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Physics The Earth and space
Class 7 Beginner About 10 min Premium

Phases of the Moon: from Aunsi to Purnima

Move the Moon round the Earth, day by day through its month, and watch how much of it we see lit from Nepal, from Aunsi (new moon) to Purnima (full moon) and back.

Core concept

The Sun always lights half of the Moon; as the Moon goes round the Earth in about 29.5 days, we see more or less of that lit half, which gives the phases

Topic: The Earth and space

The Moon makes no light of its own: it shines because sunlight falls on it, and the Sun always lights exactly half of it. The Moon goes round the Earth once in about 29.5 days. When it is between the Earth and the Sun, its lit half faces away from us and we cannot see it: this is the new moon, Aunsi. As the Moon moves on, we see more and more of its lit half (waxing), until it is on the far side of the Earth from the Sun and we see all of it: the full moon, Purnima. Then we see less and less (waning) until the next Aunsi. The Nepali calendar's festivals follow these phases: Dashain's Tika and Tihar's Laxmi Puja fall on set days of the Moon's month.

Key words explained

What you will learn

  • Explain that the Moon shines by reflecting sunlight, and that the Sun always lights half of it.
  • Explain the phases of the Moon from its position round the Earth.
  • Name and put in order the new moon, crescent, half moon, gibbous and full moon.
  • Explain the difference between waxing and waning.

Before you start

  • Know that the Moon goes round the Earth.

Materials

  • A ball on a stick (the Moon) and a bright lamp (the Sun) in a dark room

Equipment

  • A calendar that shows Aunsi and Purnima, to check the Moon in the real sky

Procedure

  1. Put the Moon at day 0 (Aunsi) and record.
  2. Move it to day 15 (Purnima) and record.
  3. Record the two half moons, at day 7 and day 22.
  4. Record at least six different days of the Moon's month.

How this simulation is simplified

  • The Moon goes round the Earth in 29.5 days, starting at the new moon on day 0.
  • The lit part is the fraction of the Moon's disc we see lit, as a percentage.
  • The Moon is seen from Nepal, in the northern half of the Earth: a waxing Moon is lit on the right.
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