CBSE Important Questions Class 8 Science Chapter 11 – Keeping Time with the Skies

Class 8 Science Chapter 11 Keeping Time with the Skies is part of the new NCERT Curiosity textbook. It explains how people have measured time by watching the sky. Students learn why we have day and night, why the Moon seems to change its shape, the eight phases of the Moon, the difference between a lunar month and a solar year, how lunar, solar and luni-solar calendars work, and why a leap year and an intercalary month are needed.

These CBSE important questions for Class 8 Science Chapter 11 cover why the Moon changes its shape, the positions of the Sun, Earth and Moon in the different phases, the Moon slowly moving away from the Earth, why the Moon is not visible every evening, the number of phases in a lunar month, why a leap year is needed, and solar and lunar eclipses. Every question has a clear step-by-step answer in simple English, so students can practise on their own, parents can check the answers, and teachers can use them for class tests.

Q1. 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?

Answer: Luni-solar calendars will need an intercalary month less often.
At present, 12 lunar months add up to about 354 days, which is about 11 days short of the solar year of 365.25 days. This gap is made up by adding an extra (intercalary) month roughly every three years.
As the Moon moves farther away, each lunar month becomes a little longer. So 12 lunar months will add up to more than 354 days, and the yearly gap with the solar year will become smaller. If the gap falls from 11 days to, say, 9 days a year, it will take longer for enough difference to build up. So the extra month will be needed less frequently.

Q2. Explain why the Moon appears to change its shape throughout the month. How do the relative positions of the Sun, Earth and Moon affect what we see from Earth?

Answer: The Moon does not make its own light; it only reflects sunlight. Exactly half of the Moon is always lit by the Sun, but how much of that lit half we can see from the Earth keeps changing as the Moon goes round the Earth in about 29.5 days.
• When the Moon is between the Earth and the Sun, its lit side faces away from us, so we see nothing. This is the new moon.
• When the Earth is between the Sun and the Moon, the whole lit side faces us, so we see a complete bright disc. This is the full moon.
• At positions in between, we see only a part of the lit half, which gives the crescent, quarter and gibbous shapes.

Q3. Explain why we see different shapes of the Moon in the sky even though the Moon itself does not change its shape.

Answer: The Moon is always a sphere and its shape never really changes. What changes is the part of its sunlit half that we can see from the Earth, because the angle between the Sun, the Earth and the Moon keeps changing as the Moon orbits us.
When the Moon lies between the Earth and the Sun, we face its dark side and see nothing (new moon). When the Earth lies between the Sun and the Moon, we face its fully lit side (full moon). In other positions we see only part of the lit half, giving the crescent and gibbous shapes. So only our view changes, not the Moon.

Q4. The Moon is gradually moving away from Earth at approximately 3.8 cm per year. What will be the eventual consequence millions of years in the future?
(a) Total solar eclipses will become impossible
(b) Lunar eclipses will become more frequent
(c) Earth's day will become shorter
(d) The Moon will collide with Mars

Answer: (a) Total solar eclipses will become impossible
A total solar eclipse happens only because the Moon, seen from the Earth, looks almost exactly as big as the Sun and can cover it completely. As the Moon moves farther away, it will look smaller and smaller in our sky. A day will come when it can no longer hide the Sun fully, so only annular (ring) eclipses will be seen and total solar eclipses will end.

Q5. Meera observed the Moon every evening at 7 p.m. for 30 days. Sometimes it was in the east, sometimes in the west, and on some days it was not visible at all. Why was the Moon not visible every evening at 7 p.m.?
(a) The Moon stops orbiting Earth on some days
(b) Clouds always block the Moon on those days
(c) The Moon's position changes due to its orbital motion
(d) The Moon only appears at night, never in the evening

Answer: (c) The Moon's position changes due to its orbital motion
As the Moon goes round the Earth, it rises about 50 minutes later each day. So at a fixed time such as 7 p.m. it is in a different part of the sky every evening, and on some days it has not yet risen or has already set. Around the new moon it is also in the sky together with the Sun, so it cannot be seen at all.

Q6. A group of students made these observations: during a new moon the Moon and the Sun are in the same direction; during a full moon, when the Sun sets in the west the Moon rises in the east; during the first quarter the Moon is visible in the afternoon and evening sky. How many main phases does the Moon complete in one month?

Answer: The Moon completes eight main phases in one lunar month of about 29.5 days.
These are: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter and waning crescent, after which the cycle begins again.
'Waxing' means the lit part is growing bigger each night and 'waning' means it is getting smaller.

Q7. Read the Assertion (A) and Reason (R) and choose the correct option.
Assertion (A): People on different parts of the Earth see nearly the same phase of the Moon on a given day.
Reason (R): The Moon's revolution period is much longer than the Earth's rotation period.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

Answer: (a) Both A and R are true, and R is the correct explanation of A
The Moon takes about 29.5 days to go round the Earth, while the Earth spins once in only 24 hours. So in the time it takes everyone on Earth to be carried round and see the Moon, the Moon has hardly moved along its orbit. The angle between the Sun, Earth and Moon stays almost the same, so the phase looks nearly the same from every country on the same day.

Q8. What is the difference between a lunar month and a solar year? Why does a lunar calendar fall out of step with the seasons?

Answer: A lunar month is the time the Moon takes to complete one cycle of phases, about 29.5 days. Twelve such months make a lunar year of about 354 days.
A solar year is the time the Earth takes to go once round the Sun, about 365.25 days. The seasons follow this solar year.
A purely lunar calendar is about 11 days shorter every year. So its months slowly slip backwards through the seasons: a festival that falls in summer one year will fall in spring after some years. Luni-solar calendars, such as the Indian calendar, fix this by adding an extra month every few years.

Q9. Why do we have a leap year? What would happen if we did not add a leap day?

Answer: The Earth takes about 365 days and 6 hours (365.25 days) to go once round the Sun, but our ordinary calendar year has only 365 days. So every year we are short by about 6 hours.
In four years these 6 hours add up to one full day, and that day is added as 29 February in a leap year.
If we did not add it, the calendar would slowly drift away from the seasons. In about 100 years it would be out by nearly 25 days, and after several centuries the summer months would fall in winter.

Q10. What is a solar eclipse and a lunar eclipse? Write one difference between them.

Answer: A solar eclipse happens when the Moon comes between the Sun and the Earth and its shadow falls on the Earth, so the Sun is hidden from us. It can happen only on a new moon day.
A lunar eclipse happens when the Earth comes between the Sun and the Moon and the Earth's shadow falls on the Moon. It can happen only on a full moon day.
Difference: in a solar eclipse the Moon's shadow falls on the Earth, while in a lunar eclipse the Earth's shadow falls on the Moon. A lunar eclipse can also be seen from a much larger part of the Earth and lasts much longer.

Q11. Why does the Moon always show us the same side?

Answer: The Moon turns once on its own axis in exactly the same time that it takes to go once round the Earth, which is about 27.3 days. Because its rotation and its revolution keep perfect step with each other, the same face is always turned towards us.
This is called tidal locking. It is why we never see the far side of the Moon from the Earth, and it was first photographed only when spacecraft went round the Moon.

Q12. How did ancient people use the Sun and the Moon to measure time? Give two examples.

Answer: Long before clocks were invented, people watched the sky to keep track of time.
1. Using the Sun: the day was divided by the position of the Sun and the length of shadows. A sundial works on this idea: as the Sun moves across the sky, the shadow of a rod moves over a marked dial and shows the hour.
2. Using the Moon: the repeating cycle of the Moon's phases gave people the idea of a month. Many festivals in India, such as Diwali and Holi, are still fixed by the phases of the Moon.
From these observations grew our ideas of the day, the month and the year.

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