What causes a ring or halo around the Moon?

Ever noticed a large ring surrounding a bright full Moon? It's all an optical illusion.

Winter halo Tom Wildoner, Pennsylvia, USA, 29 December 2020 Equipment: Canon 6D DSLR, Canon EF 17–40mm f4 USM lens
Published: April 6, 2022 at 9:15 am
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So you've looked up at the Moon tonight and noticed there's a ring or 'halo' around it. What could be causing this?

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The ring around the Moon - or 'lunar halo' is caused by the refraction of moonlight from ice crystals in the upper atmosphere.

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Cirrus clouds are found at high altitudes and resemble tufts of hair or cotton wool. Credit: Doug Armand / Getty Images
Cirrus clouds are found at high altitudes and resemble tufts of hair or cotton wool. Credit: Doug Armand / Getty Images

Cirrus clouds, at a height of 6,000m (20,000ft) or more in the atmosphere, contain tiny ice crystals that originate from the freezing of water droplets.

These ice crystals refract, or bend, light just like a camera lens. It is the shape of the ice crystals that focuses the light into a halo around the Moon.

Since the ice crystals are normally all hexagonal, the Moon’s ring is almost always the same size; it has a diameter of 22º.

Moon and Mars inside a Halo Andrew Morl, Grassholme Observatory, County Durham, 30 October 2020. Equipment: Nikon D7200 DSLR, Sigma 8–16mm lens
Moon and Mars inside a lunar halo, by Andrew Morl, Grassholme Observatory, County Durham, 30 October 2020. Equipment: Nikon D7200 DSLR, Sigma 8–16mm lens

Sometimes it is also possible to detect a second ring, 44º in diameter.

In folklore, a ring around the Moon forecasts bad weather on the way, and in many cases this may be true.

The ice crystals that produce the halo and their attendant cirrus clouds normally precede a warm front by one or two days.

A halo around the Moon captured by Mohammed Aissa Moussa from Ghardaia, Algeria. Mohammed used a Canon EOS 5D Mark II DSRL camera and 15mm fisheye lens.
A halo around the Moon captured by Mohammed Aissa Moussa from Ghardaia, Algeria. Equipment: Canon EOS 5D Mark II DSRL camera, 15mm fisheye lens.

Typically, a warm front will be associated with a low-pressure system, which is commonly referred to as a storm.

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Some folk tales say that the number of stars within the Moon’s halo indicates the number of days before bad weather will arrive. Give it a try next time you see a halo.

Authors

Garry E. Hunt is a former NASA Scientist who was a co-host on BBC's The Sky at Night for over 20 years.
Garry HuntScientist

Garry E. Hunt is a former NASA Scientist who was a co-host on BBC's The Sky at Night for over 20 years.

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