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Why does the Moon show different phases?

Class 7 ScienceNCERT chapter: Earth, Moon, and the Sunइसे हिंदी में पढ़ें

Exam-ready answer

The Moon has no light of its own; it shines by reflecting sunlight. The Sun always lights up one half of the Moon. As the Moon revolves around the Earth, we see different portions of this lit half from the Earth. These changing shapes, from new moon to full moon and back, are called the phases of the Moon.

Explanation

The Moon is not a source of light. Sunlight falls on it and is reflected towards us. At any time, exactly half of the Moon faces the Sun and is bright, while the other half is dark. The Moon itself never changes shape; only the part of the bright half that faces the Earth changes.

The Moon revolves around the Earth. When it is between the Earth and the Sun, its dark side faces us and we cannot see it. This is the new moon (Amavasya). As it moves along its path, a thin crescent appears, then a half moon, then a gibbous moon. When the Earth is between the Sun and the Moon, we see the entire lit half. This is the full moon (Purnima).

The days when the visible bright part grows are called the waxing phase (Shukla Paksha), and the days when it shrinks are the waning phase (Krishna Paksha). One full cycle from one new moon to the next takes about 29.5 days. A simple model with a lamp as the Sun and a ball moved around your head as the Moon shows all these phases.

Points to remember

  1. The Moon shines by reflected sunlight; it has no light of its own.
  2. Half of the Moon is always lit by the Sun; we see different parts of this half as it revolves around the Earth.
  3. New moon (Amavasya): the lit side faces away from Earth, so the Moon is not visible.
  4. Full moon (Purnima): the whole lit half faces Earth, so we see a complete bright disc.
  5. Waxing (Shukla Paksha) and waning (Krishna Paksha) together make one cycle of about 29.5 days.

Exam tip

Draw a simple diagram with the Sun on one side, the Earth in the middle and the Moon at four positions. Label new moon, first quarter, full moon and last quarter.

Questions students also ask

Is the phase of the Moon caused by the Earth's shadow?
No. Phases happen because we see different amounts of the Moon's sunlit half as it moves around the Earth. The Earth's shadow falls on the Moon only during a lunar eclipse, which is a different event.
Why do we always see the same face of the Moon?
The Moon takes the same time to spin once on its axis as it takes to go once around the Earth. Because of this, the same side of the Moon always faces the Earth.
What is a crescent moon and a gibbous moon?
A crescent moon is when less than half of the Moon's face looks bright, like a thin curved strip. A gibbous moon is when more than half, but not all, of the face looks bright.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.