Keeping Time with the Skies – Important Questions and Answers
Premium CBSE Class 8 Science revision post based on the chapter Keeping Time with the Skies. Includes Moon phases, waxing and waning periods, lunar, solar and luni-solar calendars, Indian National Calendar, festivals, artificial satellites, exam questions, case studies, HOTS, diagram-based questions, FAQs, and an interactive quiz.
Short Introduction
For thousands of years, humans have observed the sky to understand time. The daily rising and setting of the Sun gave us the idea of a day. The changing phases of the Moon gave us the idea of a month. The cycle of seasons gave us the idea of a year. This chapter explains how the Moon changes its appearance, why the Moon is sometimes visible during the day, how calendars were formed, why festivals fall on different dates, and why artificial satellites are launched into space.
Chapter Overview
Phases of the Moon
The changing shapes of the bright portion of the Moon seen from Earth are called phases of the Moon.
Waxing and Waning
The increasing bright portion is waxing, while the decreasing bright portion is waning.
Day, Month and Year
A day is based on Earth’s rotation, a month on Moon’s phases, and a year on Earth’s revolution around the Sun.
Calendars
Lunar, solar and luni-solar calendars were developed using repeated astronomical cycles.
Festivals and Astronomy
Many festivals are linked to lunar phases, solar movement, or luni-solar calendar adjustments.
Artificial Satellites
Artificial satellites help in communication, navigation, weather monitoring, disaster management and scientific research.
Important Time Cycles
Based on Earth’s rotation and the Sun returning to its highest position in the sky.
Based on the Moon’s phase cycle of about 29.5 days.
Based on Earth’s revolution around the Sun and the cycle of seasons.
This is about 11 days shorter than a solar year.
Important Keywords with Meanings
| Keyword | Meaning |
|---|---|
| Phase of the Moon | The changing shape of the bright portion of the Moon as seen from Earth. |
| Full Moon | The day when the Moon appears as a full bright circle; also called Purnima. |
| New Moon | The day when the Moon is not visible; also called Amavasya. |
| Waxing period | The period when the bright portion of the Moon increases. |
| Waning period | The period when the bright portion of the Moon decreases. |
| Shukla Paksha | The waxing period of the Moon in Indian calendars. |
| Krishna Paksha | The waning period of the Moon in Indian calendars. |
| Crescent phase | A Moon phase in which less than half of the illuminated portion is visible. |
| Gibbous phase | A Moon phase in which more than half of the illuminated portion is visible. |
| Illuminated portion | The part of the Moon that receives sunlight. |
| Non-illuminated portion | The part of the Moon that does not receive sunlight. |
| Mean solar day | The average time between the Sun’s highest position on one day and the next day; about 24 hours. |
| Lunar month | The time taken by the Moon to complete one cycle of phases; about 29.5 days. |
| Solar year | The time taken by Earth to complete one revolution around the Sun; about 365¼ days. |
| Lunar calendar | A calendar based mainly on the Moon’s phase cycle. |
| Solar calendar | A calendar based on the cycle of seasons and Earth’s revolution around the Sun. |
| Luni-solar calendar | A calendar that uses Moon phases but adjusts to remain synchronised with seasons. |
| Leap year | A year in which one extra day is added to keep a solar calendar synchronised with seasons. |
| Adhika Maasa | An extra month added in some luni-solar calendars every few years. |
| Indian National Calendar | A solar calendar used by the Government of India along with the Gregorian calendar. |
| Shaka Era | The era used in the Indian National Calendar. |
| Uttarayan | The apparent northward movement of the Sun from December to June. |
| Dakshinayan | The apparent southward movement of the Sun from June to December. |
| Artificial satellite | A human-made object launched into space to orbit Earth or another celestial body. |
| Space debris | Non-functional satellites, rocket parts, or fragments left in space. |
Very Short Answer Questions
1. What are phases of the Moon?
Answer: The changing shapes of the bright portion of the Moon as seen from Earth are called phases of the Moon.
2. What is Full Moon day called in India?
Answer: Full Moon day is called Purnima.
3. What is New Moon day called in India?
Answer: New Moon day is called Amavasya.
4. What is the waning period of the Moon?
Answer: The period when the bright portion of the Moon decreases is called the waning period.
5. What is the waxing period of the Moon?
Answer: The period when the bright portion of the Moon increases is called the waxing period.
6. What is Krishna Paksha?
Answer: Krishna Paksha is the waning period of the Moon.
7. What is Shukla Paksha?
Answer: Shukla Paksha is the waxing period of the Moon.
8. How long does one full cycle of Moon phases take?
Answer: One full cycle of Moon phases takes about 29.5 days, nearly a month.
9. Does the Moon produce its own light?
Answer: No, the Moon shines by reflecting sunlight.
10. Why do we see only part of the Moon on some days?
Answer: We see only the illuminated portion of the Moon that faces Earth.
11. Are Moon phases caused by Earth’s shadow?
Answer: No, Moon phases are not caused by Earth’s shadow.
12. What causes a lunar eclipse?
Answer: A lunar eclipse is caused when Earth’s shadow falls on the Moon.
13. When can a lunar eclipse occur?
Answer: A lunar eclipse can occur only on a full Moon day.
14. When can a solar eclipse occur?
Answer: A solar eclipse can occur only on a new Moon day.
15. What is a mean solar day?
Answer: The average time between the Sun’s highest position on one day and the next is called a mean solar day.
16. What is the duration of a mean solar day?
Answer: The duration of a mean solar day is about 24 hours.
17. What is a lunar calendar based on?
Answer: A lunar calendar is based on the Moon’s phase cycle.
18. What is a solar calendar based on?
Answer: A solar calendar is based on the cycle of seasons and Earth’s revolution around the Sun.
19. What is a luni-solar calendar?
Answer: A luni-solar calendar uses Moon phases and also adjusts to stay synchronised with seasons.
20. What is Adhika Maasa?
Answer: Adhika Maasa is an extra month added in some luni-solar calendars every few years.
21. What is the Indian National Calendar?
Answer: It is a solar calendar used by the Government of India along with the Gregorian calendar.
22. What is Earth’s natural satellite?
Answer: The Moon is Earth’s natural satellite.
23. What are artificial satellites?
Answer: Artificial satellites are human-made objects launched into space to orbit Earth or another body.
24. Name one Indian space organisation.
Answer: ISRO, the Indian Space Research Organisation.
Short Answer Questions
1. Why is the Moon sometimes visible during the day?
Answer: The Moon can be visible during the day because it reflects sunlight and its position in the sky changes daily. Sometimes moonrise occurs in the afternoon, so the Moon can be seen in daylight.
2. Why does the Moon appear different on different days?
Answer: The Moon appears different because we see different fractions of its illuminated portion as it revolves around Earth. The Moon’s shape itself does not change.
3. Why is New Moon not visible from Earth?
Answer: On New Moon day, the illuminated portion of the Moon faces away from Earth. The side facing Earth is non-illuminated, so the Moon is not visible.
4. Why is the Full Moon fully visible?
Answer: On Full Moon day, the entire illuminated portion of the Moon faces Earth. Therefore, the Moon appears as a full bright circle.
5. What is the difference between waxing and waning Moon?
Answer: In the waxing period, the bright portion of the Moon increases from New Moon to Full Moon. In the waning period, the bright portion decreases from Full Moon to New Moon.
6. What is the difference between crescent and gibbous phases?
Answer: In the crescent phase, less than half of the illuminated portion of the Moon is visible. In the gibbous phase, more than half of the illuminated portion is visible.
7. Why does the Moon rise about 50 minutes later each day?
Answer: As Earth completes one rotation in 24 hours, the Moon also moves ahead in its orbit around Earth. Earth must rotate a little more for the Moon to appear in nearly the same position, so moonrise is delayed by about 50 minutes.
8. Why are Moon phases not caused by Earth’s shadow?
Answer: Moon phases occur because the Moon revolves around Earth and we see different parts of its illuminated half. Earth’s shadow causes lunar eclipses, not regular Moon phases.
9. Why do eclipses not occur every month?
Answer: Eclipses do not occur every month because the Moon’s orbit is slightly tilted with respect to Earth’s orbit around the Sun. Therefore, Sun, Earth and Moon are not perfectly aligned every month.
10. How is a day defined using the Sun?
Answer: A day is based on the apparent daily motion of the Sun caused by Earth’s rotation. The average time taken by the Sun to return to its highest position in the sky is about 24 hours.
11. How is a month defined using the Moon?
Answer: A month is based on the Moon’s phase cycle. The Moon takes about 29.5 days to complete one cycle from one phase back to the same phase.
12. How is a year defined using the Earth’s motion?
Answer: A year is based on Earth’s revolution around the Sun and the cycle of seasons. Earth takes nearly 365¼ days to complete one revolution around the Sun.
13. Why do lunar calendar months not stay synchronised with seasons?
Answer: A lunar year has about 354 days, while seasons repeat in about 365 days. This 11-day difference makes lunar months shift with respect to seasons.
14. Why are leap years needed in solar calendars?
Answer: Earth takes nearly 365¼ days to complete one revolution around the Sun. The extra quarter day adds up to about one day every four years, so a leap day is added to keep the calendar synchronised with seasons.
15. Why are artificial satellites launched?
Answer: Artificial satellites are launched for communication, navigation, weather monitoring, disaster management, mapping, and scientific research.
Long Answer Questions
1. Explain how the phases of the Moon occur.
Answer: The Moon does not produce its own light. It shines by reflecting sunlight. Half of the Moon facing the Sun is illuminated, while the other half remains non-illuminated. As the Moon revolves around Earth, the portion of the illuminated half that faces Earth changes. Sometimes we see the whole illuminated portion, sometimes only part of it, and sometimes none of it. These changing shapes of the bright portion are called phases of the Moon.
2. Describe the waxing and waning periods of the Moon.
Answer: The waning period begins after Full Moon. During this period, the bright portion of the Moon gradually decreases from a full circle to half Moon, then to crescent, and finally to New Moon. In India, this is called Krishna Paksha. After New Moon, the bright portion increases from crescent to half Moon and then to Full Moon. This increasing period is called waxing period or Shukla Paksha.
3. Explain why the Moon is not seen at the same position every day.
Answer: The Moon revolves around Earth while Earth rotates on its axis. In 24 hours, Earth completes one rotation, but during that time the Moon moves ahead in its orbit. Therefore, Earth must rotate a little more before the Moon appears in nearly the same position in the sky. This is why the Moon rises about 50 minutes later each day and appears shifted on successive days.
4. Differentiate between Moon phases and lunar eclipse.
| Moon Phases | Lunar Eclipse |
|---|---|
| They occur due to changing Sun-Moon-Earth orientation as the Moon revolves around Earth. | It occurs when Earth’s shadow falls on the Moon. |
| They occur regularly every month. | It is rare and brief. |
| They include crescent, half, gibbous, full and new Moon. | The Moon becomes partially or fully darkened by Earth’s shadow. |
| They are not caused by Earth’s shadow. | It is directly caused by Earth’s shadow. |
| They are part of the natural monthly cycle. | It can happen only on a full Moon day. |
5. Explain how day, month and year are based on astronomical cycles.
Answer:
- Day: A day is based on Earth’s rotation. The Sun appears to return to its highest position in the sky after about 24 hours.
- Month: A month is based on the cycle of Moon phases, which takes about 29.5 days.
- Year: A year is based on Earth’s revolution around the Sun and the cycle of seasons, which takes about 365¼ days.
6. Explain lunar, solar and luni-solar calendars.
Answer:
- Lunar calendar: It is based mainly on Moon phases. One lunar month is about 29.5 days, and 12 lunar months make about 354 days.
- Solar calendar: It is based on Earth’s revolution around the Sun and the cycle of seasons. The Gregorian calendar is a solar calendar.
- Luni-solar calendar: It uses Moon phases for months but adds adjustments, such as an extra month, to remain synchronised with the solar year and seasons.
7. Why is Adhika Maasa added in luni-solar calendars?
Answer: A lunar year of 12 lunar months is about 354 days long, while a solar year is about 365 days. Thus, a lunar year is shorter by about 11 days. After every 2–3 years, this difference becomes close to a full month. To keep lunar months in step with the seasons, an extra month called Adhika Maasa or intercalary month is added in some luni-solar calendars.
8. Explain the Indian National Calendar.
Answer: The Indian National Calendar is a solar calendar used by the Government of India along with the Gregorian calendar. It has 365 days in a regular year. The year begins on 22 March, the day after the spring equinox. In a leap year, it begins on 21 March. The first month is Chaitra. In a regular year, the second to sixth months have 31 days, while the remaining months have 30 days. Leap years are matched with the Gregorian calendar by adding one day to Chaitra.
9. Why do many Indian festivals fall on different Gregorian dates every year?
Answer: Many Indian festivals are based on lunar or luni-solar calendars. Since lunar months are based on Moon phases and do not exactly match the Gregorian solar calendar, festival dates shift every year in the Gregorian calendar. Luni-solar calendars add an intercalary month every few years, so the shift is usually corrected and remains within a limited range. Purely lunar calendar festivals can shift more steadily through Gregorian months.
10. Explain the importance of artificial satellites.
Answer: Artificial satellites are human-made objects launched into space to orbit Earth or other celestial bodies. They help in communication, navigation, weather monitoring, disaster management, mapping, city planning and scientific research. ISRO has launched many satellites such as Cartosat for mapping and AstroSat for astronomical observations. Satellites also support missions to the Moon, Sun and Mars.
Case-Study Based Questions
Case Study 1: Moon During Daytime
Meera saw the Moon during the day at a kite festival. She was surprised because she thought the Moon appears only at night.
Q1. Can the Moon be visible during daytime?
Answer: Yes, the Moon can be visible during daytime depending on its position in the sky.
Q2. Why does the Moon shine?
Answer: The Moon shines by reflecting sunlight.
Q3. Why does the Moon appear in different shapes?
Answer: We see different parts of its illuminated portion as it revolves around Earth.
Case Study 2: Full Moon to New Moon
A student observes that after Full Moon, the bright portion of the Moon becomes smaller each day and after about two weeks the Moon is not visible.
Q1. What is this decreasing period called?
Answer: It is called the waning period.
Q2. What is this period called in India?
Answer: It is called Krishna Paksha.
Q3. What is the day when the Moon is not visible called?
Answer: It is called New Moon day or Amavasya.
Case Study 3: Measuring a Day
A student fixes a vertical stick in sunlight and marks the shadow tip every minute from 11:00 a.m. to 1:10 p.m. The shortest shadow is observed around noon.
Q1. What does the shortest shadow indicate?
Answer: It indicates the time when the Sun is at its highest position in the sky.
Q2. What is the average time between two successive highest positions of the Sun?
Answer: About 24 hours.
Q3. What is this duration called?
Answer: It is called the mean solar day.
Case Study 4: Festival Dates
Diwali, Holi and Eid-ul-Fitr do not fall on the same Gregorian date every year. A student wonders why this happens.
Q1. Why do these dates shift?
Answer: Many festivals are based on lunar or luni-solar calendars, which do not exactly match the Gregorian calendar.
Q2. On which lunar phase does Diwali fall?
Answer: Diwali falls on the New Moon of the month of Kartika.
Q3. On which lunar phase does Holi fall?
Answer: Holi falls on the Full Moon of Phalguna.
Case Study 5: Moving Specks in the Night Sky
Before sunrise, a student sees a tiny point of light moving quickly across the sky. It does not blink like most stars.
Q1. What could the moving point be?
Answer: It could be an artificial satellite.
Q2. At what height do many satellites orbit Earth?
Answer: Many orbit about 800 km above Earth’s surface.
Q3. Write two uses of artificial satellites.
Answer: They are used for communication and weather monitoring.
Assertion-Reason Questions
Choose the correct option:
A. Both Assertion and Reason are true and Reason is the correct explanation.
B. Both Assertion and Reason are true but Reason is not the correct explanation.
C. Assertion is true but Reason is false.
D. Assertion is false but Reason is true.
1. Assertion: The Moon’s shape itself does not change. Reason: We see different parts of its illuminated portion from Earth.
Answer: A
2. Assertion: Moon phases are caused by Earth’s shadow. Reason: Earth’s shadow causes lunar eclipses.
Answer: D
3. Assertion: The Moon can be visible during the day. Reason: The Moon reflects sunlight and its position changes daily.
Answer: A
4. Assertion: New Moon is not visible from Earth. Reason: The illuminated portion of the Moon does not face Earth on New Moon day.
Answer: A
5. Assertion: A waxing Moon is easiest to spot at sunset. Reason: The bright portion of the Moon increases after New Moon.
Answer: B
6. Assertion: The Moon rises about 50 minutes later every day. Reason: The Moon moves ahead in its orbit while Earth rotates.
Answer: A
7. Assertion: A lunar year is shorter than a solar year. Reason: Twelve lunar months add up to about 354 days.
Answer: A
8. Assertion: Leap years help solar calendars remain synchronised with seasons. Reason: Earth takes about 365¼ days to revolve around the Sun.
Answer: A
9. Assertion: Luni-solar calendars add an extra month every few years. Reason: This helps keep lunar months in step with seasons.
Answer: A
10. Assertion: Artificial satellites are useful for society. Reason: They help in communication, navigation, weather monitoring and disaster management.
Answer: A
Competency-Based and Critical Thinking / HOTS Questions
1. Amol was born on 6 May on a Full Moon day. Will his birthday fall on Full Moon every year?
Answer: No. The Gregorian calendar is solar, while Full Moon depends on the lunar phase cycle of about 29.5 days. Therefore, the Full Moon does not occur on the same Gregorian date every year.
2. If the Moon is seen overhead at sunset, what phase is likely seen?
Answer: A half Moon phase is likely. At sunset, the waxing half Moon is commonly visible high in the sky.
3. If the Moon is seen in the sky from sunset to sunrise, which phase is it?
Answer: It is Full Moon because on Full Moon day, the Moon is nearly opposite the Sun and can be visible through the night.
4. Why is the crescent Moon always seen with its bright side facing the Sun?
Answer: The Moon shines by reflecting sunlight. The illuminated part must face the Sun because sunlight falls from that direction.
5. If the Moon’s revolution around Earth becomes slower, will luni-solar calendars need Adhika Maasa more often or less often?
Answer: If the Moon’s revolution becomes slower, lunar months become longer. Twelve lunar months would become closer to the solar year, so an intercalary month may be needed less often.
6. Why must at least two of 37 full Moons in three years fall in the same Gregorian month?
Answer: Three Gregorian years have 36 months. If there are 37 full Moons, then by the pigeonhole principle, at least one month must contain two full Moons.
7. If leap years were stopped, why would calendar dates slowly shift with respect to seasons?
Answer: Without leap years, the extra quarter day each year would not be corrected. After many years, calendar dates would drift away from the seasons.
8. Why do festivals based on purely lunar calendars move earlier by about 11 days each Gregorian year?
Answer: A lunar year is about 354 days, while the Gregorian solar year is about 365 days. The difference is about 11 days, so purely lunar festivals move earlier by about 11 days each year.
9. Why do satellites appear as moving points of light in the night sky?
Answer: Satellites reflect sunlight while orbiting Earth. Since they move quickly in orbit, they appear as moving points of light.
10. Why is space debris a problem?
Answer: Space debris can collide with working satellites and damage them. Large pieces may also fall back to Earth, so countries are working to reduce this danger.
Diagram-Based Questions
Diagram 1: Moon Phases Cycle
Question: Name the phase when the Moon appears as a full bright circle.
Answer: Full Moon or Purnima.
Diagram 2: Waxing and Waning
Question: What is the period from New Moon to Full Moon called?
Answer: Waxing period or Shukla Paksha.
Diagram 3: Why Moon Phases Occur
Question: Are Moon phases caused by Earth’s shadow?
Answer: No, they are caused by the changing relative positions of the Sun, Moon and Earth.
Diagram 4: Natural Time Units
Question: Which astronomical cycle defines a month?
Answer: The Moon’s phase cycle defines a month.
Diagram 5: Types of Calendars
Question: Which calendar uses both Moon phases and seasonal corrections?
Answer: A luni-solar calendar.
Quick Revision Box
- The Moon shines by reflecting sunlight.
- The Moon’s shape does not change; only the visible illuminated portion changes.
- Changing visible shapes of the Moon are called phases.
- Full Moon is called Purnima.
- New Moon is called Amavasya.
- Waxing means the bright portion increases.
- Waning means the bright portion decreases.
- Shukla Paksha is the waxing period.
- Krishna Paksha is the waning period.
- One Moon phase cycle takes about 29.5 days.
- Moon phases are not caused by Earth’s shadow.
- The Moon rises about 50 minutes later each day.
- A day is based on Earth’s rotation.
- A month is based on Moon phases.
- A year is based on Earth’s revolution around the Sun.
- A lunar year has about 354 days.
- A solar year has about 365¼ days.
- Luni-solar calendars add Adhika Maasa every few years.
- Artificial satellites support communication, navigation, weather and research.
- Space debris can be dangerous for working satellites.
Important Exam Tips
Interactive Quiz
Choose the correct answer and click submit.
Frequently Asked Questions
1. What are phases of the Moon?
Phases of the Moon are the changing shapes of the bright portion of the Moon seen from Earth.
2. Why does the Moon change its shape?
The Moon’s shape does not actually change. We see different parts of its illuminated portion as it revolves around Earth.
3. Are Moon phases caused by Earth’s shadow?
No. Earth’s shadow causes lunar eclipses, not regular Moon phases.
4. What is the difference between Purnima and Amavasya?
Purnima is Full Moon day, while Amavasya is New Moon day.
5. What is the difference between waxing and waning Moon?
Waxing means the bright portion is increasing, while waning means the bright portion is decreasing.
6. Why does the Moon rise later each day?
The Moon moves ahead in its orbit while Earth rotates, so it appears in nearly the same sky position about 50 minutes later each day.
7. What are lunar, solar and luni-solar calendars?
Lunar calendars follow Moon phases, solar calendars follow seasons and Earth’s revolution, and luni-solar calendars use both with adjustments.
8. Why do Indian festivals fall on different Gregorian dates?
Many festivals are based on lunar or luni-solar calendars, which do not exactly match the Gregorian solar calendar.
9. What is Adhika Maasa?
Adhika Maasa is an extra month added in some luni-solar calendars every few years to keep lunar months aligned with seasons.
10. Why are artificial satellites launched?
Artificial satellites are launched for communication, navigation, weather monitoring, disaster management, mapping and scientific research.
Final Conclusion
The chapter Keeping Time with the Skies shows how humans used repeated sky patterns to measure time. The day comes from Earth’s rotation, the month from the Moon’s phases, and the year from Earth’s revolution around the Sun. The chapter also explains why Moon phases occur, how calendars developed, why festival dates change, and how artificial satellites help modern society through communication, navigation, weather forecasting and scientific research.
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