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Glossary term: Halo

Description: A halo is an optical phenomenon caused by ice crystals in the Earth's atmosphere. Just as refraction and reflection in water droplets causes rainbows, sunlight passing through ice crystals is reflected and refracted within the ice crystals. Halos can take many different forms and can be cause by light from the Sun or the Moon. The 22 degree halo is a circular halo that forms as a thin band of light around the Sun or the Moon. The geometry of this halo is the result of the properties of the ice crystals that sunlight or moonlight passes through in the atmosphere.

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Term and definition status: This term and its definition is still awaiting approval

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

Related Media

A halo is an optical phenomenon where circles or arcs of light are visible in the sky. Halos are caused  a bright object such

Selene meets the Moon, by Sheila Wiwchar, Canada

Caption: First place in the 2021 IAU OAE Astrophotography Contest, category Sun/Moon haloes. Photographed at Kaleida, Manitoba in Canada, this fisheye image beautifully captures the rare optical phenomena encircling the moon known as the “22° halo” and the horizontal white circle passing through the moon called the “paraselenic circle”. The white band circling the whole sky at the same altitude as the moon is named after Selene, the ancient Greek Titan, famously called the goddess of the moon. The more commonly observed counterpart produced by the sun is known as the parhelic circle, named after the Greek god of the sun, Helios. Both the 22° halo and the paraselenic circle are produced due to reflection of the moonlight from near vertical surfaces of ice crystals. Parts closer to the moon are caused due to external reflections, whereas those further away are created due to internal reflections. The constellation of the big dipper at the center makes this image even more spectacular. Can you spot it?
Credit: Sheila Wiwchar/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
The Sun, surrounded by several bright circles and arcs, over a snowy, tree-lined landscape.

Winter Haloes, by Thomas Gigl, Germany

Caption: Second place in the 2021 IAU OAE Astrophotography Contest, category Sun/Moon haloes. Captured in Jochberg located in the famous Austrian ski-region of Tirol, this image shows multiple features related to ice halos, which are a more common appearance around the sun, due to its brightness, than the moon. External and internal reflection of sun rays from ice crystal faces and within different types of ice crystals lead to these halo related phenomena. The 22° halo encircles the sun, with two bright spots at the edge called Sundogs, Parhelia or Mock Suns observed to the left and right at the same height as the sun. The horizontal white band called the parhelic circle, named after the sun god Helios, passes through the sun and the Sundogs at the same angular elevation. An Upper tangent arc, a suncave parry arc and a lower tangent arc are also seen touching the top and bottom of the 22° halo. An upside down rainbow like arc or the circumzenithal arc is seen touching the bright supralateral arc, both of which are less frequently observed.
Credit: Thomas Gigl/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
The four telescope domes of VLT. One dome is open and firing laser beams. In the sky, the moon has a ring of light around it.

ESO's Very Large Telescope (VLT) at Paranal Observatory with a lunar halo

Caption: The Very Large Telescope on Cerro Paranal in Chile has four reflecting telescopes each with a primary mirror 8.2m across. In this image one dome is open and firing laser beams into the sky. These lasers create artificial stars in the sky that can be used to adjust the telescope optics to remove the blurring effect of the Earth's atmosphere. This process is known as adaptive optics. In the sky the Moon is surrounded by a bright halo. This is caused by light from the moon reflecting inside ice crystals in the Earth's atmosphere.
Credit: Juan Carlos Muñoz-Mateos/ESO
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons