects.

Keep the curiosity alive — Page 187
End-of-chapter questions

KEEP THE CURIOSITY ALIVE

Q1 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.

STATEMENT TRUE / REASON
FALSE

(i) We can only see that part of the True The Moon emits no light of its own. Only the portion
Moon which reflects sunlight that is both lit by the Sun and turned towards the Earth
towards us. can be seen.

(ii) The shadow of Earth blocks False Phases come from the changing Sun–Earth–Moon
sunlight from reaching the Moon angle as the Moon revolves. Earth's shadow on the
causing phases. Moon causes a lunar eclipse, which happens only on
some full Moon days.

(iii) Calendars are based on various True The day, the month and the year come from Earth's
astronomical cycles which repeat in rotation, the Moon's cycle of phases and Earth's
a predictable manner. revolution.

(iv) The Moon can only be seen at False The Moon rises about 50 minutes later each day, so on
night. many days it is above the horizon in daylight — as
Meera saw at the kite festival.

Tip: Statement (ii) is the single most common wrong idea about the Moon. If Earth's
shadow caused the phases, we would get one every month and the shadow's edge
would always be a circular arc of Earth's own size — neither of which we observe.

Page 22 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Q2 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.

No. His birthday returns on 6 May every year, but the full Moon does not.

One Gregorian year = 365 days

12 cycles of the Moon's phases = 12 × 29.5 = 354 days

Difference = 365 − 354 = about 11 days

Why it happens: A year after his birth, the Moon has completed 12 full cycles in 354
days and then run on for 11 more days. So on 6 May of the next year the Moon is
about 11 days past full — a waning crescent, close to new Moon. The date on the
wall calendar is fixed by the Sun; the phase is fixed by the Moon; the two cycles
simply do not divide into each other.

Did you know? They do come back together, but slowly. 235 lunar months (about
6940 days) are almost exactly 19 solar years (also about 6940 days), so the same
phase returns to the same date roughly every 19 years. Amol can expect a full Moon
birthday at ages 19, 38 and 57.

Page 23 of 43

as e
Class 8 Science Chapter 11 Keeping Time with the Skies
a g l AglaSem · NCERT Solutions

co m
m.
Name two things that are incorrect in Fig. 11.10.
e
Q3

m l as
m .co a g
l a se
a g

co m
e m . ag
g l as
a

co m
e m.
m l as
m .co a g
l a se
ag
m a s
.co agl
Fig. 11.10, page 187 — redrawn sketch of the picture printed in the textbook.

se m
g l a
a

co m
Two things in that picture cannot happen:
m .
o m l a se
g appear within the Moon's
.c km across. Anything behind it is hidden, so no star can ever
1. Stars are shown inside the dark part of the Moon. The Moon is a solid, opaque rock about

m a
se disc. Stars can be drawn all around the Moon, never on it.
3500
a
agl 2. The non-illuminated part of the Moon is shown as a visible dark disc. We can see only
se m
m a
the portion that reflects sunlight to us. On a crescent night the rest of the Moon is not black

.c
against the sky — it is simply not seen oat all, and the sky and its stars appear right up to the
a g l
se m
l a
edge of the bright crescent.

ag

co m
m .
m as e
.co a g l
se m
g l a
a c
m .
m a s e
e m . co agl
g l as
a
Incorrect (as in Fig. 11.10) Correct — only the lit crescent is seen

com
m .
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.co


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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Left: the dark half drawn as a visible disc with stars shining through it. Right: what the sky actually
looks like — the crescent alone, with the stars outside the Moon.

Check it yourself: On the next crescent night, look carefully. You may faintly see the
rest of the disc glowing — that is earthshine, sunlight reflected off the Earth onto the
Moon's night side. Even then, no star ever shows through it.

Page 25 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Q4 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, page 187 — redrawn sketch of the six pictures of the Moon, labelled A to F as
in the book.

(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

The table as printed on page 187 — the left-hand column is for you to fill in.

(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.

(i) Match each picture by asking one question: what fraction of the disc is bright? Three days is
about one-fifth of a fortnight, and a week is half of it.

Page 26 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

PICTURE PHASE OF WHY
LABEL MOON

D Three days after A thin crescent — only about a tenth of the disc is bright, and it is
New Moon growing (waxing)

E Full Moon The whole disc is bright

A Three days after Almost full, with a thin dark crescent bitten out of one edge — a
Full Moon gibbous Moon that has just begun to wane

C A week after Full Exactly half bright with a straight dividing line — the Moon is
Moon now 90° from the Sun

B Day of New Moon Completely dark — only the non-illuminated half faces the Earth

(ii) Picture F is never seen from the Earth.

Possible: the two tips are Impossible (F): bright Moon on
ends of one diameter both sides of the dark patch

The line dividing light from dark always joins two opposite points on the Moon's edge and cuts the
disc into exactly two pieces. In F the dark part is a leaf-shaped patch lying inside the disc with lit Moon
on either side of it — no arrangement of Sun, Earth and Moon can produce that.

Why it happens: Sunlight lights one complete half of the Moon, so the boundary
between the lit and unlit halves is a circle running right round the Moon through its
two poles. Seen from Earth it can look like a straight line (half Moon) or a curve
bulging one way (crescent) or the other (gibbous), but its two ends must always land
on opposite points of the Moon's rim. That is exactly the rule picture F breaks.

Page 27 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Q5 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?

(i) She saw a half Moon — exactly half the disc bright, with a straight dividing line, and the
bright half turned towards the setting Sun in the west.

Moon overhead

90°

Half Moon (bright half towards the Sun) East West

Sun setting

Overhead at sunset means the Moon is a quarter of the sky — 90° — away from the Sun, and a 90°
separation always shows a half Moon.

(ii) It is in the waxing phase (Shukla Paksha).

Why it happens: The chapter tells us the mirror case: when the bright part has
shrunk to a half circle, the Moon is overhead at sunrise — that is the waning half
Moon. Overhead at sunset is the opposite half of the month. It also fits the rule of
thumb on page 173: a waxing Moon is easiest to spot at sunset, a waning Moon at
sunrise. Over the next week its bright part will keep growing until, at full Moon, it
rises just as the Sun sets.

Q6 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?

Kaushalya is telling the truth. Ravi cannot be right.

Page 28 of 43

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Class 8 Science Chapter 11 Keeping Time with the Skies
a g l AglaSem · NCERT Solutions

Ravi: A Moon rising in the east at the very moment the Sun is setting in the west is 180°
co m
se m.
away from the Sun. At 180° the entire lit half faces us — that is a full Moon, not a crescent. A

o m g l a
What Ravi.cdescribes is geometrically impossible.
crescent is only about 30°–45° from the Sun, so it can never be on the opposite horizon.

m a
l a se A gibbous Moon is roughly 135° from the Sun. In the afternoon the Sun is in the
agwestern half of the sky, so a point low in the east is about that far from it. A waxing gibbous
Kaushalya:

m
Moon does indeed rise in the east in the mid-afternoon — the chapter itself notes moonrise

. co ag
m
around 2:00–4:00 p.m., which is why the Moon can be spotted in daylight.

l a se
ag are locked together. Once you know how much
Why it happens: Phase and position
of the Moon is lit, you already know how far it must be from the Sun in the sky, and

. com
therefore roughly when it rises. Full Moon rises at sunset; the waxing gibbous rises a

m a s
few hours before sunset; the waxing crescent rises in the late morningem and is seen
co west just after sunset.
low in.the a gl
a s em
a gl
a s
comcalendars need an intercalary month more agl
Scientific studies show that the Moon is getting farther away from the Earth and
.
Q7

em
slower in its revolution. Will luni-solar
a s
agl
often or less often?

com

m .
e
Less often.

m l as
m .co a g
l a se At present: 12 lunar months = 12 × 29.5 = 354 days
ag Shortfall from the solar year = 365 − 354 = 11 days per year

se m
com
An extra month is needed when the shortfall adds up to about 29.5 days
g l a
m . a
ase
Time taken = 29.5 ÷ 11 ≈ 2.7 years — the chapter's "every 2–3 years"

agl
Suppose the Moon slowed until one cycle took 30 days:
co m
m .
as e
com = 365 − 360 = only 5 days per year
12 lunar months = 12 × 30 = 360 days

.Shortfall a g l
se m
g l a
a Time taken = 30 ÷ 5 = 6 years
c
m .
m a s e
e m . co agl
g l as
a

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m .
m ase
.co


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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Why it happens: A slower Moon means a longer cycle of phases, so 12 lunar months
come to more days than they do now and the lunar year sits closer to the solar year.
The annual shortfall — the whole reason an Adhika Maasa exists — becomes smaller,
so it takes many more years to build up to a full month. The correction is then
needed less frequently.

Did you know? This is a real, measured effect. Laser reflectors left on the Moon by
lunar missions show that it is moving away from us by about 4 cm a year, and as it
moves out its revolution slows.

Q8 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.

Count the months available and compare them with the number of full Moons.

Months in 3 years of a solar calendar = 3 × 12 = 36 months
Number of full Moons = 37

37 is greater than 36

Suppose, on the contrary, that no two full Moons shared a month. Then each of the 37 full
Moons would need a month of its own, so we would need at least 37 different months. But only
36 months exist in those three years. That is impossible, so the supposition is wrong: at least
one calendar month must contain two full Moons.

Why it happens: The reasoning is the pigeonhole idea — if you place 37 objects into
36 boxes, some box must hold more than one. The astronomy behind it is that a
lunar cycle of 29.5 days is shorter than almost every calendar month, so full Moons
arrive slightly faster than months do and one is bound to be caught twice. Over 3
years there are 3 × 365 = 1095 days, giving 1095 ÷ 29.5 ≈ 37 full Moons — exactly the
number in the question.

Did you know? A second full Moon within one calendar month is popularly called a
"blue Moon". Nothing about it is blue, and it is a fact about our calendar rather than
about the Moon.

Page 30 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Q9 On a particular night, Vaishali saw the Moon in the sky from sunset to sunrise.
What phase of the Moon would she have noticed?

She would have seen the full Moon (Purnima).

Why it happens: To be above the horizon for the whole night, the Moon must rise
exactly as the Sun sets and set exactly as the Sun rises. That is only possible if the
Moon is directly opposite the Sun in the sky — a separation of 180°. At 180° the
Earth is looking at the Moon from almost the same direction as the sunlight, so the
entire illuminated half is turned towards us: a full bright circle. This is position A, Day
1 in Fig. 11.5.

Any other phase would fail. A half Moon is only 90° from the Sun and so is up for only about half
the night; a crescent is close to the Sun and sets soon after it, or rises just before it.

Q10 If we stopped having leap years, in approximately how many years would the
Indian Independence day happen in winter?

About 700 to 750 years — roughly seven centuries.

Seasons repeat in about 365¼ days; a calendar with no leap years counts only 365 days

Drift each year = ¼ day = 0.25 day

So the calendar slips 1 day behind the seasons every 4 years

15 August is now in the rainy season. Winter weather comes about 6 months earlier in the

seasonal cycle:

Days to slip = 365 ÷ 2 ≈ 183 days

Years needed = 183 × 4 = about 730 years

Page 31 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Why it happens: Without the extra day, each calendar year ends about six hours
before the Earth has actually finished its orbit. The calendar therefore runs slightly
fast, and a fixed date such as 15 August arrives a little earlier in the seasonal cycle
each year. After roughly 730 years it would arrive where mid-February stands today
— a winter date. It would take about 4 × 365 ≈ 1460 years for the seasons to come
all the way back to where they started.

Tip: This is precisely the problem the leap year was invented to prevent, and why the
Gregorian calendar goes further and skips leap years in 1700, 1800 and 1900 while
keeping them in 1600 and 2000. Those small corrections keep the seasons pinned to
their dates for thousands of years.

Q11 What is the purpose of launching artificial satellites?

Artificial satellites are launched because a machine placed in orbit can see, hear and reach far
more of the Earth than anything on the ground can.

PURPOSE WHAT IT DOES FOR US

Communication Carries telephone, television and internet signals between places too far apart to be linked
directly

Navigation Lets a receiver on the ground, in a ship or in an aircraft fix its exact position

Weather monitoring Tracks clouds, storms and cyclones from above, so warnings can be issued in time

Disaster Maps floods, cyclones and earthquakes so relief can be sent where it is needed
management

Scientific research Studies the Earth, the Sun, the planets, stars and other celestial objects from above the
atmosphere

India's own missions show each of these at work. Cartosat takes high-quality images of the
Earth to improve maps, plan cities and handle natural disasters, and the Bhuvan platform uses
them to show terrain, soil, land use and vegetation. AstroSat observes stars and other celestial
objects. Chandrayaan 1, 2 and 3 went to the Moon, Aditya L1 studies the Sun and
Mangalyaan went to Mars. ISRO also lets Indian students build and launch small satellites such
as AzaadiSat, InspireSat-1 and Jugnu.

Page 32 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Did you know? After their useful life these satellites and their rocket parts become
space junk. Small pieces burn up as they fall through the atmosphere, but large ones
can reach the ground and any of them can collide with a working satellite — so
countries are now working together to remove this debris.

Q12 On which periodic phenomenon are the following measures of time based: (i) day
(ii) month (iii) year?

MEASURE OF PERIODIC PHENOMENON IT IS BASED ON LENGTH
TIME

(i) Day The rotation of the Earth about its own axis, which makes the Sun 24 hours (the
return to its highest point in the sky — found from the shortest mean solar day)
shadow

(ii) Month One complete cycle of the phases of the Moon, caused by the Moon's About 29.5 days
revolution around the Earth

(iii) Year The revolution of the Earth around the Sun, which gives one About 365¼ days
complete cycle of seasons

Why it happens: Each unit is one turn of a different wheel — the Earth spinning, the
Moon going round the Earth, and the Earth going round the Sun (Fig. 11.8). Because
none of the three wheels turns a whole number of times while another turns once,
calendars have to be adjusted: leap days for the year, and an Adhika Maasa for luni-
solar months.

Discover, design, and debate — Page 188

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Class 8 Science Chapter 11 Keeping Time with the Skies
a g l AglaSem · NCERT Solutions

Science • Society • Interdisciplinary Projects
co m
e m.
com as
DISCOVER, DESIGN, AND DEBATE PROJECTS

. a g l
e m
s Moon’s crescent always faces towards the Sun (Fig. 11.12). On days when you
Q1 laThe
g
a 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

. com
always crosses the illuminated part of the Moon first and clearly shows us that we
ag
a s emThe line joining the tips of the crescent would
see sunlight reflected off the Moon.

a l the Moon.
correspond to the diametergof

co m
em.
m l as
m .co a g
l a se
a g
m a s
m.co agl
l a se
a g

co m
m .
m as e
.co a g l
se m
g l a
a
se m
com g l a
m . a
ase
agl

co m
m .
m as e
.co a g l
se m
g l a
a c
Fig. 11.12, page 188 — redrawn sketch of the textbook illustration: the Sun low over the
m .
m a s e
agl
fields and the Moon higher up in the evening sky.

. co
e m
g l as
a

co m
m .
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.co


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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

How to do it. Just after sunset (for a waxing crescent) stand where you can see both the glow of
the Sun near the western horizon and the crescent. Point one finger at the Sun, then sweep it
along the shortest path in the sky to the Moon. Never look straight at the Sun — point at it, do
not stare at it.
What you will find. Every single time, the finger reaches the bright edge of the crescent before
it reaches the dark part. The bulging outer edge of the crescent is always turned Sunward.

Why it happens: This is direct evidence that the Moon shines by reflected sunlight
rather than by its own. The half of the Moon lit up is the half facing the Sun, so the
bright side of the disc we see must be the side nearer the Sun in the sky. If the Moon
glowed by itself there would be no reason for its bright part to keep pointing at the
Sun.

The line joining the tips. The two tips of a crescent are the points where the light–dark
boundary meets the Moon's rim. That boundary is a great circle running round the Moon
through its poles, so its two visible ends are always at opposite points of the rim — and the line
joining opposite points of a circle is a diameter. Measuring across the horns therefore
measures the full width of the Moon, however thin the crescent is.

Try this: Hold a straight stick or a taut thread across the two tips and see how it lines
up at right angles to your finger's Sun-to-Moon path. The crescent always bends
away from the Sun, never towards it.

Q2 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?

The dates that can differ are the ones that fall between 29 February and 20 April — that is, the
last three weeks of Phalguna and the whole of Chaitra. Everything from 1 Vaisakha to 9
Phalguna maps to the same Gregorian date every single year.

Page 35 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

MONTH OF THE INDIAN DAYS GREGORIAN DATES IN A COMMON YEAR
NATIONAL CALENDAR

Chaitra 30 (31 in a 22 March – 20 April (21 March – 20 April in a
leap year) leap year)

Vaisakha 31 21 April – 21 May

Jyaistha 31 22 May – 21 June

Asadha 31 22 June – 22 July

Sravana 31 23 July – 22 August

Bhadra 31 23 August – 22 September

Asvina 30 23 September – 22 October

Kartika 30 23 October – 21 November

Agrahayana 30 22 November – 21 December

Pausa 30 22 December – 20 January

Magha 30 21 January – 19 February

Phalguna 30 20 February – 21 March (20 February – 20
March when February has 29 days)

Why it happens: The Indian National Calendar has a fixed length of 365 days, so as
long as the Gregorian year is also 365 days the two run in perfect step. The only
disturbance is the Gregorian leap day. When 29 February exists it becomes 10
Phalguna, so from 1 March onwards every Gregorian date carries a Saka day-
number one higher than usual, right up to 20 April. Then the calendar puts things
right by giving Chaitra 31 days instead of 30, so the new year begins on 21 March
and 1 Vaisakha is back on 21 April as always.

Check it yourself: Independence Day is 24 Sravana and Republic Day is 6 Magha —
in every year, leap or not, because both fall inside the fixed stretch of the year. Verify
it in Fig. 11.9 style: Sravana begins on 23 July, so 15 August is its 24th day.

Page 36 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Q3 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 based on the lunar calendar or the solar calendar or the
luni-solar calendar.

How to do it. Ask family members and classmates from different states, then check each
festival's date over three or four years on a Gregorian calendar. That test alone tells you the
calendar type: a date that hardly moves means a solar festival; a date that wanders by up to a
month means a luni-solar one; a date that drifts steadily earlier through all the seasons means
a purely lunar one.
Sample answer:

STATE NEW YEAR FESTIVAL CALENDAR IT WHEN IT FALLS
FOLLOWS

Maharashtra Gudi Padwa Luni-solar First day after the new Moon
of Chaitra (March–April)

Karnataka, Andhra Ugadi Luni-solar The same day as Gudi Padwa
Pradesh, Telangana

Gujarat Bestu Varas Luni-solar The day after Diwali, in
Kartika (October–November)

Jammu and Kashmir Navreh Luni-solar First day of Chaitra (March–
April)

Manipur Cheiraoba (Sajibu Luni-solar First day of the month of
Nongma Panba) Sajibu (April)

Punjab Vaisakhi Solar (sidereal) 13 or 14 April almost every
year

West Bengal Poila Baisakh Solar (sidereal) 14 or 15 April

Assam Bohag Bihu (Rongali Solar (sidereal) 14 or 15 April
Bihu)

Tamil Nadu Puthandu Solar (sidereal) 14 April

Kerala Vishu Solar (sidereal) 14 or 15 April

Odisha Pana Sankranti (Maha Solar (sidereal) 14 April
Vishuva Sankranti)

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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Why it happens: The pattern in the table is not an accident. All the mid-April New
Years mark the same astronomical event — the Sun entering the star pattern of
Mesha (Aries) on the sidereal solar calendar — which is why they cluster on 14 April
everywhere from Punjab to Kerala. The luni-solar New Years are tied instead to a
phase of the Moon in a named month, so their Gregorian dates move about, but
only within a month, because of the Adhika Maasa correction.

Q4 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.

Sample answer (fill your own table from the calendars you collect and compare — dates for a
festival can differ by a day between regions):

YEAR EID-UL- SHIFT FROM DIWALI (LAKSHMI SHIFT FROM
FITR PREVIOUS YEAR PUJA) PREVIOUS YEAR

2021 14 May — 4 November —

2022 3 May 11 days earlier 24 October 11 days earlier

2023 22 April 11 days earlier 12 November 19 days later — a jump

2024 11 April 11 days earlier 1 November 11 days earlier

2025 31 March 11 days earlier 20 October 12 days earlier

Eid-ul-Fitr moves back by almost exactly 11 days every year, without a single exception. On the
lunar calendar itself, though, the date does not move at all — it is always 1 Shawwal, the day
after the crescent Moon is sighted at the end of Ramazan.
Diwali mostly moves back by about 11 days too, but between 2022 and 2023 it jumped forward
by 19 days. That jump is the signature of an Adhika Maasa: an extra lunar month was inserted
in 2023 (an extra Shravana), which pushed every later festival of that year about a month ahead.

Page 38 of 43

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Class 8 Science Chapter 11 Keeping Time with the Skies
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Why it happens: Eid-ul-Fitr follows a purely lunar calendar, which makes no attempt
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to keep up with the seasons — so it slips

agla luni-solar calendar, which does the same slipping
Gregorian month. Diwali follows
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never leaves autumn while Eid-ul-Fitr travels right through the year.
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Every morning on your way
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a g l Page 39 of 43

Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

TIME OF YEAR WHERE THE SUN RISES NAME GIVEN TO THE
MOVEMENT

About 21 December (winter Farthest south of east Turning point — Uttarayan
solstice) begins

January to March Creeping northward month by Uttarayan
month

About 21 March (spring Exactly east — and the Sun sets Uttarayan continues
equinox) exactly west

About 21 June (summer solstice) Farthest north of east Turning point — Dakshinayan
begins

July to September Creeping southward Dakshinayan

About 23 September (autumn Exactly east again Dakshinayan continues
equinox)

Yes — this is exactly the Uttarayan and Dakshinayan of the 'Our scientific heritage' box. The
Taittirīya Saṁhitā records the same pattern: "Thus the Sun moves southwards for six months
and northwards for six months."

Why it happens: The Earth's axis is tilted, so as the Earth goes round the Sun the
point where the Sun's rays meet the horizon at dawn shifts north and south through
the year. Across India the swing is roughly 24° either side of due east near the
southern tip and about 27° near Delhi — a wide enough gap on the horizon that
ancient observers could see it against fixed landmarks and use it as a calendar,
without any instrument at all.

Did you know? Because the sidereal year is 20 minutes longer than the tropical year,
festivals fixed to the sidereal calendar slowly slide away from the solstice they once
marked. Makar Sankranti now moves ahead by one day every 71 years — which is
why it falls in mid-January today although it was once tied to the December solstice.

Chapter at a glance
The Moon does not shine by itself. Sunlight always lights exactly one half of it. The half
turned towards the Earth is not always the lit half, so on different days we see different
fractions of the lit half — these are the phases of the Moon.

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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

In the waning period (Krishna Paksha) the bright part shrinks from a full circle to nothing
in about two weeks; in the waxing period (Shukla Paksha) it grows back. One full Moon to
the next takes about 29.5 days.
Phase and position go together. On full Moon day the Moon is opposite the Sun (it rises at
sunset); on new Moon day it is in almost the same direction as the Sun. Because the Moon
moves about 12° eastward each day, it rises about 50 minutes later every day.
Three natural periodic events give three units of time — Earth's rotation gives the mean
solar day of 24 hours, the Moon's cycle of phases gives the month, and Earth's revolution
gives the solar year of about 365¼ days.
A lunar year of 12 lunar months is only 354 days, so it falls short of the solar year by about
11 days. Solar calendars (Gregorian, Indian National Calendar) fit the seasons and use a
leap year; luni-solar calendars count months by the Moon but add an Adhika Maasa
every 2–3 years to stay with the seasons.
Most Indian festivals are set by the Moon's phase, so their Gregorian dates move. Festivals
on a solar sidereal calendar, such as Makar Sankranti, keep almost the same date. Besides
the Moon, artificial satellites orbit the Earth — most about 800 km up, one orbit in about
100 minutes.

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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

Quick revision

TERM WHAT IT MEANS HOW YOU RECOGNISE OR WHERE IT APPEARS
MEASURE IT IN THE CHAPTER

Phases of the The changing shapes of the Caused by the change in the Fig. 11.2 and Fig. 11.5;
Moon bright part of the Moon as Sun–Moon angle in the sky as Activity 11.1 and
seen from the Earth from one the Moon revolves round the Activity 11.2
day to the next Earth — not by Earth's shadow

Waxing (Shukla The fortnight in which the A waxing Moon is easiest to Section 11.1.1, page
Paksha) and bright part grows, and the spot at sunset, a waning 172–173
waning (Krishna fortnight in which it shrinks Moon at sunrise
Paksha) periods

Full Moon The Moon is nearly opposite Rises about when the Sun sets Position A, Day 1 in
(Purnima) the Sun, so the entire lit half and sets about when the Sun Fig. 11.5
faces us rises

New Moon The Moon is in nearly the Not visible at all; it rises and Position E, Day 15 in
(Amavasya) same direction as the Sun, so sets nearly with the Sun Fig. 11.5
only its non-illuminated half
faces us

Gibbous phase / More than half of the lit Gibbous at positions B and H; Page 176
Crescent phase portion is seen / less than half crescent at positions D and F
is seen

Mean solar day The average time the Sun Mark the tip of a vertical Activity 11.3 and Table
takes from its highest point in stick's shadow; the shadow is 11.2 — 24 hours
the sky one day to its highest shortest when the Sun is
point the next day highest

Lunar month One complete cycle of the About 29.5 days; 12 of them Section 11.2.1, page
phases of the Moon make a lunar year of 354 days 179

Solar year and leap One revolution of the Earth About 365¼ days. A year Section 11.2.2, page
year round the Sun, that is, one divisible by 4 is a leap year; 180
cycle of seasons; the extra 1700, 1800, 1900 were
day that keeps the calendar skipped, 1600 and 2000 were
with the seasons kept

Lunar, solar and Calendars that follow the 354 days / 365 days / months Sections 11.2.1 to
luni-solar Moon's phases, the cycle of by the Moon with a correction 11.2.3
calendars seasons, or both together for the seasons

Adhika Maasa An extra month added to a It makes up the 11-day Page 181–182
(intercalary month) luni-solar year every 2–3 shortfall of the lunar year, so
years festivals stay in their seasons

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Class 8 Science Chapter 11 Keeping Time with the Skies AglaSem · NCERT Solutions

TERM WHAT IT MEANS HOW YOU RECOGNISE OR WHERE IT APPEARS
MEASURE IT IN THE CHAPTER

Tropical year and Spring equinox to spring The sidereal year is longer 'A step further', page
sidereal year equinox; and the time for the than the tropical year by just 180
same stars to rise again at 20 minutes
sunset

Indian National India's official solar calendar 365 days; the year begins on Fig. 11.9; adopted 21
Calendar of the Shaka Era, used with 22 March; months of 30 or 31 March 1956 CE = 1
the Gregorian calendar days, the 2nd to 6th having 31 Chaitra 1878 Saka

Artificial satellite A human-made object Most orbit about 800 km up Section 11.4; Cartosat,
launched from the Earth to and take about 100 minutes AstroSat,
orbit it for one orbit; seen as a steady Chandrayaan, Aditya
point of light moving fast L1, Mangalyaan

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