Keeping Time with the SkiesClass 8 Science NCERT Solutions
12 Solutions
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Solution 1 of 12
Q1Keep the curiosity alive
State whether the following statements are True or False.
(i)
We can only see that part of the Moon which reflects sunlight towards us.
(ii)
The shadow of Earth blocks sunlight from reaching the Moon causing phases.
(iii)
Calendars are based on various astronomical cycles which repeat in a predictable manner.
(iv)
The Moon can only be seen at night.
Solution
(i) True
Reason: The Moon does not produce its own light. It shines by reflecting sunlight. From Earth, we can only see the portion of the Moon's surface that is illuminated by the Sun and is facing us.
(ii) False
Reason: The phases of the Moon are caused by the changing viewing angle of the Moon's illuminated surface as it revolves around the Earth. The Earth's shadow falling on the Moon causes a lunar eclipse, which is a different and much rarer event.
(iii) True
Reason: Calendars are systems for organizing time, and they are fundamentally based on natural, periodic astronomical events such as the Earth's rotation (day), the Moon's revolution around the Earth (month), and the Earth's revolution around the Sun (year).
(iv) False
Reason: The Moon is often visible during the daytime. Its visibility depends on its phase and its position in the sky relative to the Sun. For example, a first-quarter moon is often visible in the afternoon sky.
Q2Keep the curiosity alive
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.
Solution
Answer: No, Amol's birthday will not fall on a full Moon day every year.
Explanation:
This is due to the difference in the lengths of a solar year and a lunar year.
- Solar Year: A birthday, like May 6th, is based on the solar calendar (Gregorian calendar), which is approximately 365.25 days long. This calendar is synchronized with the Earth's revolution around the Sun.
- Lunar Cycle: A full Moon is part of the lunar cycle, which takes about 29.5 days to complete (from one full Moon to the next).
- Difference: Twelve lunar cycles (a lunar year) take about days. This is about 11 days shorter than a solar year.
Because of this 11-day difference, the date of the full Moon shifts backward by about 11 days each year in the solar calendar. Therefore, it is very unlikely that a full Moon will occur on May 6th every year.
Q3Keep the curiosity alive
Name two things that are incorrect in Fig. 11.10.
Solution
The figure described shows a star positioned inside the horns of a crescent Moon, within the unlit portion of the lunar disk.
Here are two things that are incorrect about such a depiction:
-
A Star Cannot Be Seen Through the Moon: The Moon is a solid, opaque celestial body. A star is a distant object that lies far behind the Moon. Therefore, the Moon would block the view of any star behind it. It is physically impossible for a star to appear in front of the unlit part of the Moon.
-
Incorrect Illumination of the Unlit Portion: The part of the Moon that is not directly lit by the Sun is dark. While there is a phenomenon called "earthshine" where the dark part of the Moon is faintly visible due to sunlight reflecting off the Earth, it would not be pitch black with a bright star inside it. The depiction of a star within a perfectly dark area of the moon is incorrect.
Q4Keep the curiosity alive
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 in the pictures above. | Picture label (e.g. A, B, C, etc.) | Phase of Moon | | ---------------------------------- | ---------------------------- | | | Three days after New Moon | | | Full Moon | | | Three days after Full Moon | | | A week after Full Moon | | | Day of New Moon |
(ii)
List the picture labels of the phases of the Moon that are never seen from Earth. Hint: You can use your observations from Activity 11.1 or Fig. 11.2 as reference.
Solution
The panels are described as follows:
A: A fully illuminated Moon.
B: A waning gibbous Moon (most of the left side is lit).
C: A third-quarter Moon (left half is lit).
D: A waning crescent Moon (a sliver on the left is lit).
E: A completely dark disk.
F: A waxing crescent Moon with earthshine (faint illumination of the dark part).
(i) Here is the completed table matching the picture labels to the phases of the Moon:
| Picture label (e.g. A, B, C, etc.) | Phase of Moon |
|---|---|
| F | Three days after New Moon |
| A | Full Moon |
| B | Three days after Full Moon |
| C | A week after Full Moon |
| E | Day of New Moon |
Explanation of the matches:
- Three days after New Moon: This is a young, waxing crescent. Panel F shows this phase, including the phenomenon of earthshine which is common for thin crescents.
- Full Moon: The entire disk is illuminated. This matches panel A.
- Three days after Full Moon: The Moon is in its waning gibbous phase. This matches panel B.
- A week after Full Moon: This is the third-quarter (or waning half) Moon. This matches panel C.
- Day of New Moon: The side of the Moon facing Earth is not illuminated by the Sun. This matches the dark disk in panel E.
(ii) The picture label corresponding to a phase of the Moon that is never seen from Earth is E.
Reason:
Panel E represents the New Moon. On the day of the New Moon, the Moon is located in the same part of the sky as the Sun. Its illuminated side faces away from Earth, and the side facing us is dark. It is lost in the Sun's glare during the day and is not in the sky at night. Therefore, the New Moon is not visible to us from Earth.
Q5Keep the curiosity alive
Malini saw the Moon overhead in the sky at sunset.
(i)
Draw the phase of the Moon that Malini saw.
(ii)
Is the Moon in the waxing or the waning phase?
Solution
(i) Phase of the Moon:
The phase Malini saw is the First Quarter Moon. This is depicted as a circle with its right half illuminated and its left half dark.
(ii) Waxing or Waning Phase:
The Moon is in the waxing phase.
Explanation:
The position of the Moon in the sky relative to the Sun helps determine its phase.
- At sunset, the Sun is on the western horizon.
- For the Moon to be overhead (at its highest point in the sky) at this time, it must be approximately 90 degrees away from the Sun.
- This 90-degree angle corresponds to a half moon. Since it is visible after the New Moon phase and before the Full Moon phase (i.e., seen in the evening sky), it is the First Quarter Moon, which is part of the waxing period.
Q6Keep the curiosity alive
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?
Solution
Answer: Kaushalya is telling the truth.
Explanation:
-
Ravi's statement is incorrect.
- A crescent Moon is always positioned relatively close to the Sun in the sky.
- At sunset, the Sun is in the West. A waxing crescent would be visible in the western sky shortly after sunset, and it would also be setting, not rising.
- A waning crescent rises in the East, but it does so just before sunrise, not at sunset.
- Therefore, it is impossible to see a crescent Moon rising in the East at sunset.
-
Kaushalya's statement is correct.
- A gibbous Moon is more than a quarter of a circle away from the Sun in its orbit.
- A waxing gibbous Moon rises in the East during the afternoon, well before the Sun sets. It is then visible through the evening and sets after midnight.
- Therefore, it is entirely possible to see a gibbous Moon in the eastern sky during the afternoon.
Q7Keep the curiosity alive
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?
Solution
Answer: Luni-solar calendars will need an intercalary month less often.
Explanation:
-
Purpose of Intercalary Month: Luni-solar calendars add an extra month (intercalary month or Adhika Maasa) periodically to reconcile the shorter lunar year (about 354 days) with the longer solar year (about 365 days). This keeps the lunar-based festivals aligned with the seasons.
-
Effect of a Slower Moon: If the Moon's revolution around the Earth becomes slower, the time taken to complete one cycle of phases (a synodic month) will increase. A synodic month is currently about 29.5 days.
-
Impact on Lunar Year: If each lunar month becomes longer, then a year of 12 lunar months will also become longer.
-
Conclusion: As the length of the lunar year increases, the difference between it and the solar year will decrease. Since the calendar falls out of sync with the seasons more slowly, the need to add an intercalary month to correct this drift will arise less frequently.
Q8Keep the curiosity alive
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.
Solution
To Show: At least one month in a 3-year period must contain two full moons.
Proof using the Pigeonhole Principle:
-
Identify the 'Pigeons': The 'pigeons' are the individual events we are distributing. In this case, the pigeons are the 37 full Moons.
- Number of pigeons = 37.
-
Identify the 'Pigeonholes': The 'pigeonholes' are the containers into which the pigeons are placed. In this case, the pigeonholes are the months of the solar calendar over the 3-year period.
- Number of months in 1 year = 12.
- Number of months in 3 years = .
- Number of pigeonholes = 36.
-
Apply the Principle: The Pigeonhole Principle states that if you have more pigeons than pigeonholes, at least one pigeonhole must contain more than one pigeon.
-
Conclusion: Since we have 37 full Moons (pigeons) to place into 36 months (pigeonholes), and , at least one month must contain two full Moons.
Hence shown. (This phenomenon, the second full moon in a single calendar month, is popularly known as a "Blue Moon".)
Q9Keep the curiosity alive
On a particular night, Vaishali saw the Moon in the sky from sunset to sunrise. What phase of the Moon would she have noticed?
Solution
Answer: Vaishali would have noticed the Full Moon.
Explanation:
For the Moon to be visible in the sky for the entire duration of the night (from sunset until sunrise), it must rise in the East at approximately the same time the Sun sets in the West, and it must set in the West at approximately the same time the Sun rises in the East.
This celestial alignment occurs when the Earth is positioned between the Sun and the Moon. In this configuration, the entire face of the Moon visible from Earth is illuminated by the Sun, which corresponds to the Full Moon phase.
Q10Keep the curiosity alive
If we stopped having leap years, in approximately how many years would the Indian Independence day happen in winter?
Solution
Given:
- Indian Independence Day: August 15th (summer/monsoon season).
- Winter season in India is roughly from December to February.
- A calendar year without leap years = 365 days.
- A tropical year (the time for seasons to repeat) is approximately 365.25 days.
To Find: The approximate number of years for August 15th to shift from summer to winter.
Solution:
-
Calculate the annual drift: Without leap years, the calendar year is shorter than the tropical year. The calendar would fall behind the seasons by: days per year.
-
Calculate the required seasonal shift: For a summer date to become a winter date, the calendar needs to drift by approximately half a year. Number of days in half a year days. Let us use 183 days for approximation.
-
Calculate the total time for this shift: We can find the number of years by dividing the total required drift by the annual drift. Number of years = Number of years = Number of years = years.
Final Answer: In approximately 732 years, the Indian Independence Day (August 15th) would occur during the winter season if we stopped having leap years.
Q11Keep the curiosity alive
What is the purpose of launching artificial satellites?
Solution
Artificial satellites are human-made objects launched into orbit around the Earth (or other celestial bodies) to serve various important purposes. According to the chapter, these purposes include:
- Communication: Relaying telephone calls, television broadcasts, and internet data across large distances.
- Navigation: Providing precise location and timing information for GPS (Global Positioning System) used in vehicles, ships, aircraft, and smartphones.
- Weather Monitoring: Observing cloud patterns, storms, and atmospheric conditions to forecast weather and track climate change.
- Disaster Management: Monitoring natural disasters like floods, cyclones, and forest fires to aid in warning and relief efforts.
- Scientific Research: Studying the Earth's environment (geography, land use, pollution) and conducting astronomical observations of stars, galaxies, and other celestial objects from above the Earth's atmosphere.
- Mapping and Urban Planning: Capturing high-resolution images of the Earth's surface for creating accurate maps, planning city development, and managing natural resources, as done by India's Cartosat series of satellites.
Q12Keep the curiosity alive
On which periodic phenomenon are the following measures of time based: (i) day (ii) month (iii) year?
Solution
The basic units of time in our calendars are based on the following natural periodic phenomena:
(i) Day: The concept of a day is based on the rotation of the Earth on its own axis. The solar day, which is approximately 24 hours, is the average time it takes for the Sun to appear in the same position in the sky (for example, from noon one day to noon the next).
(ii) Month: The month is based on the revolution of the Moon around the Earth. This revolution causes the Moon to go through a complete cycle of phases (from one new moon to the next), which takes approximately 29.5 days.
(iii) Year: The year is based on the revolution of the Earth around the Sun. This journey takes approximately 365.25 days and is responsible for the cycle of the seasons.