Keeping Time with the Skies Class 8: Phases of the Moon, Calendars and Satellites

What Keeping Time with the Skies is about

Key concepts

Phases of the Moon

NCERT section 11.1.1 Phases of the Moon

Activity 11.1: from the day after full Moon, note each day at sunrise (and, after about two weeks, at sunset) the Moon's shape, whether the bright part grew or shrank, and whether it looked nearer to or farther from the Sun.

Phases are the changing shapes of the Moon's bright part seen from Earth over about a month.

• Full Moon (Purnima): a full bright circle.
• Waning period (Krishna Paksha): the bright part shrinks: gibbous, half (after about a week), crescent, then none.
• New Moon (Amavasya): the bright part cannot be seen.
• Waxing period (Shukla Paksha): the bright part grows: crescent, half (about a week), gibbous, full Moon (another week).
• Gibbous: more than half the lit part visible; crescent: less than half.

One cycle from full Moon to full Moon takes about 29.5 days.

Worked example

Put these in order starting from full Moon: crescent, new Moon, half Moon, gibbous (waning). Name this half of the cycle in Indian calendars.

Try it first, then show the solutionSolution

1. After full Moon the bright part shrinks: gibbous, then half, then crescent, then none.
2. Order: gibbous (waning), half Moon, crescent, new Moon.
3. This waning half is called Krishna Paksha.

Answer: Gibbous → half → crescent → new Moon; Krishna Paksha.

Locating the Moon in the sky

NCERT section 11.1.2 Locating the Moon

• On full Moon day the Moon is nearly opposite the Sun: as the Sun rises in the east, the Moon is setting in the west.
• On later mornings, as the Moon wanes, it appears closer to the Sun in the sky. At the half Moon (waning) it is overhead at sunrise. The crescent is closer still.
• On new Moon day it is closest to the Sun (and not visible).
• A waning Moon is easiest to see at sunrise; a waxing Moon at sunset.

The Moon rises about 50 minutes later each day (for example 14:23 on 7 April 2025 and 15:17 the next day). So the Moon is often up in the daytime: moonrise can be in the afternoon, and the Moon can be seen in the eastern sky in daylight.

Why later each day? In the 24 hours the Earth takes to spin once, the Moon moves ahead in its orbit, so the Earth must turn a little more (about 50 minutes) for the Moon to return to the same place in the sky.

Worked example

The Moon rises at 2:00 p.m. today. Roughly when will it rise a week later, and will it be easier to spot at sunset or sunrise then?

Try it first, then show the solutionSolution

1. It rises about 50 minutes later each day: $7\times 50=350$ minutes $=5$ h 50 min later.
2. Moonrise is then about 7:50 p.m.
3. A Moon rising in the afternoon and then later each evening is waxing towards full, so it is easier to spot at sunset.

Answer: About 7:50 p.m.; easier at sunset (waxing).

Why phases happen (ball-and-lamp model)

NCERT section 11.1.3 Making sense of our observations

The Moon's shape never changes; only the part we see lit does.

• The Moon has no light of its own; it reflects sunlight. The half facing the Sun is always illuminated; the other half is dark.
• As the Moon revolves around the Earth, the same side of the Moon always faces us, but that side is not always the lit half.
• The fraction of the lit half that faces Earth changes during the month, giving the phases.

Activity 11.2: a ball on a stick (Moon) held at arm's length above the head (Earth), with a lamp or torch (Sun) about 3 m away. Turning slowly anticlockwise:

• ball towards the lamp (position E): only its dark side faces you: new Moon;
• ball opposite the lamp (position A): the whole lit side faces you: full Moon;
• in between: half, gibbous (more than half) or crescent (less than half), with a curved edge between light and dark.

Because one Earth rotation (a day) is much shorter than the Moon's revolution (about a month), people everywhere on Earth see nearly the same phase on a given day.

Not Earth's shadow: phases come from changing positions of Sun, Moon and Earth. Earth's shadow on the Moon causes a lunar eclipse (only at full Moon); a solar eclipse can happen only at new Moon. Eclipses are not monthly because the Moon's orbit is slightly tilted.

Worked example

In Activity 11.2, where must you hold the ball to see it fully lit, and where to see none of the lit part? Which phases do these model?

Try it first, then show the solutionSolution

1. Fully lit: hold the ball on the side away from the lamp, so you face its whole lit half: full Moon.
2. None lit: hold the ball between you and the lamp, so only its dark half faces you: new Moon.

Answer: Opposite the lamp = full Moon; towards the lamp = new Moon.

Day, month and year

NCERT section 11.2 How Did Calendars Come into Existence?

Three natural cycles give three units of time:

• Day: the Earth's rotation makes the Sun rise in the east and set in the west. The average time for the Sun to go from its highest point one day to its highest point the next is 24 hours: the mean solar day.
• Month: the cycle of the Moon's phases, about 29.5 days.
• Year: the Earth's revolution round the Sun, about 365¼ days, one cycle of seasons (solar year).

Activity 11.3: fix a 1 m stick upright in a sunny spot; from 11:00 a.m. to about 1:10 p.m., mark the tip of the shadow every minute. The shortest shadow shows when the Sun is highest. Repeat on several days: the time between shortest shadows on consecutive days is about 24 hours (for example 12:20, 12:20, 12:19 gives 24:00 and 23:59).

Worked example

A student's shadow-stick readings for the time of the shortest shadow are 12:18 on Monday, 12:19 on Tuesday and 12:21 on Wednesday. Find the lengths of the two solar days, and explain why we use a mean solar day of 24 hours.

Try it first, then show the solutionSolution

1. Monday to Tuesday: from 12:18 to 12:19 the next day is 24 h 1 min.
2. Tuesday to Wednesday: from 12:19 to 12:21 the next day is 24 h 2 min.
3. The time from one highest point of the Sun to the next varies slightly through the year, so we use its average, the mean solar day of 24 hours.

Answer: 24 h 1 min and 24 h 2 min; the average of such days is the 24-hour mean solar day.

Lunar, solar and luni-solar calendars

NCERT section 11.2.3 Luni-solar calendars

Lunar calendar (11.2.1): 12 lunar months of about 29.5 days make a lunar year of $12\times 29.5=354$ days. Since seasons repeat in about 365 days, a lunar calendar drifts by about 11 days a year, and its months do not stay fixed to seasons.

Solar calendar (11.2.2): made to match seasons, important for farming. The Gregorian calendar has months of 30, 31 or 28 days adding to 365. The extra quarter day adds up to about a day in 4 years, so a leap year (year divisible by 4) has 29 days in February. Since this slightly overcorrects, century years are not leap years (1700, 1800, 1900) unless divisible by 400 (1600, 2000). The Gregorian calendar follows the tropical year (spring equinox to spring equinox); the sidereal year (same stars rising at sunset again) is about 20 minutes longer.

Luni-solar calendar (11.2.3): months follow the Moon, but since 12 lunar months fall about 11 days short, after 2 to 3 years the gap nears a month and an extra month (Adhika Maasa, intercalary month) is added. Month names: Chaitra, Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha, Pausha, Magha, Phalguna. Amant months end on new Moon; Purnimant months end on full Moon.

Heritage: the Sun rises a little north of east in summer and south of east in winter. Its northward shift (December to June) is Uttarayan; southward (June to December) is Dakshinayan, recorded in the *Taittirīya Saṁhitā* and *Surya Siddhanta*.

Worked example

Show why a luni-solar calendar needs an extra month roughly every 3 years.

Try it first, then show the solutionSolution

1. Lunar year $=12\times 29.5=354$ days; solar year about 365 days.
2. Shortfall each year $=365-354=11$ days.
3. In 3 years: $3\times 11=33$ days, more than one lunar month of 29.5 days, so an extra month is added.

Answer: The 11-day yearly gap adds up to over a month in about 3 years.

Indian National Calendar and festivals

NCERT section 11.3 Are Festivals Related to Astronomical Phenomena?

Indian National Calendar (11.2.4): a solar calendar of 365 days used alongside the Gregorian calendar for official purposes.

• The year starts on 22 March (the day after the spring equinox); in leap years Chaitra gets an extra day and the year starts on 21 March.
• Months have 30 or 31 days: the 2nd to 6th months have 31 days, the rest 30 ($5\times 31+7\times 30=365$).
• Recommended by the Calendar Reform Committee (set up 1952, chaired by astrophysicist Meghnad Saha) and adopted from 21 March 1956 (1 Chaitra 1878 Saka).

Festivals (11.3):

• Lunar or luni-solar festivals: Diwali (new Moon of Kartika), Holi (full Moon of Phalguna), Buddha Purnima (full Moon of Vaisakha), Dussehra (10th day of Ashwina), Eid-ul-Fitr (crescent Moon after Ramazan). Their Gregorian dates change each year.
• Luni-solar festivals shift by less than a month (the extra month corrects them); a purely lunar festival like Eid moves about 11 days earlier each year and passes through all seasons.
• Makar Sankranti, Pongal, Bihu, Vaisakhi, Poila Baisakh, Puthandu follow a solar sidereal calendar: nearly the same date every year, slowly moving ahead (Makar Sankranti by one day every 71 years) because of a slow wobble of Earth's axis.
• The Rashtriya Panchang (Positional Astronomy Centre) fixes festival dates for the whole country.

Worked example

Why does Eid-ul-Fitr move through all seasons over the years, while Diwali stays in October or November?

Try it first, then show the solutionSolution

1. Eid follows a purely lunar calendar of 354 days, so it comes about 11 days earlier each Gregorian year and keeps drifting.
2. Diwali follows a luni-solar calendar that adds an extra month every 2 to 3 years, pulling it back in step with the seasons.
3. So Diwali shifts by less than a month and stays in autumn.

Answer: Eid is lunar (no correction); Diwali is luni-solar (corrected by Adhika Maasa).

Artificial satellites

NCERT section 11.4 Why Do We Launch Artificial Satellites in Space?

The Moon is Earth's natural satellite. Artificial satellites are human-made objects placed in orbit round the Earth.

• Most orbit about 800 km up and take about 100 minutes per orbit; at night they look like fast-moving points of light.
• Uses: communication, navigation, weather monitoring, disaster management and scientific research.
• ISRO examples: Cartosat (high-quality images for maps, city planning and disasters; used by the Bhuvan platform), AstroSat (observes stars and other celestial objects); missions Chandrayaan 1, 2, 3 (Moon), Aditya L1 (Sun), Mangalyaan (Mars); student satellites such as AzaadiSat, InspireSat-1 and Jugnu.
• Vikram Sarabhai is called the Father of the Indian Space Programme; VSSC in Thiruvananthapuram is named after him.

Activity 11.4: just before sunrise or after sunset, from an open place with an adult, look for a point of light moving fast across the sky; apps and websites list passes.

Space debris: dead satellites and rocket parts crowd space and can hit working satellites; small pieces burn up in the atmosphere, larger ones may crash. Countries are working to remove it.

Worked example

A satellite takes 100 minutes per orbit. About how many orbits does it complete in a day, and why can it be seen best just after sunset?

Try it first, then show the solutionSolution

1. Minutes in a day $=24\times 60=1440$; orbits $=\frac{1440}{100}=14.4$.
2. Just after sunset, the sky is dark for us, but a satellite high above is still in sunlight and reflects it, so it shows up as a moving point of light.

Answer: About 14 orbits a day; it is sunlit while our sky is dark.

Science in real life: Keeping Time with the Skies

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Calendars: Day, Month, Year, Leap Years and Festival Dates
• Phases of the Moon and the Moon's Changing Position

Keeping Time with the Skies: formulas and key facts

Lunar month

$\approx 29.5\text{ days}$

Lunar year

$12\times 29.5=354\text{ days}$

Yearly gap

$365-354=11\text{ days}$

Moonrise delay

$\approx 50\text{ minutes later each day}$

Gregorian leap year

$\text{divisible by }4;\text{century years only if divisible by }400$

Indian National Calendar

$5\times 31+7\times 30=365\text{ days, starting 22 March}$

Typical satellite

$\approx 800\text{ km high},\approx 100\text{ min per orbit}$

Keeping Time with the Skies: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Keeping Time with the Skies

The question patterns to expect, and the method that works for each.

Tricks to remember Keeping Time with the Skies

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Shukla shines more

  Shukla (bright) Paksha: the bright part grows; Krishna (dark) Paksha: it darkens.

  Use it when: Waxing and waning names.

• Full faces away from the Sun

  Full Moon is opposite the Sun, so it rises as the Sun sets and is up all night.

  Use it when: Locating the full Moon.

• Fifty-minute Moon

  The Moon is 'late by fifty' every day.

  Use it when: Moonrise times and tides.

• 354 + 11 = 365

  Lunar year plus eleven makes a solar year; three elevens make a month.

  Use it when: Adhika Maasa and Eid drift.

• Leap year rule 4-100-400

  Every 4th year leap, every 100th not, every 400th yes.

  Use it when: Leap year questions.

Keeping Time with the Skies: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Keeping Time with the Skies

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying Earth's shadow causes the phases

  It sounds like an eclipse.

  Wrong working

  Earth's shadow covers part of the Moon, making a crescent.

  Right working

  Phases happen because we see different fractions of the Moon's sunlit half as it revolves; Earth's shadow causes only a lunar eclipse.

  ✓Do this: Phases = viewing angle; eclipse = shadow.

  Usually costs: 1-2 marks.

• ✗Mistake: Thinking the Moon is seen only at night

  It is noticed mostly at night.

  Wrong working

  The Moon never appears in the daytime.

  Right working

  The Moon rises about 50 minutes later each day and is often visible in daylight, for example a half Moon at sunrise or sunset.

  ✓Do this: Use moonrise times.

  Usually costs: 1 mark.

• ✗Mistake: Calling a year divisible by 4 always a leap year

  The century rule is forgotten.

  Wrong working

  1900 was a leap year because it divides by 4.

  Right working

  Century years are leap years only if divisible by 400: 1900 no, 2000 yes.

  ✓Do this: Apply 4-100-400.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up lunar and luni-solar

  Both use Moon months.

  Wrong working

  A lunar calendar adds an extra month every three years.

  Right working

  Only a luni-solar calendar adds Adhika Maasa; a purely lunar calendar does not, so it drifts through seasons.

  ✓Do this: Luni-solar = lunar months + solar correction.

  Usually costs: 1 mark.

• ✗Mistake: Wrong phase for 'overhead at sunset'

  Waxing and waning are confused.

  Wrong working

  A half Moon overhead at sunset is waning.

  Right working

  It is waxing (first-quarter half Moon); the waning half Moon is overhead at sunrise.

  ✓Do this: Waxing at sunset, waning at sunrise.

  Usually costs: 1 mark.

NCERT exercise guide: Keeping Time with the Skies

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Keeping Time with the Skies

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The full Moon is called
   (a) Amavasya
   (b) Purnima
   (c) Shukla Paksha
   (d) Krishna Paksha

   Hint

   Full circle.

   Answer

   (b) Purnima

2. Q2 · 1 mark · MCQExam level

   The Indian National Calendar usually begins on
   (a) 1 January
   (b) 14 April
   (c) 22 March
   (d) 21 June

   Hint

   After the spring equinox.

   Answer

   (c) 22 March

3. Q3 · 1 mark · Very short answerExam level

   Name ISRO's mission to study the Sun.

   Hint

   L1.

   Answer

   Aditya L1.

4. Q4 · 2 marks · Very short answerExam level

   How is the time of the Sun's highest point found using a stick?

   Hint

   Shadow length.

   Answer

   Mark the tip of the stick's shadow every minute around midday; the time of the shortest shadow is when the Sun is highest.

5. Q5 · 2 marks · Very short answerHarder exam level

   Why is a waning Moon easiest to spot at sunrise?

   Hint

   Position relative to the Sun.

   Answer

   After full Moon, the Moon lags further behind each day, so it rises later at night and is still up in the western or overhead sky at sunrise.

6. Q6 · 2 marks · Very short answerHarder exam level

   Show that the Indian National Calendar has 365 days in a normal year and explain what happens in a leap year.

   Hint

   Five months of 31.

   Answer

   $5\times 31+7\times 30=155+210=365$ days. In a leap year Chaitra gets 31 days (366 in all) and the year begins on 21 March.

7. Q7 · 3 marks · Short answerHarder exam level

   Why can a lunar eclipse happen only on a full Moon day, but not every full Moon day?

   Hint

   Tilted orbit.

   Answer

   A lunar eclipse needs Earth between the Sun and Moon, which happens only at full Moon. The Moon's orbit is slightly tilted relative to Earth's orbit, so at most full Moons it passes above or below Earth's shadow.

8. Q8 · 3 marks · Short answerHarder exam level

   Why are the Gregorian dates of Diwali and Holi different each year but always within about a month of each other year to year?

   Hint

   Luni-solar.

   Answer

   They follow a luni-solar calendar based on Moon phases, so their Gregorian dates change. Adding Adhika Maasa every 2 to 3 years keeps the lunar months in step with the solar year, so the shift stays under a month.

9. Q9 · 3 marks · Short answerHarder exam level

   Describe the ball-and-lamp model of the Moon's phases and what it shows.

   Hint

   Activity 11.2.

   Answer

   Hold a ball on a stick at arm's length above your head with a lamp or torch about 3 m away, and turn slowly anticlockwise. Facing the lamp, the ball looks dark (new Moon); with your back to the lamp, it looks fully lit (full Moon); in between it looks crescent, half or gibbous. It shows phases depend on how much of the lit half faces the viewer.

10. Q10 · 5 marks · Long answerHarder exam level

    Explain how the day, the month and the year arise from motions of the Earth and Moon, and describe how the length of a solar day can be measured with a stick. Why do we need leap years?

    Hint

    Three cycles.

    Answer

    Day: Earth's rotation makes the Sun return to its highest point about every 24 hours. Month: the Moon's revolution round the Earth gives a cycle of phases of about 29.5 days. Year: Earth's revolution round the Sun gives a cycle of seasons of about 365¼ days. Measuring: mark a vertical stick's shadow tip every minute near midday on consecutive days; the time between shortest shadows is about 24 hours. Leap years: the extra quarter day each year adds up to a day in 4 years, so February gets 29 days to keep the calendar with the seasons.

11. Q11 · 2 marks · Very short answerHOTS / competency

    The bright part of a crescent Moon always points towards the Sun. Explain why.

    Hint

    Reflected sunlight.

    Answer

    The Moon shines only by reflecting sunlight, so the lit side is always the side facing the Sun; the curved bright edge of the crescent therefore points towards the Sun.

12. Q12 · 3 marks · Short answerHOTS / competency

    Riya's birthday, 10 August, fell on a new Moon (Amavasya) this year. Roughly when will the new Moon closest to her birthday fall next year? Explain.

    Hint

    12 lunar months = 354 days.

    Answer

    Twelve lunar months later is 354 days, about 11 days before 10 August, so around 30 July; the following new Moon comes about 29.5 days after that, around 28 August. So her next birthday is not a new Moon; the Moon's phase on a date shifts by about 11 days each year.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is the Moon's phase called when it cannot be seen?Show answer

   New Moon (Amavasya).

2. 2.What is the gibbous phase?Show answer

   When more than half of the Moon's lit part is seen from Earth.

3. 3.About how much later does the Moon rise each day?Show answer

   About 50 minutes.

4. 4.How long does the Moon take to go through all its phases?Show answer

   About 29.5 days.

5. 5.What is the mean solar day?Show answer

   The average time (24 hours) for the Sun to return to its highest point in the sky.

6. 6.How long is a lunar year?Show answer

   About 354 days.

7. 7.What is Adhika Maasa?Show answer

   An extra (intercalary) month added every few years in luni-solar calendars.

8. 8.When does the Indian National Calendar year begin?Show answer

   On 22 March (21 March in a leap year).

9. 9.Who chaired the Calendar Reform Committee?Show answer

   Meghnad Saha.

10. 10.Name two uses of artificial satellites.Show answer

    Communication and weather monitoring (also navigation, disaster management, research).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you name the phases in order from full Moon to full Moon?
• ☐Can you explain phases with the Sun, Earth and Moon positions?
• ☐Can you say why phases are not caused by Earth's shadow?
• ☐Can you tell where the Moon is at sunrise in each waning phase?
• ☐Can you explain why the Moon rises about 50 minutes later each day?
• ☐Can you link day, month and year to natural cycles?
• ☐Can you calculate the lunar year and the 11-day gap?
• ☐Can you apply the leap year rule to 1900 and 2000?
• ☐Can you explain why Eid drifts but Diwali stays in autumn?
• ☐Can you list uses of satellites and two ISRO missions?

Keeping Time with the Skies: frequently asked questions