Why phases happen

The idea

As the Moon revolves around Earth in about a month, we see only the illuminated part of the side facing Earth; the change in the fraction of the illuminated portion seen from Earth causes the phases.

A ball and a lamp

A ball is lit by a lamp. You walk around the ball and look at it.

What do you see as you move?

• Different amounts of the lit part
• Always the same view
• Nothing at all

Go round the Earth

Visit all eight places of the Moon.

Fraction of the lit part

The Moon goes round the Earth in about a month. The side facing the Sun is lit. From Earth we see only the lit part of the side that faces us. At some places we see all of the lit part, at others only a part or none. The change in the fraction of the lit portion we see causes the phases.

The Moon revolves around the Earth in about a month. We see only the lit part of the side facing Earth. The change in the fraction of the lit part seen from Earth causes the phases.

Notes

Phases: as the Moon goes round the Earth we see different fractions of its lit half.

Check yourself

The phases of the Moon are caused by the change in the …

The Moon goes around the Earth once in about …

On new Moon day we do not see the lit part because …

On full Moon day we see …

• fraction of the lit part seen from Earth — correct. Yes!
• shape of the Moon. The Moon's shape does not change.
• Moon's own light. The Moon has no light of its own.
• a day. It takes about a month.
• a month — correct. Yes!
• a year. It takes about a month.
• the Moon is switched off. The unlit half faces the Earth.
• Earth's shadow hides it. Phases are not due to Earth's shadow. The unlit half faces Earth.
• only the unlit half faces the Earth — correct. Yes!
• the whole lit half — correct. Yes!
• none of the lit half. That is new Moon.
• only a thin crescent. On full Moon day we see all of the lit half.