Phases of the Moon: why the Moon seems to change shape

Watching the Moon for a month

The chapter asks you to keep a Moon diary (Activity 11.1). Start at sunrise on the day after a full Moon, when the Moon is easy to find in the western sky. Each day, note the date and time, shade a circle to show the bright part, and record whether the bright part grew or shrank and whether the Moon looked closer to the Sun or farther from it. After about 15 days the Moon can no longer be seen at sunrise, so watch at sunset for the next 15 days. The diary shows that the Moon looks different each day, is not visible on every day, and is not in the same place in the sky at the same time each day.

Waxing and waning

From the full Moon, the bright part shrinks to half within about a week and vanishes after another week: this two-week waning period is called Krishna Paksha. The day the Moon cannot be seen is the new Moon (Amavasya). Then the bright part grows to a half circle in a week and to a full circle in another week: the waxing period, or Shukla Paksha, ending on the full Moon (Purnima). The whole cycle, full Moon to full Moon, lasts about 29.5 days. A shape showing less than half of the lit side is a crescent; more than half is gibbous.

Why the Moon has phases

The Moon itself never changes shape. It has no light of its own and shines by reflecting sunlight, so the half facing the Sun is always lit and the other half is dark. The Moon goes round the Earth, and the same half of the Moon always faces us. But the half facing us is not always the lit half. Sometimes all of the lit half faces us, sometimes only part of it, and on new Moon day only the dark half faces us. What we call a phase is simply how much of the lit half we can see.

A model with a ball and a lamp

Activity 11.2 makes this clear. Push a stick into a small ball (the Moon) and stand in a dark place with a lamp or torch about 3 m away (the Sun); your head is the Earth. Hold the ball at arm’s length, a little above your head. Facing the lamp (position E), you see only the ball’s dark side, like the new Moon. Turn slowly anticlockwise and a thin lit edge appears, growing to a half and then more than half. With your back to the lamp (A), you see the whole lit half, like the full Moon. The line between the lit and dark parts looks curved, just like on the real Moon.

Following the Moon round its orbit

The real Moon goes round the Earth from position A back to A in about a month. At A we see all of its lit half (full Moon). At B and H we see more than half of it (gibbous), at C and G exactly half, at D and F less than half (crescent), and at E none (new Moon). From A to E it wanes, and from E back to A it waxes. Since the Earth turns once a day while the Moon takes a month to go round, people everywhere on Earth see nearly the same phase on any given day.

Where and when to look

At full Moon, the Moon sits almost opposite the Sun: while the Sun comes up in the east, the Moon goes down in the west. On the following mornings the shrinking Moon appears closer to the Sun; when it is a half circle it is overhead at sunrise, and a few days later the crescent is closer still. Look for a waxing Moon around sunset and a waning one around sunrise. Wherever it is, the bright side faces the Sun.

The Moon does not always rise when the Sun sets. While the Earth turns once, the Moon travels on along its orbit, so the Earth must turn for about 50 minutes more before the Moon is back in the same place. Moonrise times, listed in newspapers or by the Positional Astronomy Centre of the India Meteorological Department, move about 50 minutes later each day: moonrise was at 14:23 on 7 April 2025 and at 15:17 the next day. That is why the Moon is often seen in the afternoon sky.

Not the Earth’s shadow

A common mistake is to think the Earth’s shadow makes the phases. When the Earth’s shadow falls on the Moon, we get a lunar eclipse, which can happen only at full Moon; a solar eclipse can happen only at new Moon. Most months have no eclipse, because the Moon’s orbit is slightly tilted to the Earth’s orbit round the Sun.

Read on: lunar, solar and luni-solar calendars, artificial satellites, the chapter notes and the NCERT solutions.