[PDF] Class 8 Science NCERT Notes – Chapter 11: Keeping Time with the Skies (PDF, MindMap, Q&A, Quizzes)

PDF Study Notes: Chapter 11: Keeping Time with the Skies

1. The Moon’s Appearance and Phases

1.1 Understanding the Moon’s Phases 

The changing shapes of the bright portion of the Moon as seen from Earth are known as the phases of the Moon. These changes occur in a predictable cycle.

• The Cause: The Moon does not produce its own light; it shines by reflecting sunlight. One half of the Moon is always illuminated by the Sun. The phases we see depend on how much of this illuminated half is facing the Earth as the Moon revolves around our planet.

• Common Misconception: The phases are not caused by the Earth’s shadow falling on the Moon. The Earth’s shadow causes a lunar eclipse, which is a rare and brief event.

• The Cycle: The complete cycle of phases, from one full Moon to the next, takes approximately 29.5 days (nearly a month).

1.2 Key Phases and Periods

• New Moon (Amavasya): The Moon is not visible from Earth because its non-illuminated side faces us. In its orbit, the Moon is closest in the sky to the Sun at this time.

• Waxing Period (Shukla Paksha): The period after the new Moon when the visible bright portion of the Moon is increasing.

• Crescent Phase: Less than half of the illuminated portion is visible.

• Gibbous Phase: More than half of the illuminated portion is visible.

• Full Moon (Purnima): The entire illuminated portion of the Moon faces the Earth, appearing as a full bright circle. The Moon is nearly opposite the Sun in the sky during this phase.

• Waning Period (Krishna Paksha): The period after the full Moon when the visible bright portion of the Moon is decreasing. This period also includes gibbous and crescent phases as the visible light shrinks.

1.3 Observing the Moon in the Sky

• Visibility: Contrary to common belief, the Moon can often be seen during the daytime. A waxing Moon is easiest to spot at sunset, while a waning Moon is easiest to spot at sunrise.

• Position and Timing: The Moon’s position in the sky changes daily. On a full Moon day, the Moon rises around the time the Sun sets. On subsequent days, the Moon rises approximately 50 minutes later each day. This is because, as the Earth completes one 24-hour rotation, the Moon has moved further along in its orbit, and the Earth must rotate a little more for the Moon to appear in the same spot in the sky.

2. The Foundation and Types of Calendars

2.1 Natural Cycles as Units of Time 

Calendars were developed based on observing natural, periodic astronomical events.

• The Day: Based on the Earth’s rotation on its axis. A mean solar day is the average time (24 hours) it takes for the Sun to go from its highest position in the sky on one day to the highest position on the next. The Sun’s highest point corresponds to the time when an object’s shadow is shortest.

• The Month: Based on the Moon’s revolution around the Earth, which completes a cycle of phases in about 29.5 days.

• The Year: Based on the Earth’s revolution around the Sun, which causes the cycle of seasons. A solar year is approximately 365.25 days.

2.2 Solar Calendars 

These calendars are synchronized with the seasons (the solar year).

• The Gregorian Calendar: The most widely used calendar today. It has 365 days, with months adjusted to 28, 30, or 31 days.

• Leap Years: To account for the extra quarter day in a solar year, a leap day is added every four years. To maintain long-term accuracy, leap years are skipped in years divisible by 100 (like 1900) but not in years divisible by 400 (like 2000).

Tropical vs. Sidereal Year:

• Tropical Year: The time between successive spring equinoxes. The Gregorian calendar is based on this.
• Sidereal Year: The time it takes for the same stars to rise again at sunset. It is about 20 minutes longer than a tropical year.

2.3 Lunar Calendars 

These calendars are based on the cycle of the Moon’s phases.

• A lunar year consists of 12 lunar months, totaling about 354 days.
• Because a lunar year is about 11 days shorter than a solar year, seasons do not remain synchronized with the same lunar months year after year.

2.4 Luni-Solar Calendars 

These calendars combine elements of both lunar and solar systems.

• They use the Moon’s phases for months but make adjustments to stay synchronized with the seasons.
• To correct for the ~11-day shortfall each year, an extra month, known as an intercalary month or Adhika Maasa, is added every 2-3 years.
• Many traditional Indian calendars are luni-solar. They can be Amant (month starts after the new Moon) or Purnimant (month starts after the full Moon).

2.5 The Indian National Calendar

Adopted by the Government of India for official purposes alongside the Gregorian calendar.

• It is a solar calendar with 365 days.
• The year begins on March 22 (or March 21 in a leap year), the day after the spring equinox.
• Months are named from traditional Indian calendars (e.g., Chaitra, Vaisakha) and have either 30 or 31 days.
• Leap years are matched to the Gregorian system, with the extra day added to the first month, Chaitra.

3. Astronomy, Festivals, and Culture

Lunar/Luni-Solar Festivals: 

Many Indian festivals are tied to the phases of the Moon.

• Diwali: New Moon of Kartika.
• Holi: Full Moon of Phalguna.
• Eid-ul-Fitr: Sighting of the crescent Moon at the end of Ramazan.

Date Shifts 

Festivals based on a purely lunar calendar (like Eid) occur about 11 days earlier in the Gregorian calendar each year. Festivals based on luni-solar calendars (like Diwali) also shift but stay within a range of about one month due to the intercalary month correction.

• Solar Festivals: Some festivals like Makar Sankranti, Pongal, and Vaisakhi are based on a solar sidereal calendar and occur on nearly the same Gregorian date each year.

Ancient Indian Astronomy: 

Ancient observers noted the Sun’s apparent northward movement (Uttarayan) from December to June and southward movement (Dakshinayan) from June to December. Texts like the Surya Siddhanta record these observations, linking them to constellations.

4. Artificial Satellites and Space Science

Definition: Artificial satellites are human-made objects launched to orbit the Earth. They appear as fast-moving points of light in the night sky.

Purpose: They are used for communication, navigation, weather monitoring, disaster management, and scientific research.

Indian Space Program (ISRO):

• Vikram Sarabhai is known as the Father of the Indian Space Programme.
• Cartosat series: Satellites for high-quality Earth imaging used for mapping and city planning.
• AstroSat: A mission for scientific observation of stars and celestial objects.
• Other Missions: Chandrayaan (Moon), Aditya L1 (Sun), Mangalyaan (Mars).

Space Debris 

After their useful life, old satellites and rocket parts become space junk, which poses a collision risk to active satellites.

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Quiz

Answer each question in 2-3 sentences.

1. What is the fundamental reason we see phases of the Moon, and what is a common incorrect explanation?

2. Explain the difference between the waxing period (Shukla Paksha) and the waning period (Krishna Paksha).

3. Why does the Moon rise approximately 50 minutes later each day?

4. How does a luni-solar calendar, used in many parts of India, stay synchronized with both the lunar cycle and the solar seasons?

5. Describe the key features of the Indian National Calendar.

6. What is the difference between a tropical year and a sidereal year?

7. Why do festivals like Diwali occur on different dates in the Gregorian calendar each year, while festivals like Makar Sankranti occur on nearly the same date?

8. What is the primary function of ISRO’s Cartosat series of satellites?

9. According to ancient Indian astronomical observations, what are Uttarayan and Dakshinayan?

10. What is space debris and why is it a concern?

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Answer Key

1. The phases of the Moon happen because we see different amounts of the Moon’s sunlit half as it revolves around the Earth. The Moon shines by reflecting sunlight, and our viewing angle of this illuminated portion changes. A common incorrect explanation is that the Earth’s shadow causes the phases, but this phenomenon is actually a lunar eclipse.

2. The waxing period, or Shukla Paksha, is the two-week period after the new Moon when the visible bright portion of the Moon grows from nothing to a full circle. The waning period, or Krishna Paksha, is the two-week period after the full Moon when the visible bright portion shrinks from a full circle back to nothing.

3. As the Earth completes one rotation in 24 hours, the Moon also moves forward in its orbit around the Earth. Therefore, the Earth needs to rotate for an additional 50 minutes for the Moon to appear in nearly the same position in the sky as the previous day.

4. A luni-solar calendar primarily uses the Moon’s phases for months, but since a 12-month lunar year is about 11 days shorter than a solar year, it falls out of sync with the seasons. To correct this, an extra month, called an intercalary month or Adhika Maasa, is added every two to three years.

5. The Indian National Calendar is a solar calendar with 365 days, beginning on March 22nd (the day after the spring equinox). Its leap years are matched with the Gregorian calendar, and its months, taken from traditional calendars, have either 30 or 31 days.

6. A tropical year is the time between successive spring equinoxes and is the basis for the Gregorian calendar. A sidereal year is the time it takes for the Earth to complete one orbit so that the same stars rise again at sunset; it is about 20 minutes longer than a tropical year.

7. Diwali is based on a luni-solar calendar tied to the Moon’s phases, which do not perfectly align with the 365-day Gregorian solar calendar, causing its date to shift. Makar Sankranti is based on a solar sidereal calendar, which aligns much more closely with the Gregorian calendar’s solar cycle, so its date remains relatively fixed.

8. The Cartosat series of satellites, launched by ISRO, captures high-quality images of the Earth. This data is used to improve maps, plan cities, and manage natural disasters in India.

9. Uttarayan is the Sun’s apparent northward movement in the sky, which occurs from December to June. Dakshinayan is the Sun’s apparent southward movement, occurring from June to December, as observed by ancient sky-watchers.

10. Space debris, or space junk, consists of old, non-functional artificial satellites and their rocket parts that remain in orbit around the Earth. It is a concern because it crowds space and could collide with and damage working satellites.

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Essay Questions

1. Discuss the evolution of timekeeping by tracing the development of lunar, solar, and luni-solar calendars. Explain the astronomical cycles that underpin each system and describe the specific problems each type of calendar was designed to solve.

2. Analyze the statement: “The Moon’s phases are a result of geometry, not shadow.” In your response, detail the relative positions of the Sun, Earth, and Moon during the new moon, full moon, and quarter moon phases to explain why we see these different appearances from Earth.

3. Explore the deep connection between astronomical phenomena and cultural practices in India. Using specific examples from the text, explain how the cycles of the Sun and Moon have influenced the timing of various festivals, calendars, and even traditional arts.

4. Describe the role of the Indian Space Research Organisation (ISRO) in utilizing space for national development and scientific advancement. Detail the purpose of key missions like Cartosat and AstroSat, and discuss the contributions of figures like Vikram Sarabhai.

5. Compare and contrast the Gregorian calendar with the Indian National Calendar. Discuss their astronomical basis, structure (number of days, months, leap year system), and official purpose.

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Glossary

• Adhika Maasa: An extra month, also called an intercalary month, added to some luni-solar calendars every few years to keep the lunar cycle in step with the solar year.
• Amant: A type of luni-solar calendar in which the new month starts on the first day after the new Moon.
• Amavasya: The new Moon day, when the Moon is not visible from Earth.
• Artificial Satellites: Human-made objects launched from Earth to orbit the planet for purposes such as communication, navigation, and research.
• AstroSat: An ISRO mission for making scientific observations of stars and other celestial objects.
• Cartosat: A series of ISRO satellites that capture high-quality images of the Earth for mapping, city planning, and disaster management.
• Crescent phase: A phase of the Moon where less than half of the illuminated portion can be seen from Earth.
• Dakshinayan: The apparent southward movement of the Sun in the sky, observed from June to December.
• Full Moon: The phase when the entire illuminated portion of the Moon faces the Earth.
• Gibbous phase: A phase of the Moon where more than half of the illuminated portion can be seen from Earth.
• Gregorian calendar: A solar calendar widely used today, consisting of 365 days and a leap year system to synchronize with the seasons.
• Indian National Calendar: A solar calendar used by the Government of India for official purposes, beginning the day after the spring equinox.
• Intercalary month: See Adhika Maasa.
• Krishna Paksha: The waning period of the Moon, the two-week period after the full Moon when its visible bright portion decreases.
• Leap year: A year in a solar calendar to which an extra day is added (e.g., February 29th) to keep the calendar synchronized with the solar year.
• Lunar calendar: A calendar based on the cycles of the Moon’s phases, typically having a year of 354 days.
• Luni-solar calendars: Calendars that use the Moon’s phases for months but also make adjustments (like adding an intercalary month) to stay in sync with the cycle of seasons.
• Mean solar day: The average time the Sun takes to go from its highest position in the sky on one day to the next, which is 24 hours.
• New Moon: The phase when the Moon is not visible because its non-illuminated side faces the Earth.
• Phases of the Moon: The changing shapes of the bright portion of the Moon as seen from the Earth.
• Purnima: The full Moon day.
• Purnimant: A type of luni-solar calendar in which the new month starts on the day after the full Moon.
• Shukla Paksha: The waxing period of the Moon, the two-week period after the new Moon when its visible bright portion increases.
• Sidereal year: The time duration required for the same stars to rise again at sunset, used by astronomers to track Earth’s position in its orbit.
• Solar calendar: A calendar based on the cycle of seasons, which is determined by the Earth’s revolution around the Sun (approximately 365.25 days).
• Solar year: The time the Earth takes to complete one revolution around the Sun, which is nearly 365 and a quarter days.
• Space Debris: Non-functional artificial satellites and rocket parts that remain in orbit, posing a collision risk.
• Tropical year: The time between successive spring equinoxes; the basis for the Gregorian calendar.
• Uttarayan: The apparent northward movement of the Sun in the sky, observed from December to June.
• Waning period: The period when the bright part of the Moon is visible from Earth decreases.
• Waxing period: The period when the bright part of the Moon is visible from Earth increases.