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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鈥檚 appearance when viewed from the Earth changes because different parts of the Moon are illuminated at different times. This means we see the Moon鈥檚 phases from Earth as different shapes.

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Phases of the Moon

The Moon鈥檚 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 鈥 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

Image gallerySkip image gallerySlide 1 of 8, New Moon 鈥 the illuminated part is not visible at all., New Moon New Moon 鈥 the illuminated part is not visible at all.

A lasts around 29.5 days and starts with a 鈥 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鈥檚 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.

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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鈥檚 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.

Image gallerySkip image gallerySlide 1 of 8, 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 鈥 this is a new Moon., 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 鈥 this is a new Moon.

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