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

1. State whether the following statements are True or False.
(i) We can only see that part of the Moon which reflects sunlight towards us.
See AnswerTrue.
The Moon does not produce its own light; it shines by reflecting sunlight. We see the portion of the Moon that is both illuminated by the Sun and facing the Earth.

(ii) The shadow of Earth blocks sunlight from reaching the Moon causing phases.
See AnswerFalse.
This is a common misconception! The phases of the Moon are caused by the changing viewing angle of the Moon’s illuminated half as it orbits the Earth. The Earth’s shadow falling on the Moon causes a lunar eclipse, which is a much rarer event.

(iii) Calendars are based on various astronomical cycles which repeat in a predictable manner.
See AnswerTrue.
Calendars were developed by observing the predictable, repeating cycles in the sky, such as the Earth’s daily rotation (for a day), the Moon’s revolution around Earth (for a month), and the Earth’s revolution around the Sun (for a year and its seasons).

(iv) The Moon can only be seen at night.
See AnswerFalse.
The Moon can often be seen during the daytime! Its visibility during the day depends on its phase and its position in the sky relative to the Sun. For instance, a waning moon is often visible in the morning sky after sunrise.

2. 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.
See AnswerNo, Amol’s birthday will not fall on a full Moon day every year. This is because our birthday calendar (the Gregorian calendar) is a solar calendar, based on the Earth’s 365.25-day trip around the Sun. The cycle of the Moon’s phases, from one full Moon to the next, takes about 29.5 days. Since a solar year (365 days) is not a perfect multiple of the lunar month (29.5 days), the date of the full Moon shifts each year within the solar calendar. A solar year is about 11 days longer than 12 lunar months, so the full Moon will occur on a different date each May.

3. Name two things that are incorrect in Fig. 11.10.

See AnswerThe two major scientific errors in the drawing are:
1. Stars are shown inside the dark, unlit portion of the Moon. This is impossible because the Moon is a solid, opaque object. We cannot see through it to the stars behind it.
2. The crescent shape of the Moon is created by direct sunlight illuminating one part of it. The drawing incorrectly shows the crescent as a shape that you can see stars through, which misunderstands what the dark part of the Moon actually is – it’s the solid surface of the Moon that is in shadow, not an empty space.

4. 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.

Answer:

(ii) List the picture labels of the phases of the Moon that are never seen from Earth.

See AnswerFrom Earth, we never see (B) new moon phase.

5. Malini saw the Moon overhead in the sky at sunset.
(i) Draw the phase of the Moon that Malini saw.
Answer:

Malini saw the Half Moon (A week after full Moon).

(ii) Is the Moon in the waxing or the waning phase?
See AnswerThe Moon is in the waxing phase. Waxing means the illuminated portion we see from Earth is growing, which happens in the week following the New Moon, leading up to the Full Moon. The First Quarter moon rises around noon and is highest in the sky (overhead) around sunset.

6. 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?
See AnswerKaushalya is telling the truth. A waxing gibbous Moon rises in the mid-afternoon and would be visible in the eastern part of the sky during that time. Ravi’s statement is incorrect because a crescent Moon seen around sunset is a waxing crescent, which appears in the western sky near the setting Sun and then sets soon after; it does not rise in the east at sunset.

7. 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?
See AnswerLuni-solar calendars will need an intercalary month (Adhika Maasa) less often. Here’s why: An intercalary month is added to make up for the ~11 day difference between the shorter lunar year (~354 days) and the solar year (~365 days). If the Moon’s revolution slows down, the time it takes to complete its cycle of phases (the lunar month) will become longer. A longer lunar month means a longer lunar year. This would reduce the difference between the lunar and solar years, so the mismatch would accumulate more slowly, and an extra month would be needed less frequently to keep the calendar in sync with the seasons.

8. 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.
See AnswerThis can be shown with simple logic. A 3-year period in a solar calendar contains a total of 3 x 12 = 36 months. If you have 37 full Moons occurring within these 36 months, it is impossible for each full Moon to have its own separate month. By the time you assign one full Moon to each of the 36 months, you will have one full Moon left over. This 37th full Moon must therefore fall into one of the months that already has a full Moon, meaning at least one month must have two full moons.

9. On a particular night, Vaishali saw the Moon in the sky from sunset to sunrise. What phase of the Moon would she have noticed?
See AnswerVaishali would have noticed the Full Moon. The Full Moon is the only phase where the Moon is directly opposite the Sun in the sky. This means it rises in the east at roughly the same time the Sun sets in the west, stays visible all night long, and then sets in the west at roughly the same time the Sun rises in the east.

10. If we stopped having leap years, in approximately how many years would the Indian Independence day happen in winter?
See AnswerIf we stopped having leap years, our 365-day calendar would fall behind the actual seasons by about 0.25 days every year. Indian Independence Day is in August (monsoon/summer season). For it to occur in winter (let’s say December/January), the calendar would need to shift by about 4 to 5 months. A shift of one month (approx. 30 days) would take 30 days / 0.25 days/year = 120 years. Therefore, to shift by 4 months, it would take approximately 4 x 120 = 480 years. So, it would take roughly 500 years for Independence Day to fall in the winter season.

11. What is the purpose of launching artificial satellites?
See AnswerArtificial satellites are launched for many important purposes that help us in our daily lives and advance our scientific knowledge. As mentioned in the chapter, their primary uses include: communication (like TV and phone signals), navigation (like GPS), weather monitoring, disaster management, scientific research, creating improved maps and city planning.

12. On which periodic phenomenon are the following measures of time based: (i) day (ii) month (iii) year?
See Answer(i) day: The ‘day’ is based on the periodic rotation of the Earth on its own axis, which causes the cycle of daylight and darkness and the Sun’s apparent movement across the sky.
(ii) month: The ‘month’ is based on the periodic revolution of the Moon around the Earth, which results in the repeating cycle of the Moon’s phases (from new moon to full moon and back again).
(iii) year: The ‘year’ is based on the periodic revolution of the Earth around the Sun, which takes approximately 365.25 days and causes the cycle of the seasons.