What is a full Moon?

A full Moon occurs when the Moon appears as a complete circle in the sky. We see it as a full orb because the whole of the side of the Moon facing the Earth is lit up by the Sun's rays.

The Moon produces no visible light of its own, so we can only see the parts of the Moon that are lit up by other objects.

A small amount of light comes from distant stars and the reflection of light from the Earth (known as 'Earthshine'). However the main source of light for the Moon is the Sun.

The Moon appears as different shapes in the sky depending on its 'phase', from new Moon to full Moon via 'waxing' (growing) and 'waning' (shrinking) moons. These phases are determined by the relative positions of the Sun, Earth and Moon.

If the Moon is between the Earth and the Sun in its orbit, then the back side of the Moon is lit up and the side facing the Earth is in darkness. This is called a new Moon.

If the Moon is on the opposite side of the Earth to the Sun, then the near side of the Moon will be fully lit up: a full Moon

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How often does a full Moon occur?

A full Moon happens roughly every 29.5 days. This is the length of time it takes for the Moon to go through one whole lunar phase cycle.

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Is there a full Moon every month?

The Moon’s phases and the months of the year are inextricably linked - the word 'month' even takes its root from the word 'moon'. 

A month was originally defined to be either 29 or 30 days, roughly equal to the 29.5-day lunar cycle. However, some of our calendar months were later padded out with extra days, in order that 12 months would make up one complete 365-day solar year.

Because our modern calendar isn’t quite in line with the Moon’s phases, sometimes we get more than one full Moon in a month. This is commonly known as a blue moon.

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When is the next full Moon?

The next full Moon is on 20 October at 3.56pm in the UK. This is sometimes known as a 'Hunter's Moon'. 

Check the calendar below to see all the full Moon dates in 2021.

Full Moon calendar 2021

Full Moon date and time   Full Moon name
28 January (7.16pm) Wolf Moon
27 February (8.17am) Snow Moon
28 March (7.48pm) Worm Moon
27 April (4.31am) Pink Moon (supermoon)
26 May (12.13pm) Flower Moon (supermoon)
24 June (7.39pm) Strawberry Moon
24 July (3.36am) Buck Moon
22 August (1.01pm) Sturgeon Moon
21 September (12.54am) Corn/Harvest Moon
20 October (3.56pm) Hunter's Moon
19 November (8.57am) Beaver Moon
19 December (4.35am) Cold Moon

All times show the time of the full Moon at the Royal Observatory's home in London, either in GMT or BST depending on the time of year

Why is there a full Moon in the middle of the day?

Some of the times included in the table show full moons happening in the middle of the day. How can this be?

While you can often see the Moon even during the day, it may at first seem odd to think of a full Moon occurring during daylight hours. However, there is a straightforward explanation.

The reason this happens is that the time refers to the exact moment when the Sun and Moon are aligned on opposite sides of the Earth. This moment is known as the 'syzygy' of the Sun-Earth-Moon system, and can happen at any time day or night.

The Moon will still look full either on the night before or the evening after the exact moment of 'full Moon'.

When is the next lunar eclipse?

What is a Supermoon?

The distance between the Moon and the Earth varies, because the Earth is not right at the centre of the Moon’s orbit and the Moon’s orbit is not a circle (it’s an ellipse).

The moment when the Moon is closest to the Earth is called a lunar perigee. When the Moon is furthest away it is known as a lunar apogee.

If the lunar perigee occurs very close to a full Moon, then we see what is known as a Supermoon. If a lunar apogee occurs very close to a full Moon then we see a Micromoon.

Learn more about supermoons

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2021 guide to the night sky

Plan your stargazing year ahead with the Royal Observatory's astronomy calendar