NCERT Solutions Curiosity Chapter 11 .1.3 Making sense of our observations — In-text Questions
Book page 17411 Updated on2026-09-05
Q1.
Why does the illuminated portion of the Moon seen from the Earth decrease when it appears closer to the Sun?
Answer
Because the fraction we can see depends entirely on the angle from which we view the lit half, and that angle is the Moon–Sun separation in the sky.
Sunlight always lights exactly one half of the Moon — the half turned towards the Sun (Fig. 11.3). That never changes. What changes is where the Earth is standing when it looks at that lit half.
When the Moon is far from the Sun in the sky (full Moon, about 180°), we are looking at the Moon from nearly the same direction the sunlight is coming from, so the lit half is turned almost squarely towards us. We see all of it.
As the Moon comes closer to the Sun in the sky, we begin to see the lit half more and more edge-on. Less and less of it is turned our way, so the bright shape narrows: gibbous, then half, then a thin crescent.
At new Moon (about 0°) we are looking at the Moon from almost exactly the opposite side to the sunlight. The whole lit half faces away and only the dark half faces us.
The Sun lights the same half of the Moon at every position. As the Moon moves towards the Sun's direction in the sky, the Earth sees that lit half more and more edge-on, so the bright shape shrinks from full to crescent to nothing.
Careful: This has nothing to do with Earth's shadow. Earth's shadow falling on the Moon is a lunar eclipse, which can happen only on a full Moon day and does not happen every month, because the Moon's orbit is slightly tilted to the Earth's orbit round the Sun.