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

Question 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.
Answer: True. The Moon does not create its own light; it shines because it bounces sunlight off its surface. We observe the portion of the Moon that is lit by the Sun and also turned toward Earth.
In simple words: The Moon has no light of its own. We see only the part that gets sunlight and faces us.

Exam Tip: Remember that the Moon reflects light - it is not a light source. This is key to understanding why we see only part of it lit at different times.

Question 1. (ii) The shadow of Earth blocks sunlight from reaching the Moon causing phases.
Answer: False. Many people mistakenly think this, but it is not correct. Moon phases happen because we see different amounts of the Moon's lit half as it moves around Earth. When Earth's shadow falls on the Moon, this causes a lunar eclipse, which is a much rarer occurrence.
In simple words: Moon phases are about which part we can see lit up, not Earth's shadow blocking it. Earth's shadow only blocks the Moon during a lunar eclipse, which is rare.

Exam Tip: Do not confuse moon phases with lunar eclipses - these are two different things. Phases happen every month, but eclipses are much less common.

Question 1. (iii) Calendars are based on various astronomical cycles which repeat in a predictable manner.
Answer: True. Calendars were created by noticing regular patterns in the sky that happen over and over. These include Earth's daily spin on its axis (which gives us a day), the Moon's trip around Earth (which gives us a month), and Earth's journey around the Sun (which gives us a year and seasons).
In simple words: Calendars are built on sky patterns that repeat - Earth spinning gives days, the Moon going around gives months, and Earth going around the Sun gives years.

Exam Tip: Be ready to name at least three cycles: Earth's rotation (day), Moon's revolution (month), and Earth's revolution (year).

Question 1. (iv) The Moon can only be seen at night.
Answer: False. The Moon can frequently be seen during the day. Whether we can see it during daylight depends on which moon phase it is in and where it sits in the sky compared to the Sun. For example, a waning moon is often visible in the morning sky after the Sun rises.
In simple words: The Moon can be seen during the day too. It depends on what phase it is and where it is in the sky.

Exam Tip: Students often think the Moon only appears at night. Point out that visibility depends on the phase and position, not just the time of day.

Question 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.
Answer: No, Amol's birthday will not fall on a full Moon day every year. Our regular birthday calendar (the Gregorian calendar) is based on Earth's year, which is about 365.25 days long as it orbits the Sun. The Moon's phase cycle, from one full Moon to the next, takes roughly 29.5 days. Since a year of 365 days does not divide evenly into lunar months of 29.5 days, the date when the full Moon occurs changes each year. In fact, a solar year is about 11 days longer than 12 lunar months, so the full Moon shifts to a different date each May.
In simple words: The calendar year and the Moon's cycle do not match up evenly. So the full Moon happens on different dates in May each year.

Exam Tip: Show the math: 365 ÷ 29.5 is not a whole number, which proves the mismatch. This is why lunar and solar calendars drift apart.

Question 3. Name two things that are incorrect in Fig. 11.10.
Answer: The two main scientific mistakes in the drawing are:
(1) Stars appear inside the dark part of the Moon. This cannot happen because the Moon is a solid, opaque object. We cannot see through it to reach the stars that are behind it.
(2) The crescent shape of the Moon is made by direct sunlight hitting one side of it. The drawing wrongly shows the crescent as if you can look through it and see stars, which is wrong. The dark part is actually the solid Moon surface in shadow, not empty space that light can pass through.
In simple words: Stars cannot show through the Moon because the Moon is solid. Also, the dark part is not empty - it is the Moon's surface in shadow.

Exam Tip: When asked to spot errors in moon diagrams, check two things: can you see stars through the Moon (no, it is solid), and is the dark part shown correctly (it is shadow, not space).

Question 4. (i) Look at the pictures of the Moon in Fig. 11.11, and write the correct panel number corresponding to the phases of the Moon shown in the pictures.
Answer: Three days after New Moon - D
Full Moon - E
Three days after Full Moon - A
A week after Full Moon - C
Day of New Moon - B
In simple words: Match each moon picture to its phase name by looking at how much of it is lit up and in which direction the light comes from.

Exam Tip: Learn the order of moon phases and what each one looks like. Waxing phases grow from left to right; waning phases shrink from left to right (in the Northern Hemisphere).

Question 4. (ii) List the picture labels of the phases of the Moon that are never seen from Earth.
Answer: The New Moon phase (B) is never seen from Earth. During the New Moon, the Moon is between Earth and the Sun, so the side facing us is completely dark and receives no sunlight. We cannot observe it because it blends in with the bright Sun in the sky.
In simple words: We never see the New Moon because it sits between Earth and the Sun, so the side we see gets no light.

Exam Tip: The New Moon is invisible from Earth, which is why it marks the start of the lunar month in many calendars. Only the lit phases are visible.

Question 5. (i) Malini saw the Moon overhead in the sky at sunset. Draw the phase of the Moon that Malini saw.
Answer: Malini saw the Half Moon (also called the First Quarter or a week after the Full Moon). The sketch shows a half-lit circle with the right half bright and the left half dark.
In simple words: When the Moon is straight overhead at sunset, it is in the half-moon phase - exactly half lit and half dark.

Exam Tip: The First Quarter moon rises around noon and reaches its highest point (overhead) near sunset. This timing is a key clue for identifying this phase.

Question 5. (ii) Is the Moon in the waxing or the waning phase?
Answer: The Moon is in the waxing phase. Waxing means the bright part we see from Earth is growing. This happens in the week right after the New Moon and continues until we reach the Full Moon. The First Quarter moon, which rises around noon, is highest in the sky (overhead) roughly at sunset during this waxing period.
In simple words: Waxing means the lit part gets bigger each night. It happens between the New Moon and the Full Moon.

Exam Tip: Remember: waxing = growing, waning = shrinking. The position at sunset helps you tell which phase it is.

Question 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?
Answer: Kaushalya is telling the truth. A waxing gibbous Moon comes up in the mid-afternoon and is visible in the eastern part of the sky at that time. Ravi's claim is not correct. A crescent Moon we see at sunset is a waxing crescent, which shows up in the western sky close to the setting Sun and then sets quickly right after. It does not rise in the east at sunset.
In simple words: The gibbous Moon can be seen in the afternoon in the east. A crescent Moon at sunset is in the west, not the east.

Exam Tip: Match moon phases to their typical sky positions and times: early phases in the west near sunset, later phases in the east in early morning or afternoon in the east.

Question 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?
Answer: Luni-solar calendars will need an intercalary month (called Adhika Maasa) less often. An intercalary month is added to bridge the gap - roughly 11 days - between the shorter lunar year (about 354 days) and the solar year (about 365 days). If the Moon moves more slowly in its orbit, the time it takes to go through its full phase cycle (the lunar month) will get longer. A longer lunar month means the lunar year will also be longer. This would shrink the difference between the lunar and solar years. With a smaller mismatch, the yearly gap piles up more slowly, so an extra month would be required less frequently to keep the calendar matched with the seasons.
In simple words: If the Moon moves slower, its month gets longer, so the gap between lunar and solar years gets smaller. That means fewer extra months are needed.

Exam Tip: Focus on the logic: slower Moon - longer lunar month - smaller yearly gap - fewer intercalary months needed. Show your reasoning step by step.

Question 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.
Answer: This can be proven using basic logic. A 3-year period in a solar calendar has a total of 3 × 12 = 36 months. We have 37 full Moons occurring within these 36 months. It is not possible for each full Moon to fit into its own separate month. After you place one full Moon in each of the 36 months, one full Moon will be left over. This 37th full Moon must go into a month that already has a full Moon in it. As a result, at least one month must contain two full Moons.
In simple words: Three years have 36 months, but there are 37 full Moons. Since you run out of months, at least two full Moons must fall in the same month.

Exam Tip: This is a pigeonhole principle problem. Show the math clearly: 3 × 12 = 36 months, but 37 full Moons means one must double up.

Question 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?
Answer: Vaishali would have observed the Full Moon. The Full Moon is the only phase where the Moon is located directly across from the Sun in the sky. During this phase, the Moon comes up in the east at roughly the same time the Sun goes down in the west. It stays visible all through the night and then sets in the west at roughly the same time the Sun comes up in the east.
In simple words: Only the Full Moon is visible all night long. It rises when the Sun sets and sets when the Sun rises.

Exam Tip: The Full Moon is the only phase visible the entire night because it is opposite the Sun. All other phases rise or set before the sun does.

Question 10. If we stopped having leap years, in approximately how many years would the Indian Independence day happen in winter?
Answer: If leap years were no longer used, our 365-day calendar would slip behind the true seasons by about 0.25 days each year. Independence Day falls in August, which is a monsoon and summer season. For it to occur in winter (around December or January), the calendar would need to drift by about 4 to 5 months. A shift of one month (around 30 days) would take 30 ÷ 0.25 = 120 years. To shift by 4 months, it would need about 4 × 120 = 480 years. Therefore, it would take roughly 500 years for Independence Day to move into the winter season.
In simple words: Without leap years, the calendar drifts about one day every four years. Shifting by 4-5 months would take around 500 years.

Exam Tip: Show the calculation: 0.25 days drift per year × needed shift = total years. This demonstrates why leap years are essential.

Question 11. What is the purpose of launching artificial satellites?
Answer: Artificial satellites are sent into orbit for many critical purposes that benefit our lives and expand scientific knowledge. Their key uses include sending communication signals (such as television and phone broadcasts), aiding in navigation systems (like GPS), observing weather patterns, managing disasters, performing scientific research, and generating detailed maps for urban and regional planning.
In simple words: We launch satellites for many jobs: sending TV and phone signals, giving directions with GPS, tracking weather, helping during disasters, and making maps.

Exam Tip: List at least three main uses of satellites: communication, navigation, and weather monitoring. These are most commonly tested.

Question 12. On which periodic phenomenon are the following measures of time based: (i) day (ii) month (iii) year?
Answer:
(i) Day: The 'day' is based on Earth's spinning motion on its own axis. This rotation creates the repeating cycle of light and darkness and makes the Sun seem to move across the sky.
(ii) Month: The 'month' is based on the Moon's movement around Earth. This revolution creates the repeating pattern of the Moon's phases, which go from new moon to full moon and back to new moon again.
(iii) Year: The 'year' is based on Earth's movement around the Sun. This revolution takes about 365.25 days and creates the repeating cycle of seasons.
In simple words: Days come from Earth spinning. Months come from the Moon circling Earth. Years come from Earth circling the Sun.

Exam Tip: Remember: rotation of Earth = day, revolution of Moon around Earth = month, revolution of Earth around Sun = year. These three motions define our time units.