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Keeping Time with the Skies: questions with answers

Class 8 Science, Chapter 11 (NCERT). 15 questions by concept. Open “Show answer” for the answer and why.

How to teach this chapter · Common misconceptions & worked visual for this chapter

Locating the Moon in the sky

Question 1

Someone knows only that tonight's Moon is a thin crescent. Does that narrow down where in the sky to look for it?

  1. (a)Yes, because a crescent always lies close to the Sun in the sky.
  2. (b)No, because the Moon can be anywhere at any phase.
  3. (c)Yes, because a crescent stays in the sky all night, as a full Moon does.
  4. (d)No, because the phase says nothing about where the Moon will be.
Show answer

(a) Yes, because a crescent always lies close to the Sun in the sky.

A thin crescent lies near the Sun’s direction in the sky. A waxing crescent is sought after sunset; a waning crescent before sunrise. Phase therefore helps narrow the search.

The Moon can be seen during the day

Question 2

Moonrise can fall between about 2:00 and 4:00 in the afternoon. How often, then, is the Moon in the sky during daylight?

  1. (a)Very rarely, and only by chance.
  2. (b)Only during an eclipse.
  3. (c)Never, because the Moon is only above the horizon while the Sun is down.
  4. (d)On many days of every month.
Show answer

(d) On many days of every month.

The Moon can be above the horizon during daylight on many days of a month. Its orbital motion changes its position relative to the Sun, so daylight sightings are normal.

The Moon rises about fifty minutes later each day

Question 3

Does the Moon rise near sunset every day?

  1. (a)Yes, the Moon rises at sunset every night.
  2. (b)No. A full Moon rises roughly near sunset, but moonrise occurs at other times during the month.
  3. (c)Yes, apart from the nights when it does not rise at all.
  4. (d)No, and the shift from night to night comes from the Earth going round the Sun.
Show answer

(b) No. A full Moon rises roughly near sunset, but moonrise occurs at other times during the month.

A full Moon is roughly opposite the Sun in the sky, so it rises near sunset. Other phases have different moonrise times. Exact times depend on location and date.

The Sun's rising point shifts through the year

Question 4

At a place in India with a clear, level horizon, does the Sun rise at the same horizon point throughout the year?

  1. (a)No, and its northward movement through the year is called Dakshinayan.
  2. (b)No. The sunrise point shifts north and south, passing approximately due east near the equinoxes.
  3. (c)No, and it rises in a different direction each day at random.
  4. (d)Yes, it rises exactly in the east every day.
Show answer

(b) No. The sunrise point shifts north and south, passing approximately due east near the equinoxes.

The sunrise point changes gradually through the year. For a clear, level horizon, sunrise is approximately due east near the equinoxes and north or south of east at other times.

The day as a unit of time

Question 5

Which repeating interval is averaged to define the 24-hour mean solar day?

  1. (a)Two successive sunrises.
  2. (b)Two successive positions of the Sun in its journey round the Earth.
  3. (c)Two successive moonrises.
  4. (d)Two successive moments when the Sun is at its highest position in the sky.
Show answer

(d) Two successive moments when the Sun is at its highest position in the sky.

A solar day runs from one local solar noon to the next. These intervals vary slightly through the year. Their average defines the 24-hour mean solar day.

The month and the year as units of time

Question 6

How long does the Earth take to complete one journey round the Sun?

  1. (a)Exactly 30 days times twelve, matching the phase cycle.
  2. (b)Exactly 365 days.
  3. (c)Nearly 365 and a quarter days.
  4. (d)Exactly 360 days.
Show answer

(c) Nearly 365 and a quarter days.

A year is nearly 365 and a quarter days, rather than exactly 365 days. Gregorian leap days compensate for this difference, with century exceptions to the four-year rule.

The phases are not caused by the Earth's shadow

Question 7

At which phase of the Moon can a lunar eclipse take place?

  1. (a)At any phase at all.
  2. (b)At full Moon.
  3. (c)At new Moon.
  4. (d)At every full Moon, because a lunar eclipse occurs every month.
Show answer

(b) At full Moon.

A lunar eclipse occurs when the Moon passes through Earth’s shadow. The required Sun–Earth–Moon arrangement occurs at full Moon, but most full Moons miss the shadow.

The phases of the Moon

Question 8

About how long is the interval from one new Moon to the next?

  1. (a)About 29.5 days.
  2. (b)About a year.
  3. (c)About a week.
  4. (d)Exactly 30 days, because every lunar phase cycle has that fixed length.
Show answer

(a) About 29.5 days.

The Moon completes a phase cycle in about 29.5 days. About a week separates successive quarter stages; that is only part of the whole cycle.

The Moon shines by reflecting sunlight

Question 9

A star lies directly behind the unlit part of a crescent Moon. Can it be seen from the Earth?

  1. (a)Yes, because the unlit part is see-through.
  2. (b)Yes, because the Moon is crescent-shaped and the star lies outside it.
  3. (c)No, because stars stop emitting light whenever the Moon passes in front of them.
  4. (d)No, because the Moon is solid and blocks it, lit or not.
Show answer

(d) No, because the Moon is solid and blocks it, lit or not.

The dark portion belongs to the same opaque Moon as the bright crescent. It blocks light from a star directly behind it; darkness does not make it transparent.

Why the phases happen

Question 10

At new Moon, approximately how much of the Moon’s surface receives direct sunlight?

  1. (a)None of it, which is why nothing can be seen.
  2. (b)Half of it, but the Moon has spun so that its dark side faces us.
  3. (c)All of it, but the Earth's shadow hides it.
  4. (d)Half, mainly on the hemisphere facing away from Earth.
Show answer

(d) Half, mainly on the hemisphere facing away from Earth.

At new Moon, the Sun illuminates approximately half the lunar surface. That illuminated hemisphere faces mainly away from Earth. The phase concerns our view, not sunlight switching off.

Festivals and the calendars they follow

Question 11

At which Moon phases do Diwali and Holi fall, and how do the yearly date shifts of Indian festivals compare?

  1. (a)New Moon and full Moon; identical shifts
  2. (b)Full Moon for both; identical shifts
  3. (c)Full Moon for both; different shifts
  4. (d)New Moon and full Moon; different shifts
Show answer

(d) New Moon and full Moon; different shifts

Diwali is associated with new Moon and Holi with full Moon. Indian festivals follow different calendar rules, so their Gregorian dates do not all shift alike.

Solar calendars and leap years

Question 12

A calendar counts every year as exactly 365 days. Will it stay in step with the seasons?

  1. (a)Yes, because abandoning leap days would not affect its alignment with the seasons.
  2. (b)Yes, because 365 days is exactly the length of the Earth's journey.
  3. (c)No, because the solar year is 366 days long and needs an extra day every year.
  4. (d)No, because the Earth's journey takes about a quarter of a day longer than that.
Show answer

(d) No, because the Earth's journey takes about a quarter of a day longer than that.

The Earth takes nearly 365 and a quarter days to go round the Sun, so a 365-day calendar loses about a quarter of a day each year. Left uncorrected that error builds up, and the calendar slips against the seasons.

Lunar calendars

Question 13

A festival follows a purely lunar calendar with twelve lunar months and no intercalary month. Will it stay in the same season year after year?

  1. (a)No, it moves through the seasons, shifting about 11 days earlier each year.
  2. (b)Yes, because a lunar calendar keeps in step with the seasons.
  3. (c)No, but it returns to its season after two years.
  4. (d)Yes, because a lunar year and a solar year are the same length.
Show answer

(a) No, it moves through the seasons, shifting about 11 days earlier each year.

The seasons follow the solar year, and a lunar year is about 11 days shorter. So a lunar month slips earlier against the seasons every year, and over about 33 years it works its way right round them.

Luni-solar calendars and the intercalary month

Question 14

Does a luni-solar calendar lose step with the seasons the way a purely lunar one does?

  1. (a)Yes, because it is built on lunar months.
  2. (b)Yes, but only by a day or two each century.
  3. (c)No, because the extra month brings it back into step every few years.
  4. (d)No, because it adds an extra day rather than a month.
Show answer

(c) No, because the extra month brings it back into step every few years.

Twelve lunar months are shorter than a solar year. A luni-solar calendar adds an extra month periodically. This prevents the steady seasonal drift of a purely lunar calendar.

The Indian National Calendar

Question 15

How long are the months of the Indian National Calendar?

  1. (a)28 to 31 days
  2. (b)30 or 31 days
  3. (c)30 days in every month
  4. (d)29 or 30 days
Show answer

(b) 30 or 31 days

The Indian National Calendar is solar. Its months have 30 or 31 days; they do not copy Gregorian month lengths or follow Moon phases.

Common misconception in Keeping Time with the Skies

Open full misconception guide →

Does the Moon itself change shape?

Wrong

The Moon physically gets thinner during a crescent phase.

Why it feels right. Only a sliver is visible, so the object looks smaller.

Right

We see a changing portion of the Moon’s sunlit half.

Worked example

Both discs show the same round Moon. A different fraction of the lit half is visible from Earth.
  1. Move a ball around your eye position with a lamp fixed.
  2. The ball stays round as different amounts of its lit half face you.