Class 8 Science Chapter 11 – Keeping Time with the Skies from NCERT Curiosity with easy Notes and complete Book Solutions. Get simple explanations, important concepts, questions and answers, and revision material for effective learning, practice and exam preparation.
Keeping Time with the Skies
Class 8 Science – Chapter 11
Short Easy Notes, Concepts and Book Solutions
Probe and ponder
Easy Notes and Concepts
1. Why does the Moon change its appearance?
2. Phases of the Moon
3. Locating the Moon
- On a full Moon day, the Moon is nearly opposite the Sun.
- At sunrise, the full Moon is nearly setting in the west.
- A waning Moon is easiest to spot around sunrise.
- A waxing Moon is easiest to spot around sunset.
- The Moon rises about 50 minutes later each day.
- The Moon does not rise and set at exactly the same time every day.
4. Why are Moon phases not caused by Earth’s shadow?
5. New Moon and Full Moon
The illuminated side faces away from Earth, so we cannot see the bright portion.
The illuminated side faces Earth, so we see the full bright circle.
More than half of the illuminated Moon is visible.
6. Day, Month and Year
Based on Earth’s rotation and the apparent daily cycle of the Sun.
Based on the cycle of the Moon’s phases.
Based on Earth’s revolution around the Sun and the cycle of seasons.
7. Solar Day
8. Lunar Calendar
- One lunar month is about 29.5 days.
- 12 lunar months make about 354 days.
- This is about 11 days shorter than the solar year.
- Therefore, lunar months slowly shift with respect to seasons.
9. Solar Calendar
10. Luni-solar Calendar
11. Indian National Calendar
- The Indian National Calendar is a solar calendar.
- It is used along with the Gregorian calendar for several official purposes.
- Its year normally begins on 22 March.
- In a leap year, the year begins on 21 March.
- Its months are based on traditional Indian calendar names.
12. Festivals and Astronomy
- Diwali – new Moon of Kartika.
- Holi – full Moon of Phalguna.
- Buddha Purnima – full Moon of Vaisakha.
- Eid-ul-Fitr – after sighting the crescent Moon at the end of Ramazan.
- Dussehra – tenth day in the month of Ashwina.
13. Artificial Satellites
- Communication
- Navigation
- Weather monitoring
- Disaster management
- Scientific research
- Mapping and observing Earth
14. Important Indian Space Missions
- Cartosat: Earth observation and mapping.
- AstroSat: Study of stars and other celestial objects.
- Chandrayaan: Missions to study the Moon.
- Aditya L1: Study of the Sun.
- Mangalyaan: Mission to Mars.
15. Space Junk
Activity 11.1 – Let us explore
- Spot the Moon at sunrise in the western direction on the first day after the full Moon.
- Make a table similar to Table 11.1 in your notebook. Document the following:
- Date
- When you saw the Moon (at sunrise or sunset)?
- Shade the corresponding Circle with pencil to show the bright portion of the Moon as shown in Fig. 11.1.
- From the second day onwards also document the following.
- Is the size of bright portion of the Moon increasing or decreasing from the previous day.
- Whether the Moon appears closer to or farther from the Sun in the sky than the day before.
- After about 15 days, you may not be able to see the Moon at sunrise or sunset. For the next 15 days, carry out this activity at sunset.
Activity 11.2 – Let us explore
- Take a small soft ball and insert a stick into it (Fig. 11.4a). This represents the Moon.
- Go to a dark open place (at night), and ask a teacher or guardian to shine a torchlight towards you from about 3 m to represent light coming from the Sun or stand near an electric lamp. Your head represents the Earth.
- Now hold the ball at arm’s length in one hand such that it is slightly above your head as shown in Fig. 11.4b. Keep the ball at position E towards the direction of the lamp. Does the portion of the ball facing you appear to be illuminated or not?
- Turn around slowly, in the anti-clockwise direction, with your arm outstretched as shown in Fig. 11.4b and keep looking at the ball. Does the shape of the illuminated portion change? Is the line separating the illuminated and non-illuminated portions of the ball curved?
- Was your observation similar to the changing shape of the illuminated portion of ball shown in Fig. 11.4c? The shape of the illuminated portion of the ball, as seen by you, changes depending on where the ball is with respect to the lamp.
Activity 11.3 – Let us measure a day!
- Find a small flat area in a ground which receives sunlight during the day. Fix a 1 m stick vertically in it as shown in Fig. 11.7.
- Start observing at 11:00 a.m. Every minute, mark a dot on the ground at the tip of the stick’s shadow. Keep marking dots until around 1:10 p.m.
- Identify when the shadow was shortest and find out its time by counting the number of dots. Record this time in Table 11.2. Repeat this exercise for the next few days.
- Find the duration of the solar day by finding a difference in time on two consecutive days as shown in Table 11.2.
Activity 11.4 – Let us identify
- Spotting an artificial satellite is a night sky watching activity like we have done previously. Just before sunrise or after sunset, go to a location, accompanied by an adult, that has a clear view of the sky, without any obstruction of trees or tall buildings.
- To identify satellites in the sky, look for any moving object in the sky that appears as a point of light with steady or flickering brightness and is moving very fast across the sky. You can see them with the naked eye or with binoculars.
- You may use mobile apps or websites that provide details of satellites visible in your location and when they will be passing above you in the sky.
Our scientific heritage
For thousands of years, people—including those in India—have been observing the sky and developing calendars. People in ancient times did not know that Earth revolves around the Sun, and lacked modern instruments. Yet through years of careful sky observations, they noticed patterns and cycles in natural events. Hence, they could determine that the length of the year was approximately 365 days allowing them to create calendars.
For example, careful observation reveals that the Sun does not always rise exactly in the East. In summer, it rises a little northward of East and in winter a little southward of East. These extremes happen on the solstices, around June 21 and December 21 each year. The Sun’s apparent northward movement from December to June is called Uttarayan, and its apparent southward movement from June to December is called Dakshinayan.
This cycle repeats every year and is closely linked to the changing seasons.
Be a scientist
Meghnad Saha (1893–1956)
Meghnad Saha was a pioneering astrophysicist of India who studied stars and their temperatures and developed a mathematical equation, famously known as the Saha equation.
He was also the chairperson of the Calendar Reform Committee.
Vikram Ambalal Sarabhai (1919–1971)
Vikram Sarabhai, a researcher in space science and nuclear physics is known as the Father of the Indian Space programme.
He pioneered the effort to launch the first artificial satellites.
Keep the curiosity alive
(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.
(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
- Day of New Moon – completely dark/not visible.
- Three days after New Moon – waxing crescent.
- Full Moon – completely bright circle.
- Three days after Full Moon – waning gibbous.
- A week after Full Moon – waning half Moon.
(ii) List the picture labels of the phases of the Moon that are never seen from Earth. Hint: You can use your observations from Activity 11.1 or Fig. 11.2 as reference.
(i) Draw the phase of the Moon that Malini saw.
(ii) Is the Moon in the waxing or the waning phase?
- (i) day: Earth’s rotation and the daily cycle of the Sun.
- (ii) month: Cycle of the Moon’s phases.
- (iii) year: Earth’s revolution around the Sun and the cycle of seasons.
Discover, design, and debate
- Andhra Pradesh – Ugadi
- Karnataka – Ugadi
- Maharashtra – Gudi Padwa
- West Bengal – Poila Baisakh
- Punjab – Vaisakhi
- Assam – Bohag Bihu
- Kerala – Vishu
- Tamil Nadu – Puthandu
- Gujarat – Bestu Varas
- Odisha – Pana Sankranti
These festivals are associated with regional calendars and cultural traditions. Some follow solar calendars, while others are connected with lunar or luni-solar calendars.
Prepare some questions based on your learnings so far.
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Reflect on the questions framed by your friends and try to answer …
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Quick Revision
| Topic | Remember |
|---|---|
| Moonlight | The Moon reflects sunlight. |
| Moon phases | They occur because we see different portions of the illuminated Moon. |
| Full Moon | The entire illuminated portion facing Earth is visible. |
| New Moon | The illuminated portion facing Earth is not visible. |
| Waxing | Bright portion increases. |
| Waning | Bright portion decreases. |
| Month | Based on the Moon’s phase cycle. |
| Day | Based on Earth’s rotation and the daily solar cycle. |
| Year | Based on Earth’s revolution and seasons. |
| Lunar calendar | Follows the Moon’s cycle. |
| Solar calendar | Follows the solar year and seasons. |
| Luni-solar calendar | Uses both lunar and solar cycles. |
| Artificial satellite | Human-made object launched to orbit a celestial body. |
| Space debris | Unwanted human-made objects left in space. |
Snapshots – Chapter at a Glance
- The illuminated part of the Moon changes its shape from day to day through phases, like the new Moon, crescent, and full Moon.
- The phases of the Moon happen because we see different parts of the illuminated portion of the Moon as it moves around the Earth.
- A full cycle of phases of the Moon takes about a month.
- The various cycles observed in nature resulted in the creation of calendars.
- Lunar calendars follow the Moon’s cycle, while solar calendars follow the cycle of seasons, which depend upon the position of the Earth in its orbit around the Sun. Luni-solar calendars adapt to both the cycles.
- Artificial satellites are human-made which are launched from the Earth. They provide important information for our well-being and space-science studies.
Prepare with this NCERT Curiosity worksheet for Chapter 11. The worksheet includes important questions and revision exercises to help students understand key concepts, practise what they have learned, and prepare effectively for class tests and exams.
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