SCIENCE CLASS- 8
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
Explanation:
The Moon does not produce its own light. It reflects sunlight. We can see only the illuminated part of the Moon that faces the Earth.
(ii) The shadow of Earth blocks sunlight from reaching the Moon causing phases.
Answer: False
Correct Statement:
The phases of the Moon are caused by the changing positions of the Earth, Moon and Sun, not by the Earth's shadow.
Explanation:
The Earth's shadow causes a lunar eclipse, not the regular phases of the Moon.
(iii) Calendars are based on various astronomical cycles which repeat in a predictable manner.
Answer: True
Explanation:
Calendars are based on the regular and predictable motions of the Earth and Moon.
(iv) The Moon can only be seen at night.
Answer: False
Correct Statement:
The Moon can be seen both during the day and at night depending on its phase and position.
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.
Explanation:
The Full Moon occurs according to the lunar cycle, which is about 29.5 days long.
The calendar date 6th May follows the solar calendar.
Since the lunar month and solar calendar do not match exactly, the Full Moon does not occur on the same date every year.
Question 3. Name two things that are incorrect in Fig. 11.10.
Answer:
The following two things are incorrect:
- Stars are shown very close to the Moon. In reality, stars are extremely far away.
- Clouds are shown behind and in front of the Moon in an unrealistic manner. Clouds are present in the Earth's atmosphere, whereas the Moon is far beyond them in space.
Question 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 above.
| Phase of Moon | Picture Label |
|---|---|
| Three days after New Moon | A |
| Full Moon | E |
| Three days after Full Moon | F |
| A week after Full Moon | D |
| Day of New Moon | B |
(ii) List the picture labels of the phases of the Moon that are never seen from Earth.
Answer:
None of the phases are completely invisible except the New Moon.
The phase represented by B (New Moon) is generally not visible from Earth because the illuminated side faces away from us.
Question 5. Malini saw the Moon overhead in the sky at sunset.
(i) Draw the phase of the Moon that Malini saw.
Answer:
Malini would have seen the First Quarter Moon (Half Moon).
(ii) Is the Moon in the waxing or waning phase?
Answer:
The Moon is in the waxing phase.
Explanation:
A First Quarter Moon is seen high in the sky around sunset and is a waxing phase.
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.
Explanation:
A waxing gibbous Moon can be seen in the eastern sky during the afternoon.
A crescent Moon is generally seen close to sunrise or shortly after sunset and not rising exactly in the East when the Sun is setting.
Question 7. Scientific studies show that the Moon is getting farther away from the Earth and slower in its revolution. Will lunisolar calendars need an intercalary month more often or less often?
Answer:
Lunisolar calendars will need an intercalary month more often.
Explanation:
If the Moon moves farther away, its orbital period increases.
This increases the difference between lunar months and the solar year, requiring more frequent adjustment through intercalary months.
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:
There are only 12 months in a year.
In 3 years:
12 × 3 = 36 months
But there are 37 Full Moons.
Since there are more Full Moons than months, at least one month must contain two Full Moons.
This follows the Pigeonhole Principle.
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 a Full Moon.
Explanation:
A Full Moon rises around sunset and sets around sunrise, remaining visible throughout the night.
Question 10. If we stopped having leap years, in approximately how many years would the Indian Independence Day happen in winter?
Answer:
Every year the calendar would shift by about 0.25 day.
To shift by about 182 days (half a year):
182 ÷ 0.25 = 728 years
Answer:
Indian Independence Day would occur in winter after approximately 728 years.
Question 11. What is the purpose of launching artificial satellites?
Answer:
Artificial satellites are launched for various purposes:
- Communication
- Weather forecasting
- Navigation and GPS services
- Scientific research
- Remote sensing
- Disaster management
- Defence and security
- Astronomical observations
Question 12. On which periodic phenomenon are the following measures of time based?
(i) Day
Answer:
A day is based on the rotation of the Earth about its axis.
(ii) Month
Answer:
A month is based on the revolution of the Moon around the Earth and the cycle of lunar phases.
(iii) Year
Answer:
A year is based on the revolution of the Earth around the Sun.
Important Concepts from the Chapter
- The Moon shines due to reflected sunlight.
- Phases of the Moon occur because of changing relative positions of the Earth, Moon and Sun.
- A lunar month is approximately 29.5 days.
- The New Moon is generally invisible.
- A Full Moon is visible throughout the night.
- Calendars are based on repeating astronomical cycles.
- Leap years help synchronize the calendar year with Earth's revolution around the Sun.
- Artificial satellites are used for communication, navigation, weather forecasting and scientific research.