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NCERT Solutions Curiosity Chapter 11 .1.3 Making sense of our observations — Activity 11.2: Let us explore

Book page 17411 Updated on2026-09-05

Q1.
Does the portion of the ball facing you appear to be illuminated or not?
Answer

At position E — the ball held out towards the lamp — the side facing you is not illuminated. You see a dark ball.

Why it happens: The lamp lights the far half of the ball, the half pointing away from your eye. The half turned towards you lies in the ball's own shadow. Your head is the Earth, the ball is the Moon and the lamp is the Sun, so this arrangement — Moon between Earth and Sun — is exactly new Moon day (Amavasya).
Q2.
Does the shape of the illuminated portion change?
Answer

Yes, and it changes smoothly and in a fixed order as you turn anticlockwise:

E (no bright part) → F thin crescent → G half → H gibbous → A whole bright disc → B gibbous → C half → D crescent → back to E
Why it happens: The lamp never changes what it lights — exactly half the ball, all the time. What changes is your line of sight. Turning your body swings your eye round to a new angle, so a different fraction of the lit half is turned towards you. That is the whole mechanism of the Moon's phases, reproduced in a dark room.
Tip: Hold the ball a little above your head. If you hold it exactly level, your own head gets in the way at position A — which is, in miniature, why we do not get a lunar eclipse every month.
Q3.
Is the line separating the illuminated and non-illuminated portions of the ball curved?
Answer

Yes — at every position except the two half-lit ones (C and G), where it looks perfectly straight.

Why it happens: On the ball itself the boundary between light and dark is a circle running right round the ball. When you look at a circle from an angle, you do not see a circle — you see a flattened circle, that is, an ellipse, and half of that ellipse is a curved line. Only when your eye lies in the plane of that circle, at the half-lit positions, does the ellipse squash down to a straight line across the middle.

This is why crescent and gibbous Moons have a curved edge on the inside while a half Moon has a straight one. It is a clue you can use: a straight inner edge means the Moon is exactly 90° from the Sun in the sky.

Q4.
Was your observation similar to the changing shape of the illuminated portion of ball shown in Fig. 11.4c?
Answer

Yes. The sequence you see matches Fig. 11.4c position by position, and each position matches a real phase of the Moon in Fig. 11.5:

Position of the ballWhere it is with respect to the lampWhat you seeMatching phase of the Moon
AOpposite the lampWhole disc brightFull Moon (Purnima), Day 1
B and HSlightly to one side of AMore than half brightGibbous phase
C and GAt right angles to the lampExactly half bright, straight edgeHalf Moon
D and FSlightly to one side of ELess than half brightCrescent phase
ETowards the lampNo bright part at allNew Moon (Amavasya), Day 15
Why it happens: The activity reproduces the real geometry faithfully: one fixed light source, a sphere lit on one half only, and a moving observer. Because the answer depends only on the Sun–Earth–Moon angle, a ball, a torch and a dark courtyard are enough to derive the phases of the Moon.
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