11.1 Phases of the Moon

Probe and Ponder

At the Makar Sankranti kite festival, Meera looked up — and saw the Moon shining in broad daylight! She’d always thought the Moon came out only at night. And it wasn’t a full circle, but a partial shape. She knew the Moon is a sphere that shines by reflecting sunlight, so why isn’t the whole Moon visible every night? (Not a lunar eclipse — those are rare and brief.) What makes the Moon’s shape change?

Activity 11.1 — Moon-watching for a month Starting the day after a full Moon, spot the Moon each day (at sunrise for the first two weeks, sunset for the next two) and shade in how much of it is bright. Note whether the bright part is growing or shrinking, and whether the Moon is moving closer to or farther from the Sun in the sky. Over a month, a clear pattern appears.

The cycle of shapes

Waxing and Waning The bright portion of the Moon shrinks from a full circle to nothing over about two weeks — the waning period — and then grows back to full over the next two weeks — the waxing period.

Full Moon (a full bright circle): called Purnima.
New Moon (not visible at all): called Amavasya.
Between them: a crescent (less than half lit) and a gibbous (more than half lit).

In India, the waning fortnight is Krishna Paksha, the waxing fortnight Shukla Paksha. The full cycle from one full Moon to the next takes about a month.

What “Phases” Means The phases of the Moon are the changing shapes of the bright portion of the Moon as seen from Earth, from day to day. (Remember: the Moon itself is always a full sphere — only the bright part we can see changes.)

Where to Look, and When The phase tells you where to find the Moon. At full Moon it is nearly opposite the Sun (rising in the East as the Sun sets in the West). A waxing Moon is easiest to spot at sunset, a waning Moon at sunrise. And the Moon rises about 50 minutes later each day — so sometimes it rises in the afternoon, which is exactly how Meera could see it during daylight!