NCERT Solutions for Class 8 Science Chapter 11 – Keeping Time with the Skies

NCERT Solutions for Class 8 Science Chapter 11 – Keeping Time with the Skies help students understand how celestial bodies such as the Sun, Moon, and stars have been used to measure time since ancient times. This chapter connects astronomy with daily life, explaining how natural cycles form the basis of calendars and timekeeping.

The chapter introduces students to the movement of celestial objects, phases of the Moon, solar and lunar cycles, and the development of calendars. It builds curiosity about space while strengthening scientific observation and reasoning skills.

Each answer is explained clearly to ensure conceptual clarity and effective exam preparation.

1.0NCERT Solutions for Class 8 Science Chapter 11 PDF Download

Students can download the NCERT Solutions for Class 8 Science Chapter 11 PDF for convenient learning and quick revision. The solutions are prepared by ALLEN’s experienced faculty members, ensuring accuracy, clarity, and alignment with NCERT guidelines.

2.0NCERT Questions with Solutions Class 8 Science Chapter 11 – Detailed Explanation

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.
(ii) The shadow of Earth blocks sunlight from reaching the Moon, causing phases.
(iii) Calendars are based on various astronomical cycles which repeat predictably.
(iv) The Moon can only be seen at night.

Solution

(i) True: We can only see the part of the Moon that reflects sunlight towards Earth.

(ii) False: The Earth’s shadow causes lunar eclipses, not the regular phases of the Moon.

(iii) True: Calendars are based on repeating astronomical events like day-night, Moon phases, and seasons.

(iv) False: The Moon can also be seen during the daytime, depending on its phase and position in the sky.

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.

Solution No, Amol’s birthday does not fall on a full Moon day every year. This is because the Moon’s phases follow a lunar cycle of about 29.5 days, while the calendar year follows the solar cycle of about 365 days. So, the date of the full Moon changes each year in the Gregorian calendar.

3. Name two things that are incorrect in the Figure.

Solution: Two incorrect things are:

• Stars are shown near the Moon during the daytime, which is incorrect because stars are not visible in the daytime sky.

• The Moon’s dark part is shaded incorrectly to show a phase. The shadow in the figure suggests it’s caused by Earth’s shadow, which is not true for regular Moon phases; they are caused by the Moon’s position relative to the Earth and Sun, not a shadow.

4. Look at the pictures of the Moon in the figure and answer the following questions.

(i) Write the correct panel number corresponding to the phases of the Moon shown in the pictures above.

(ii) List the picture labels of the phases of the Moon that are never seen from Earth.

Solution

(i) Picture B – On a new Moon day, only the non-illuminated side of the Moon faces the Earth, so it is not visible from Earth.

Picture F – This is not a phase of the Moon. So, it can never be seen from Earth.

5. 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) At sunset, the Moon is overhead only during the first quarter (a week after New Moon), when the right half is illuminated. So, we need to draw a half Moon (right half bright, left half dark).

(ii) The Moon is in the waxing phase because at sunset a half Moon (first quarter) is seen overhead, and this happens only when the bright part of the Moon is increasing.

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?

Solution Kaushalya is telling the truth because gibbous Moons can be seen in the East during the afternoon.

Ravi’s statement is incorrect because crescent Moons do not rise in the East at sunset. A crescent Moon appears just after the New Moon (waxing crescent) or just before the New Moon (waning crescent). Waxing crescent is visible after sunset in the western sky, not rising in the east, and waning crescent rises just before sunrise, not at sunset.

7. 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: Luni-solar calendars will need an intercalary month more often as the Moon moves farther and slower, and it takes longer to complete a cycle. So, a lunar year becomes even shorter compared to the solar year.

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.

Solution A solar calendar has 12 months × 3 years = 36 months. 

There are 37 full Moons in 36 months.

Therefore, at least one month must have two full Moons.

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?

Solution As the Moon is visible all night long only on a Full Moon, it is a Full Moon.

10. If we stopped having leap years, in approximately how many years would the Indian Independence Day happen in winter?

Solution

One leap year adds about 1 day every 4 years.
Without leap years, the calendar shifts by 1 day every 4 years.
There are roughly 183 days between 15 August and winter (mid-February).

183 × 4 = 732 years

So, in approximately 730–732 years, 15 August would occur in winter.

11. What is the purpose of launching artificial satellites?

Solution

Artificial satellites are launched for:
• Communication
• Navigation
• Weather monitoring
• Scientific research
• Disaster management
• Earth observation

12. On which periodic phenomenon are the following measures of time based?

(i) Day
(ii) Month
(iii) Year

Solution

(i) Day → Earth’s rotation
(ii) Month → Moon’s revolution (phases of the Moon)
(iii) Year → Earth’s revolution around the Sun

3.0Key Topics Covered in Class 8 Science Chapter 11 – Keeping Time with the Skies

Key Topics Covered

• The Moon’s Appearance: We only see the part of the Moon that reflects sunlight toward Earth. The Moon is visible not just at night, but also during the daytime depending on its phase and position.

Phases of the Moon: These are caused by the Moon's position relative to the Earth and Sun, not by Earth's shadow.

• Waxing Period (Shukla Paksha): The bright part of the Moon is increasing.
• Waning Period (Krishna Paksha): The bright part of the Moon is decreasing.
• Astronomical Timekeeping: Calendars are based on predictable repeating cycles:
• Day: Based on Earth's rotation.
• Month: Based on the Moon's revolution and its phases.
• Year: Based on Earth's revolution around the Sun.
• Artificial Satellites: Man-made objects launched into orbit for purposes like communication, weather monitoring, and navigation.

4.0Benefits of Studying Class 8 Science Chapter 11 – Keeping Time with the Skies

• Understanding the Universe: Helps students differentiate between the actual mechanics of the solar system and common visual misconceptions (like stars appearing during the day).
• Cultural and Practical Literacy: Explains the logic behind different types of calendars, such as the Gregorian and the luni-solar systems.
• Modern Technological Awareness: Provides a foundation for how artificial satellites facilitate daily activities like GPS navigation and cellular communication.
• Analytical Skills: Encourages scientific reasoning through puzzles, such as calculating long-term calendar shifts or identifying incorrect astronomical illustrations.