Phases of the Moon

Key Terms

The Moon does not produce its own light. We can see the Moon because it reflects light from the Sun.
The Moon orbits the Earth, and as this happens the Moon’s appearance when viewed from the Earth changes because different parts of the Moon are illuminated at different times. This means we see the Moon’s phases from Earth as different shapes.

The Moon’s appearance changes over time when viewed from Earth. Sometimes, the Moon is not easily seen in the sky and at other times it can appear as a thin , a full circle – or somewhere in between.

A Moon phase is the shape of the visible part of the Moon, and this changes gradually over the course of a .

A lunar month

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New Moon
New Moon – the illuminated part is not visible at all.
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Waxing crescent Moon
Waxing crescent Moon – less than half of the visible surface is illuminated (starts as a thin, curved sliver) and getting larger.
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First quarter Moon
First quarter Moon – half of the visible surface is illuminated.
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Waxing gibbous Moon
Waxing gibbous Moon – more than half of the visible surface is illuminated - and getting larger.
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Full Moon
Full Moon – the entire visible surface is illuminated.
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Waning gibbous Moon
Waning gibbous Moon – more than half of the visible surface is illuminated - and getting smaller.
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Third quarter Moon
Third quarter Moon – half of the visible surface is illuminated.
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Waning crescent Moon
Waning crescent Moon – less than half of the visible surface is illuminated – and getting smaller.

A lasts around 29.5 days and starts with a – when the Moon is not easily seen in the sky because the light from the Sun lands on the side of the Moon which is facing away from Earth.

As the Moon moves around Earth, the illuminated section of the Moon’s surface starts to face towards the Earth, and we see a thin appear.

Over the course of a lunar month, the Moon goes through the following phases:

A diagram showing the phases of the Moon — Image caption,
There are eight phases of the Moon beginning with the new Moon, followed by waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, third quarter and finally waning crescent. This cycle repeats once every 29.5 days.

Did you know?

What causes lunar phases?

The Moon is non-luminous, meaning that it does not produce light. We see the Moon because it reflects light from the Sun, and half of the Moon’s surface is always illuminated. The amount of this illuminated portion that we can see from Earth varies, depending on the angle that we are viewing from. This changes as the Moon orbits Earth, and Earth orbits around the Sun.

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When the moon is between Earth and the Sun, the illuminated part of the Moon is facing away from Earth so we cannot see it at all – this is a new Moon.
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As the Moon moves around Earth, we begin to see some of the illuminated part of the Moon – this is a crescent Moon. As the Moon continues around its orbit the crescent grows in size, known as waxing, as more of the illuminated part of the Moon becomes visible.
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When the Moon is one quarter of the way around its orbit, we see half of the visible surface of the Moon illuminated – a first quarter Moon.
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As the Moon continues around its orbit, we see more of the illuminated part of the Moon. More than half of the Moon’s visible surface is illuminated – this is a gibbous moon.
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When the Moon is on the opposite side of Earth to the Sun, the illuminated part of the Moon is facing towards Earth – so we see a full Moon. This occurs when the Moon is halfway around its orbit, approximately two weeks after the start of the lunar month.
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As the Moon continues to move around its orbit, less of the illuminated part of the Moon is visible – this is a waxing gibbous Moon.
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When the Moon is three quarters of the way around its orbit, we see half of the visible surface of the Moon illuminated again – a third quarter Moon.
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As the Moon continues through its orbit it continues the size of the illuminated part of the Moon which is visible continues to decrease. Less than half of the illuminated part of the Moon is now visible – a waning crescent Moon. The lunar cycle starts again with a new Moon, when the Moon has returned to its original position, between the Sun and Earth.