Class 8 Science · Original EZT practice
Keeping Time with the Skies (Class 8 Chapter 11): summary and practice
Before clocks and phones, people read time off the sky. This chapter shows how the Moon's changing face gave us the month, how the Sun gave us the year, why our festivals wander across dates, and why hundreds of artificial satellites now circle above us. All questions are original EZT tutor practice following the chapter's own section order. Attempt each before reading the model answer.
10 original worked examples, 10 fresh self-check questions and 4 mixed questions, with explained answers. These are not copied textbook exercises, an official chapter index or a full answer key. No affiliation with NCERT is claimed.
Official sources for topic and edition context, checked 18 October 2026:
1. Phases of the Moon: where the month comes from
The Moon takes about 29.5 days - nearly a month - to complete one full cycle of phases as seen from Earth. The fully lit disc is the full Moon (Purnima); the day it is not visible is the new Moon (Amavasya). The waxing period is Shukla Paksha and the waning period is Krishna Paksha.
2. Three families of calendars
A lunar calendar counts 12 lunar months, about 354 days. A solar calendar keeps 365 days and adds leap days. A luni-solar calendar keeps lunar months but inserts an extra month every few years so festivals stay near their seasons.
3. The Indian National Calendar
The Indian National Calendar is a solar calendar of 365 days whose months carry traditional Indian names. The year begins on 22 March in a regular year, and on 21 March in a leap year when Chaitra receives the extra day.
4. Festivals and the sky
Many Indian festivals are fixed by the Moon, so their Gregorian dates shift each year. Luni-solar calendars keep them near the same season; purely lunar calendars can drift right across the seasons.
5. Artificial satellites
The Moon is Earth's natural satellite, and hundreds of artificial satellites now orbit above us. They support communication, navigation, weather monitoring, disaster management and scientific research.
Choose, solve, check, then try a fresh question.
Before calculating, read the method prompt and name the condition that makes it useful. Write your own attempt. Open the steps only when you need them, and use the check to catch a wrong sign, unit or interpretation. Finish with the fresh question before reading its answer.
Predict the next full Moon
Choose the method: Use the roughly 29.5-day phase cycle, not a calendar month of 30 or 31 days.
EZT practice question
A full Moon is seen tonight. Roughly how many days until the next full Moon?
Show the worked solution
- One complete phase cycle takes about 29.5 days.
- That is nearly a month.
- So the next full Moon comes about 29 to 30 days later.
Answer: About 29-30 days later, or roughly a month.
Check: The Moon's phase cycle is close to, but not exactly, one calendar month.
Watch for this: Do not say exactly 30 days; the natural cycle is about 29.5 days.
Now try it yourself
If a full Moon fell on the 5th of a month, around which date would you expect the next new Moon?
Check your answer
Around the 19th-20th.
New Moon comes about half a cycle, or 14-15 days, after full Moon.
Identify waxing after a new Moon
Choose the method: Start at Amavasya and follow whether the visible lit part grows or shrinks.
EZT practice question
Tonight is a new Moon. Will the Moon be waxing or waning over the next two weeks, and what is that period called?
Show the worked solution
- At new Moon, the lit part is not visible from Earth.
- After Amavasya, the lit part grows night by night.
- A growing Moon is waxing; this is the Shukla Paksha.
Answer: It will be waxing, in the Shukla Paksha, moving towards the next full Moon.
Check: New Moon to full Moon is the growing half of the cycle.
Watch for this: Do not call this Krishna Paksha; that is the shrinking half after full Moon.
Now try it yourself
What are the Hindi names for the full-Moon and new-Moon days?
Check your answer
Purnima and Amavasya.
Purnima is full Moon and Amavasya is new Moon.
Explain why the Moon shows phases
Choose the method: Separate the Moon's sunlit half from the part of that half we can see from Earth.
EZT practice question
Why does the Moon appear to change shape through the month?
Show the worked solution
- The Sun lights one half of the Moon.
- As the Moon travels around Earth, we see changing amounts of that lit half.
- The visible share grows and shrinks in a regular cycle.
Answer: Phases happen because our view of the Moon's sunlit half changes as it orbits Earth.
Check: The Moon itself is not being cut into pieces; only the visible lit fraction changes.
Watch for this: Do not explain ordinary phases as Earth's shadow; that describes a lunar eclipse, not the monthly cycle.
Now try it yourself
About how long does one full phase cycle take?
Check your answer
About 29.5 days.
That is the time from one full Moon to the next.
Measure the lunar year's shortfall
Choose the method: Subtract the 12-lunar-month year from the solar year.
EZT practice question
By roughly how many days does a lunar year fall short of a solar year?
Show the worked solution
- Twelve lunar months total about 354 days.
- A solar year is about 365 days.
- 365 - 354 = 11 days.
Answer: About 11 days.
Check: The 11-day gap explains why a purely lunar calendar slides through the seasons.
Watch for this: Do not compare one lunar month with one calendar month; compare the full years.
Now try it yourself
Which calendar family adds an entire extra month now and then?
Check your answer
Luni-solar calendars.
The extra month pulls lunar festivals back near their seasons.
Apply the Gregorian leap-year rule
Choose the method: Use divisibility by 4, skip most century years, then restore years divisible by 400.
EZT practice question
Was the year 1900 a leap year in the Gregorian calendar? What about 2000?
Show the worked solution
- Both 1900 and 2000 are divisible by 4.
- Century years normally skip the leap day, so 1700, 1800 and 1900 were not leap years.
- Every 400 years the leap day is added back, so 1600 and 2000 were leap years.
Answer: 1900 was not a leap year; 2000 was a leap year.
Check: 2000 passes the special divisible-by-400 rule; 1900 does not.
Watch for this: Do not stop at divisibility by 4 for century years.
Now try it yourself
Why does February have 29 days in some years?
Check your answer
The extra quarter-day each year adds up to about one day in four years.
The leap day keeps the calendar in step with Earth's revolution.
Recall the Indian National Calendar year start
Choose the method: Name the first month and connect leap years to the extra day in Chaitra.
EZT practice question
What is the first month of the Indian National Calendar, and on which Gregorian date does its year usually begin?
Show the worked solution
- The months carry traditional Indian names, beginning with Chaitra.
- In a regular year, the new year begins on 22 March.
- In a leap year, Chaitra receives the extra day and the year begins on 21 March.
Answer: Chaitra; the year usually begins on 22 March, and on 21 March in leap years.
Check: This is a solar calendar, so it stays aligned with the seasons.
Watch for this: Do not call the Indian National Calendar lunar just because its month names are traditional.
Now try it yourself
In a regular year, which months have 31 days?
Check your answer
The second to sixth months.
The rest have 30 days in a regular year.
Explain Diwali's changing Gregorian date
Choose the method: Place the festival on a luni-solar calendar and remember the intercalary month.
EZT practice question
Why does Diwali fall on different Gregorian dates each year but never drift into, say, the summer?
Show the worked solution
- Diwali is fixed to the new Moon of Kartika.
- The calendar behind it is luni-solar, not purely Gregorian.
- An intercalary month is added every few years, keeping the lunar months near the same seasons.
Answer: Its Gregorian date shifts because it is Moon-based, but the intercalary month keeps Kartika near the same season, so the shift stays under about a month.
Check: The answer explains both the yearly shift and the lack of seasonal drift.
Watch for this: Do not say the date is random; it follows a regular lunar rule.
Now try it yourself
Which festival depends on actually sighting the crescent Moon?
Check your answer
Eid-ul-Fitr.
It is celebrated after the crescent Moon is sighted at the end of Ramazan.
Match festivals with Moon positions
Choose the method: Separate full-Moon festivals, new-Moon festivals and crescent sighting.
EZT practice question
Name the Moon timing for Diwali, Holi, Buddha Purnima and Eid-ul-Fitr.
Show the worked solution
- Diwali falls on the new Moon of Kartika.
- Holi falls on the full Moon of Phalguna.
- Buddha Purnima falls on the full Moon of Vaisakha.
- Eid-ul-Fitr follows sighting of the crescent Moon at the end of Ramazan.
Answer: Diwali: new Moon; Holi and Buddha Purnima: full Moon; Eid-ul-Fitr: crescent sighting after Ramazan.
Check: Each festival is tied to a sky observation, not a fixed Gregorian date.
Watch for this: Do not place Diwali on a full Moon; it is a new-Moon festival.
Now try it yourself
Name two festivals fixed to a full Moon.
Check your answer
Holi and Buddha Purnima.
Holi is the full Moon of Phalguna and Buddha Purnima is the full Moon of Vaisakha.
Estimate satellite orbits per day
Choose the method: Convert one day to minutes, then divide by the time for one orbit.
EZT practice question
A satellite takes about 100 minutes per orbit. Roughly how many orbits does it complete in a day?
Show the worked solution
- One day has 24 x 60 = 1440 minutes.
- Divide 1440 by 100 minutes per orbit.
- 1440 / 100 = 14.4, so the satellite completes about 14 full orbits.
Answer: About 14 orbits a day.
Check: Fourteen orbits take 1400 minutes, leaving 40 minutes towards the next orbit.
Watch for this: Do not divide 24 by 100; first convert the day into minutes.
Now try it yourself
Name three uses of artificial satellites.
Check your answer
Communication, navigation and weather monitoring.
They also support disaster management and scientific research.
Match ISRO missions to their jobs
Choose the method: Read the mission name as a clue to whether it studies Earth, stars, the Moon, the Sun or Mars.
EZT practice question
Which ISRO missions or series match Earth imaging, astronomy, the Moon, the Sun and Mars?
Show the worked solution
- Cartosat satellites provide high-quality Earth images for the Bhuvan platform.
- AstroSat observes stars.
- Chandrayaan missions explore the Moon.
- Aditya L1 studies the Sun and Mangalyaan went to Mars.
Answer: Earth imaging: Cartosat/Bhuvan; astronomy: AstroSat; Moon: Chandrayaan; Sun: Aditya L1; Mars: Mangalyaan.
Check: Each name is paired with its observing target or destination.
Watch for this: Do not treat every satellite as a communication satellite; scientific missions have different jobs.
Now try it yourself
Which ISRO series images the Earth for maps and city planning, and which platform shows those images?
Check your answer
Cartosat; Bhuvan.
Cartosat imagery feeds ISRO's Bhuvan mapping platform.
Try a mixed set
Choose a method or explanation before opening each answer.
1. How long is one full Moon-phase cycle?
Show answer and reason
About 29.5 days.
That is roughly one month.
2. Which paksha runs from new Moon to full Moon?
Show answer and reason
Shukla Paksha.
The lit part grows during this waxing period.
3. Is the Indian National Calendar lunar or solar?
Show answer and reason
Solar.
It has 365 days and begins around the spring equinox.
4. Roughly how high do many artificial satellites orbit?
Show answer and reason
About 800 km above Earth's surface.
Many take roughly 100 minutes per orbit from that altitude.
Try it yourself
Work these on paper before checking any notes. Answers are deliberately not shown here.
- A full Moon is on the 12th. Around which date is the next new Moon?
- Why does a purely lunar festival drift across seasons?
- Is 2100 a Gregorian leap year?
- How many 100-minute orbits fit into two days?
Know the rule but lose track of the steps?
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Questions about this practice page
- Why does the Moon show phases at all?
- We see only the sunlit half of the Moon, and as the Moon travels around Earth the visible share of that lit half grows and shrinks - giving the cycle from new Moon to full Moon and back in about 29.5 days.
- What is the difference between Shukla Paksha and Krishna Paksha?
- Shukla Paksha is the waxing half of the cycle, when the lit part grows from new Moon to full Moon; Krishna Paksha is the waning half, when it shrinks from full Moon back to new Moon.
- Why do we need leap years?
- Earth takes about 365 and a quarter days to orbit the Sun. The quarter-days add up to roughly one day every four years, so an extra day keeps the calendar in step with the seasons.
- Is the Indian National Calendar the same as the traditional Indian calendars?
- No - it is a solar calendar of 365 days starting 22 March with months named after traditional months; the traditional calendars behind most festivals are lunar or luni-solar.
- Are these the NCERT exercise questions for Keeping Time with the Skies?
- No. EZT pages are original practice on this chapter - use them to prepare for the textbook exercises, not as a substitute for your book.