--- title: "NCERT Solutions Class 8 Science Ch 11 Keeping Time with the Skies" url: https://www.swavid.com/science/class/8/chapter/keeping-time-with-the-skies/ncert-solutions dateModified: 2026-10-07T16:16:25+00:00 --- # NCERT Solutions Class 8 Science Ch 11 Keeping Time with the Skies This chapter's questions cover concepts related to the phases of the Moon, the motion of the Earth and the Moon, the creation and use of different calendars, and astronomical observations. Free PDF (8 pages): https://www.swavid.com/api/seo/pdf/ncert/science/class-8/swavid-ncert-solutions-class-8-science-chapter-11-keeping-time-with-the-skies-a218162f5e.pdf ## Probe and ponder ### Question 1 *3 marks · Short answer* Have you ever seen the Moon during the day? Why do you think it is sometimes visible when the Sun is up? **Solution** 1. The Moon is visible during the day because it reflects sunlight towards the Earth, just like it does at night. 2. During certain phases and positions in its orbit, the Moon is far enough from the Sun in the sky to be seen during daylight hours. 3. Its moonrise time changes by about 50 minutes each day, which sometimes causes moonrise to happen in the afternoon. **Answer:** The Moon is sometimes visible during the day because it reflects sunlight and its position in the sky makes it observable during daylight hours. > Common mistake: Thinking the Moon disappears or cannot reflect sunlight when the Sun is up. ### Question 2 *3 marks · Short answer* Imagine you lived on the Moon instead of Earth. What would you mean by a day, a month or a year? **Solution** 1. A day on the Moon would mean one full rotation of the Moon relative to the Sun, which takes about 29.5 Earth days. 2. A month representing the lunar phase cycle would not apply in the same way because an observer on the Moon always sees the same side of the Earth undergoing phases over about 29.5 Earth days. 3. A year on the Moon would mean one complete revolution of the Moon (along with the Earth) around the Sun, which takes one Earth year (approximately 365 and a quarter days). **Answer:** A lunar day is about 29.5 Earth days, a month corresponds to the Earth's phase cycle as viewed from the Moon, and a year is equal to one Earth year. > Common mistake: Confusing a lunar day with the Moon's rotation time on its own axis (27.3 days) instead of the solar day (29.5 days). ### Question 3 *3 marks · Short answer* What would happen if Earth had two moons instead of one? How would that change the night sky? **Solution** 1. If Earth had two moons, both would go through their own independent cycles of phases depending on their positions relative to the Sun and Earth. 2. The night sky would have two illuminated bodies changing shapes, leading to more moonlight on certain nights. 3. It would also create more complex tidal patterns and frequent overlapping shadows or eclipses. **Answer:** The night sky would feature two independent moons going through phases, leading to brighter nights and more complex celestial patterns. > Common mistake: Assuming both moons would always stay in the same phase. ### Question 4 *3 marks · Short answer* If we didn’t have clocks or calendars, how else could we measure time? **Solution** 1. Time could be measured by observing natural periodic events such as the daily rising and setting of the Sun. 2. The length and direction of shadows cast by objects could be used to track the time of day. 3. Longer durations could be measured by tracking the changing phases of the Moon or the repeating cycle of seasons. **Answer:** Time can be measured using natural periodic events like the daily motion of the Sun, shadow lengths, Moon phases, and seasonal changes. > Common mistake: Listing modern time-measuring instruments instead of natural phenomena. ### Question 5 *2 marks · Very short answer* Share your questions **Solution** 1. Why does the crescent Moon always point towards the Sun? 2. How do different cultures track time using the stars? **Answer:** Questions related to observable astronomical phenomena and time-keeping. > Common mistake: Leaving the question blank or asking unrelated non-scientific questions. ## Keep the curiosity alive ### Question 1 *1 mark · True or false* 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. **Part (i)** 1. The Moon shines by reflecting sunlight, and we can only see the illuminated portion that reflects light towards us. Answer (i): True **Part (ii)** 1. The Earth's shadow on the Moon causes a lunar eclipse, not the phases of the Moon. Answer (ii): False. The phases of the Moon occur due to the relative change in orientation of the Sun, Moon, and Earth as the Moon revolves around the Earth. **Part (iii)** 1. Calendars are designed based on predictable natural periodic astronomical cycles like days, months, and years. Answer (iii): True **Part (iv)** 1. The Moon can often be seen during the day depending on its phase and position in the sky. Answer (iv): False. The Moon can be seen both during the day and at night. **Answer:** (i) True, (ii) False, (iii) True, (iv) False > Common mistake: Confusing lunar eclipses with moon phases. ### Question 2 *3 marks · Short answer* 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. **Solution** 1. A full Moon cycle takes about 29.5 days (a lunar month), and 12 lunar months make up a lunar year of 354 days. 2. The Gregorian calendar is a solar calendar based on a solar year of 365 days, which is about 11 days longer than a lunar year. 3. Therefore, the full Moon dates shift by about 11 days each year in the Gregorian calendar, so his birthday will not fall on a full Moon day every year. **Answer:** No, his birthday will not fall on a full Moon day every year because the lunar cycle does not match the solar calendar year. > Common mistake: Assuming lunar months and solar calendar months have the same duration and align every year. ### Question 3 *3 marks · Short answer* Name two things that are incorrect in Fig. 11.10. **Solution** 1. In Fig. 11.10, the clouds are shown behind the Moon, whereas clouds are part of Earth's atmosphere and must always appear in front of celestial objects. 2. The illuminated side of the crescent Moon in the drawing faces away from the Sun's position, but the bright side of the Moon always faces towards the Sun. **Answer:** 1. Clouds are shown behind the Moon instead of in front. 2. The illuminated side of the crescent Moon does not face the Sun. > Common mistake: Forgetting that clouds are atmospheric and lie well below space objects. ### Question 4 *3 marks · Short answer* Look at the pictures of the Moon in Fig. 11.11, and answer the following questions. (i) Write the correct panel number corresponding to the phases of the Moon shown in the pictures above. (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. **Part (i)** 1. Three days after New Moon corresponds to a waxing crescent, which matches panel B. 2. Full Moon corresponds to a fully illuminated disc, which matches panel A. 3. Three days after Full Moon corresponds to a waning gibbous, which matches panel F. 4. A week after Full Moon corresponds to a half circle waning phase, which matches panel E. 5. Day of New Moon corresponds to a completely dark disc, which matches panel D. Answer (i): Three days after New Moon: B; Full Moon: A; Three days after Full Moon: F; A week after Full Moon: E; Day of New Moon: D **Part (ii)** 1. Only one half of the Moon is illuminated by the Sun and the other half remains dark. 2. Since the same side of the Moon always faces the Earth, we can never see the completely dark half or portions facing away. Answer (ii): Panel C represents the phase where the completely dark side faces Earth, which is never seen. **Answer:** Refer to the table and list of unseen phases in the parts. > Common mistake: Confusing the waxing and waning crescent or gibbous phases. ### Question 5 *3 marks · Short answer* 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? **Part (i)** 1. When the Moon is overhead at sunset, half of its illuminated portion is visible from Earth, forming a half circle known as the first quarter. Answer (i): A half circle (first quarter Moon). **Part (ii)** 1. A waxing Moon is easiest to spot at sunset when it is increasing in its bright portion after a new moon. Answer (ii): Waxing phase. **Answer:** (i) Half circle (First quarter). (ii) Waxing phase. > Common mistake: Confusing sunset positions of waxing and waning moons. ### Question 6 *3 marks · Short answer* 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? **Solution** 1. Ravi said he saw a crescent Moon rising in the East when the Sun was setting. A waxing crescent Moon is visible in the western sky at sunset and sets soon after, so it does not rise in the East at sunset. 2. Kaushalya said she saw the gibbous Moon during the afternoon in the East. A waxing gibbous Moon rises in the afternoon in the eastern sky. 3. Therefore, Kaushalya is telling the truth. **Answer:** Kaushalya is telling the truth because a waxing gibbous Moon can be seen in the eastern sky during the afternoon. > Common mistake: Thinking all phases of the moon rise only at night. ### Question 7 *3 marks · Short answer* Scientific studies show that the Moon is getting farther away from the Earth and slower in its revolution. Will luni-solar calendars need an intercalary month more often or less often? **Solution** 1. A luni-solar calendar adds an intercalary month (Adhika Maasa) every few years to make up for the short fall of 12 lunar months compared to a solar year. 2. As the Moon gets slower in its revolution and moves farther away, the time taken for one lunar month increases. 3. Since the lunar month becomes longer, the total number of lunar months in a solar year decreases, meaning luni-solar calendars will need an intercalary month less often. **Answer:** Luni-solar calendars will need an intercalary month less often because the lunar month becomes longer. > Common mistake: Confusing how a slower revolution affects the length of the lunar month and the frequency of intercalary months. ### Question 8 *3 marks · Proof* 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. **Solution** 1. Given: A total of 37 full moons occur during a period of 3 years in a solar calendar. 2. To prove: At least two of the 37 full moons must happen during the same month of the solar calendar. 3. A solar calendar year consists of 12 months. 4. For 3 years, the total number of months is $3 \times 12 = 36$ months. 5. By the pigeonhole principle, if 37 full moons are distributed among 36 months, at least one month must contain more than one full moon. 6. Hence proved, at least two of the 37 full moons must happen during the same month. **Answer:** Since there are 36 months in 3 years and 37 full moons, at least two full moons must occur in the same month. > Common mistake: Failing to calculate the total number of months correctly over the 3-year period. ### Question 9 *3 marks · Short answer* On a particular night, Vaishali saw the Moon in the sky from sunset to sunrise. What phase of the Moon would she have noticed? **Solution** 1. State what is given: Vaishali saw the Moon in the sky continuously from sunset to sunrise. 2. The idea used: The full Moon rises in the east almost as the Sun sets in the west and remains visible throughout the night until sunrise. 3. Result: Vaishali would have noticed the full Moon (or Purnima) phase. **Answer:** Full Moon phase (or Purnima). > Common mistake: Confusing the full Moon with other phases that are only visible for a part of the night. ### Question 10 *3 marks · Short answer* If we stopped having leap years, in approximately how many years would the Indian Independence day happen in winter? **Solution** 1. The Earth takes approximately 365 and a quarter days to complete one revolution around the Sun, which defines a solar year. 2. Skipping leap years means the calendar loses about one quarter of a day every year, causing the calendar dates to shift earlier relative to the seasons by one day every 4 years. 3. Winter in India occurs around December, which is about 4 to 5 months (or roughly 140 to 150 days) away from August. 4. Multiplying the required shift of about 140 to 150 days by 4 years per day gives approximately 560 to 600 years. **Answer:** Approximately 560 to 600 years > Common mistake: Confusing the direction of seasonal shift or incorrectly calculating the cumulative days per year without accounting for the quarter-day excess. ### Question 11 *3 marks · Short answer* What is the purpose of launching artificial satellites? **Solution** 1. Artificial satellites are human-made objects launched from the Earth to orbit around it. 2. They help us in many essential activities such as communication, navigation, and weather monitoring. 3. They are also used for disaster management and scientific research about the Earth and space. **Answer:** Communication, navigation, weather monitoring, disaster management, and scientific research. > Common mistake: Listing only communication and forgetting applications like weather monitoring and navigation. ### Question 12 *2 marks · Very short answer* On which periodic phenomenon are the following measures of time based: (i) day (ii) month (iii) year? **Solution** 1. The day is based on the apparent periodic motion of the Sun due to the rotation of the Earth around its own axis. 2. The month is based on the cycle of the phases of the Moon as it revolves around the Earth. 3. The year is based on the Earth's revolution around the Sun and the resulting cycle of seasons. **Answer:** (i) Rotation of Earth on its axis, (ii) Revolution of Moon around Earth, (iii) Revolution of Earth around Sun. > Common mistake: Confusing Earth's rotation with its revolution. ## Frequently asked questions ### How many exercises and total questions are in Class 8 Science Chapter 11 Keeping Time with the Skies? This chapter in the new NCERT book features a total of 17 questions across its sections. There are 5 questions in Probe and Ponder and 12 questions in Keep the Curiosity Alive. SwaVid provides step-by-step solutions and a free PDF for all these questions right on this page. ### Which topics do the questions in this chapter cover? The questions cover important concepts like the phases of the Moon, measuring time without clocks or calendars, and luni-solar calendars. Other topics include lunar cycles versus solar calendar years, moonrise and phase observations, and the effect of having two moons on the night sky. ### What are the hardest question types in this chapter and how should I approach them? Proof-based and conceptual questions regarding orbital changes and natural periodic phenomena are usually considered the trickiest. To approach them, focus on understanding the underlying astronomical movements rather than just memorising facts. You can refer to the detailed explanations on SwaVid's page to master these challenging topics. ### How can I write answers for full marks in Class 8 Science Chapter 11 exams? To secure full marks, structure your answers clearly by defining key astronomical terms and explaining natural periodic phenomena step by step. Mentioning specific observations like the Moon overhead at sunset or moonrise timings adds valuable depth. SwaVid's solutions on this page demonstrate the exact formatting needed for top scores. ### Is the free PDF for Keeping Time with the Skies available for the 2026-27 session? Yes, the complete chapter solutions aligned with the new NCERT book for the 2026-27 session are available here. You can easily access and download SwaVid's free PDF directly from this page to aid your revision. ## Related pages - [Class 8 Science chapters](https://www.swavid.com/science/class/8) Solutions written by SwaVid, a personal AI tutor for Class 6 to 10 Maths and Science. Practise this chapter free: https://www.swavid.com/start/student