CLASS 8 · SCIENCE · CHAPTER 11

Keeping Time with the Skies

Simple answers to the opening prompts, chapter observations and activities, all twelve end questions, reflection prompts, and all three projects.

33 worked answersComplete chapter answers

Read the NCERT questions ↗← All chapters

33 worked answers · short steps first, explanations when you need them

Probe and Ponder

Textbook pages 170

Question Opening prompt 1

Why can the Moon sometimes be seen during the day?

Steps only

The Moon reflects sunlight and is sometimes above the horizon during the day.

Answer

The Moon reflects sunlight and is sometimes above the horizon during the day.

Question Opening prompt 2

What would a day, month, and year mean if you lived on the Moon?

Steps only

They would be measured using repeating cycles of rotation and orbit.

Answer

They would be measured using repeating cycles of rotation and orbit.

Question Opening prompt 3

What might change in the night sky if Earth had two moons?

Steps only

We would see two changing moon paths; tides could also be affected.

Answer

We would see two changing moon paths; tides could also be affected.

Question Opening prompt 4

How could people measure time without clocks or calendars?

Steps only

Use cycles of daylight, Moon phases, shadows, or seasons.

Answer

Use cycles of daylight, Moon phases, shadows, or seasons.

Question Opening prompt 5

Write one question about the Moon, calendars, or satellites.

Steps only

Example: Why does moonrise get later each day?

Answer

Example: Why does moonrise get later each day?

Chapter Investigations

Textbook pages 171–186

Question Activity 11.1

What should be recorded during a month of Moon watching?

Steps only

Date, time, shape, waxing or waning, and apparent distance from the Sun.

Answer

Date, time, shape, waxing or waning, and apparent distance from the Sun.

Question Activity 11.2

What does the ball-and-lamp activity show about Moon phases?

Steps only

Phases happen because we see different portions of the Moon’s sunlit half.

Answer

Phases happen because we see different portions of the Moon’s sunlit half.

Question Activity 11.3

How can a stick’s shadow help estimate the length of a solar day?

Steps only

The shortest shadow marks solar noon; successive solar noons are about 24 hours apart.

Answer

The shortest shadow marks solar noon; successive solar noons are about 24 hours apart.

Question Activity 11.4

How can an artificial satellite be spotted in the sky?

Steps only

Look for a steady moving point of light; use an adult and a tracking app.

Answer

Look for a steady moving point of light; use an adult and a tracking app.

Question Moonrise concept

Why does the Moon rise about 50 minutes later each day?

Steps only

The Moon moves in its orbit, so Earth needs about 50 more minutes to line up again.

Answer

The Moon moves in its orbit, so Earth needs about 50 more minutes to line up again.

Question Calendar concept

Why do lunar and solar calendars sometimes need an adjustment?

Steps only

A lunar year is shorter, so a luni-solar calendar sometimes adds a month.

Answer

A lunar year is shorter, so a luni-solar calendar sometimes adds a month.

Question Indian National Calendar

How is the Indian National Calendar arranged?

Steps only

It is a solar calendar; Chaitra begins March 22, or March 21 in leap years.

Answer

It is a solar calendar; Chaitra begins March 22, or March 21 in leap years.

Question Festival concept

Why do many festival dates shift in the Gregorian calendar from year to year?

Steps only

Moon-based dates shift against the solar Gregorian calendar.

Answer

Moon-based dates shift against the solar Gregorian calendar.

Question Satellite uses

What are artificial satellites used for?

Steps only

Communication, navigation, weather, mapping, disaster support, and research.

Answer

Communication, navigation, weather, mapping, disaster support, and research.

Question Lunar month

How long does one cycle of Moon phases take?

Steps only

About 29.5 days.

Answer

About 29.5 days.

Keep the Curiosity Alive

Textbook pages 187–189

Question 1

Mark each statement true or false: we see reflected sunlight from the Moon; Earth’s shadow causes phases; calendars use predictable cycles; the Moon is visible only at night.

Steps only

(i) True; (ii) False; (iii) True; (iv) False.

Answer

(i) True;
(ii) False;
(iii) True;
(iv) False.

Question 2

If Amol was born on May 6 during a full Moon, will May 6 be a full Moon every year?

Steps only

No. Lunar phases shift about 11 days against the Gregorian date each year.

Answer

No. Lunar phases shift about 11 days against the Gregorian date each year.

Question 3

Name two things to check for errors in a diagram of Moon phases.

Steps only

The Moon’s shape does not change, and Earth’s shadow does not cause its phases.

Answer

The Moon’s shape does not change, and Earth’s shadow does not cause its phases.

Question 4

How can the six Moon pictures be matched to the named dates?

Steps only

Three days after New Moon: D; Full Moon: E; three days after Full Moon: A; a week after Full Moon: C; New Moon: B.

Answer

D; E; A; C; B, in the same order as the table.

Question 4(ii)

Which picture label shows the phase that is not visible from Earth?

Steps only

Picture B is the New Moon; its Earth-facing side is dark.

Answer

Picture B (New Moon) is usually not visible from Earth.

Question 5

What phase is overhead at sunset, and is it waxing or waning?

Steps only

First quarter; waxing.

Answer

First quarter; waxing.

Question 6

Who could be correct: a crescent Moon rising at sunset in the east, or a gibbous Moon seen in the east in the afternoon?

Steps only

The gibbous Moon in the afternoon is possible; a crescent at sunset would be in the west, not rising in the east.

Answer

The gibbous Moon in the afternoon is possible; a crescent at sunset would be in the west, not rising in the east.

Question 7

If the Moon’s revolution becomes slower, will a luni-solar calendar need an extra month more often or less often?

Steps only

Less often, because longer lunar months reduce the yearly difference.

Answer

Less often, because longer lunar months reduce the yearly difference.

Question 8

There are 37 full Moons in three solar years. Show that at least two happen in the same named month.

Steps only

12 × 3 = 36; with 37 full Moons, at least one month has four.

Answer

12 × 3 = 36; with 37 full Moons, at least one month has four.

Question 9

What phase is the Moon if it is visible from sunset to sunrise?

Steps only

Full Moon.

Answer

Full Moon.

Question 10

If leap years stopped, about how long until India’s Independence Day falls in winter?

Steps only

About 500 years: roughly 125 days × 4 years per day of drift.

Answer

About 500 years, using roughly one day of seasonal drift every four years.

Question 11

Why do we launch artificial satellites?

Steps only

They provide communication, navigation, weather, mapping, and research services.

Answer

They provide communication, navigation, weather, mapping, and research services.

Question 12

What repeating event is used to define a day, month, and year?

Steps only

Day: Earth’s rotation; month: Moon phases; year: Earth’s revolution around the Sun.

Answer

Day: Earth’s rotation; month: Moon phases; year: Earth’s revolution around the Sun.

Question Question-making activity

Write questions using why, when, how long, where, and why not about this chapter.

Steps only

Answers vary; use clear questions about the chapter ideas.

Answer

Answers vary; use clear questions about the chapter ideas.

Question Reflection activity

How can you answer a classmate’s question about the Moon or timekeeping?

Steps only

Answers vary; support them with a simple reason or observation.

Answer

Answers vary; support them with a simple reason or observation.

Discover, Design, and Debate

Textbook pages 188–189

Question Project 1

Research New Year celebrations in ten Indian states and identify the calendar each follows.

Steps only

Make a table and verify each festival’s date and calendar locally.

Answer

Make a table and verify each festival’s date and calendar locally.

Question Project 2

How can you compare the Gregorian dates of Eid-ul-Fitr and Diwali over five years?

Steps only

Eid shifts earlier in the solar year; Diwali stays near the same season.

Answer

Eid shifts earlier in the solar year; Diwali stays near the same season.

Question Project 3

How can you observe the Sun’s sunrise position over a year?

Steps only

Record the sunrise position monthly from one safe, fixed viewpoint.

Answer

Record the sunrise position monthly from one safe, fixed viewpoint.