EN हिं

NCERT Solutions Curiosity Chapter 11 .1 How Does the Moon’s Appearance Change and Why? — Activity 11.1: Let us explore

Book page 17211 Updated on2026-09-05

Q1.
Analyse the data recorded by you in Table 11.1. Did the Moon appear different each day?
Answer

Yes. The shape of the bright part is a little different every single day, and the change goes in one direction for a whole fortnight before it reverses.

Starting at sunrise the day after full Moon, this is what your Table 11.1 should show:

Roughly which dayAppearance of the MoonBright portion compared to the previous dayMoon–Sun separation compared to the previous day
Day 1Full bright circle——
Days 2–7Gibbous, shrinkingDecreasedCloser
About Day 8Half circleDecreasedCloser
Days 9–14Crescent, getting thinnerDecreasedCloser
About Day 15Not visible (new Moon)Decreased to nothingClosest to the Sun
Days 16–29Crescent → half → gibbous → full againIncreasedFarther
Why it happens: The first fortnight is the waning period (Krishna Paksha) and the second is the waxing period (Shukla Paksha). Since the whole cycle takes about 29.5 days, the visible shape changes by only about one-fifteenth of the disc from one day to the next — small, but quite easy to see if you sketch it.
Q2.
Was the Moon visible on all days?
Answer

No. For a day or two around new Moon (Amavasya) you will not find the Moon at all.

Why it happens: Two things go wrong for the observer at once on that day. First, only the non-illuminated half of the Moon is turned towards the Earth, so there is nothing bright to see. Second, the Moon is then in almost the same direction as the Sun, so it rises and sets with the Sun and is drowned in daylight.

This is also why the activity asks you to change your observing time midway. During the first fortnight the Moon is waning, so it is easiest to catch at sunrise; during the second fortnight it is waxing and is easiest to catch at sunset. In between, near new Moon, it is not available at either time.

Q3.
Did the Moon appear at the same position in the sky as on the previous day?
Answer

No. At the same clock time each day the Moon has clearly shifted — it appears further east, and steadily closer to the Sun through the waning fortnight.

The Moon completes 360° round the Earth in about 29.5 days
Daily eastward shift = 360° ÷ 29.5 ≈ 12.2° per day
The Earth turns 360° in 24 hours = 15° per hour
Extra turning needed = 12.2° ÷ 15° per hour ≈ 0.81 h = about 49 minutes
Why it happens: While the Earth is completing one rotation, the Moon has not stayed put — it has run ahead in its own orbit. The Earth must turn a little further before the Moon comes back over the same spot, and "a little further" works out to about 50 minutes (Fig. 11.6).

Over the fortnight the pattern at sunrise is very regular: on full Moon day the Moon is setting in the west; about a week later, when it is a half circle, it is overhead; a few days after that the thin crescent is low in the east, close to the Sun.

Was this helpful? Report an error