Case Study Based Questions: Keeping Time with the Skies | Science Class 8 PDF Download

Case Study 1

During a camping trip, Ananya noticed the Moon in the sky at sunset, appearing as a thin crescent. The next evening, the crescent seemed slightly larger and was positioned farther from the Sun. She also learned from a local guide that fishermen plan their trips based on Moon phases because tides change with them. Ananya wondered why the Moon’s shape changed nightly, why it moved relative to the Sun, and how it influenced tides.

Questions

1. Why did the Moon’s crescent shape grow larger each evening? (2 marks)
   Solution:
   The Moon’s shape grows during the waxing phase as more of its illuminated portion, reflecting sunlight, faces Earth. Each day, the Moon’s position shifts, showing a larger crescent.

2. Why did the Moon appear farther from the Sun each evening? (2 marks)
   Solution:
   As the Moon orbits Earth, its position relative to the Sun changes daily. During the waxing phase, it moves farther from the Sun in the sky, visible at sunset.

3. How do Moon phases affect tides, helping fishermen plan trips? (3 marks)
   Solution:
   Moon phases influence tides due to its gravitational pull. During new and full Moon phases, tides are higher (spring tides) because the Sun and Moon’s gravity align. Fishermen use these patterns to plan trips, as high tides affect fishing conditions and boat navigation.

4. What phase was the Moon in when Ananya saw it as a crescent at sunset? (2 marks)
   Solution:
   The crescent Moon at sunset indicates the waxing crescent phase, occurring a few days after the new Moon, when the illuminated portion starts to grow.

5. Why can the Moon be seen during the day, as Ananya observed? (2 marks)
   Solution:
   The Moon is visible during the day when its illuminated portion faces Earth and is above the horizon while the Sun is up, often during waxing or waning crescent phases.

Case Study 2

At a village festival celebrating Makar Sankranti, Rohan saw kites filling the sky and noticed the Moon visible during the day. His grandmother explained that this festival occurs around January 14 each year, tied to the Sun’s position, unlike Diwali, which shifts dates. Rohan also read about ISRO’s Chandrayaan missions, which study the Moon. He wondered why Makar Sankranti’s date is stable, why Diwali’s date changes, and how satellites like Chandrayaan help us understand the Moon.

Questions

1. Why does Makar Sankranti occur on nearly the same date each year? (2 marks)
   Solution:
   Makar Sankranti follows a solar sidereal calendar, tied to the Sun’s position at the winter solstice, aligning closely with the Gregorian calendar’s tropical year, keeping its date stable.

2. Why does Diwali’s date change in the Gregorian calendar? (2 marks)
   Solution:
   Diwali is based on a luni-solar calendar, tied to the new Moon of Kartika. The lunar year is shorter than the solar year, causing dates to shift annually unless adjusted by an intercalary month.

3. How do ISRO’s Chandrayaan missions contribute to studying the Moon? (3 marks)
   Solution:
   Chandrayaan missions, like Chandrayaan-1, 2, and 3, study the Moon’s surface, composition, and atmosphere using satellites and landers. They collect data on lunar soil, water presence, and topography, enhancing our understanding of the Moon’s geology and aiding future exploration.

4. Why was the Moon visible during the day at the festival? (2 marks)
   Solution:
   The Moon is visible during the day when its illuminated portion faces Earth and it’s above the horizon, often during waxing or waning phases, as seen during Makar Sankranti.

5. How does a luni-solar calendar differ from a solar calendar? (2 marks)
   Solution:
   A luni-solar calendar tracks Moon phases for months but adds an intercalary month every few years to sync with the solar year’s seasons. A solar calendar, like the Gregorian, adjusts months to match the 365-day solar year.

Case Study 3

During a science class, Priya conducted an activity to track the Moon’s phases. She observed the Moon at sunrise, noting it was a waning gibbous. Over days, it shrank to a crescent and moved closer to the Sun in the sky. She also learned that ancient Indians used the Sun’s position to mark Uttarayan and Dakshinayan. Priya wondered why the Moon’s position shifted, how its phases formed, and how the Sun’s movement defined these periods.

Questions

1. Why did the Moon’s position shift closer to the Sun each day? (2 marks)
   Solution:
   As the Moon orbits Earth, its position relative to the Sun changes daily. During the waning phase, it moves closer to the Sun in the sky, observed at sunrise.

2. How do the Moon’s phases form, as Priya observed? (2 marks)
   Solution:
   The Moon’s phases occur because only the illuminated portion, reflecting sunlight, is visible from Earth. As the Moon orbits, different parts of this portion face Earth, changing its shape from gibbous to crescent.

3. How did ancient Indians use the Sun’s position for Uttarayan and Dakshinayan? (3 marks)
   Solution:
   Ancient Indians observed the Sun’s apparent movement: northward from December to June (Uttarayan) and southward from June to December (Dakshinayan). These correspond to solstices, marking seasonal changes, used to time agricultural and cultural events like festivals.

4. What phase was the Moon in when Priya saw it as a waning gibbous? (2 marks)
   Solution:
   The waning gibbous phase occurs after the full Moon, when the illuminated portion shrinks but is still more than half visible, as Priya observed at sunrise.

5. Why don’t lunar eclipses occur every full Moon? (2 marks)
   Solution:
   Lunar eclipses only occur when the Moon passes through Earth’s shadow on a full Moon, but the Moon’s orbit is slightly tilted relative to Earth’s, so alignment is rare, happening only a few times a year.

Case Study 4

While stargazing, Vikram noticed a bright point moving quickly across the sky, which his teacher identified as an artificial satellite. He learned about ISRO’s AstroSat, which observes stars, and how ancient Indians used star positions to track time. Vikram also read that the Moon’s revolution is slowing, affecting lunar calendars. He wondered how satellites move, how stars helped ancient timekeeping, and how the Moon’s slowing affects calendars.

Questions

1. Why did the satellite Vikram saw move quickly across the sky? (2 marks)
   Solution:
   Artificial satellites, orbiting Earth at about 800 km, complete an orbit in roughly 100 minutes, appearing as fast-moving points of light due to their high speed and reflective surfaces.

2. How did ancient Indians use stars for timekeeping? (2 marks)
   Solution:
   Ancient Indians tracked stars rising at sunset, noting their yearly cycle due to Earth’s revolution. This sidereal year helped define calendars, aligning festivals with celestial patterns.

3. How does the Moon’s slowing revolution affect luni-solar calendars? (3 marks)
   Solution:
   The Moon’s revolution slowing increases the time for a lunar month, requiring less frequent intercalary months in luni-solar calendars to sync with the solar year. Over long periods, calendars need adjustments to account for this gradual change in lunar cycles.

4. What is the purpose of ISRO’s AstroSat satellite? (2 marks)
   Solution:
   AstroSat observes stars and celestial objects, collecting data on their temperature, composition, and behavior, contributing to astrophysical research and understanding the universe.

5. How does the sidereal year differ from the tropical year? (2 marks)
   Solution:
   The sidereal year, based on Earth’s revolution relative to fixed stars, is about 20 minutes longer than the tropical year, based on the cycle of seasons from equinox to equinox.