The Secrets of Moondogs

By Nancy Atkinson - January 18, 2013 at 12:55 AM UTC | Planetary Science
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You've probably heard of -- and likely have even seen -- a

Sundog

, the atmospheric phenomenon that creates rainbow-colored blobs or arcs of light on either side of the Sun. But did you know the Moon can have 'dogs' too? Also known as mock moons, false moons, or the scientific name of paraselenae, Moondogs aren't seen as often as Sundogs. However, the conditions needed to create either of the 'dogs' are about the same. This great image of a Moondog, above, by

Jeff Schultz

is a perfect example of the kind of night you might see this gorgeous phenomenon.

What are the secrets to how Moondogs are produced?

[caption id="attachment_99537" align="aligncenter" width="580" caption = "A bright Moondog on January 20, 2012 seen in Wiltshire, England. Credit: Richard Fleet"]

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Moondogs are seen most often in the winter when ice crystals may be present in the atmosphere. But they can appear anytime of the year when hexagon-shaped ice crystals might be high in the sky, or also when thin cirrus or cirrostratus clouds are just right. What happens is that the ice crystals or clouds refract the moonlight, creating blobs or arcs of light to the left and right of the Moon, or sometimes just on one side.

Also, the Moon usually needs to be full or nearly full, along with being low in the sky for the effect to be produced. The angular separation of the light blobs from the Moon is usually 22 degrees.

Sometimes, a full halo of light around the Moon will also appear in conjunction with the Moondogs; other times smaller arcs of light will be part of the effect, but often Moondogs appear without any other effects.

Moondogs can appear colorful like a Sundog or 'shine' with a light similar to the Moon.

[caption id="attachment_99513" align="aligncenter" width="580" caption = "A Moondog seen over Adelaide, Australia on November 28, 2012. Credit: Ian Musgrave."]

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Moondogs are seen in both hemispheres, as this image from

Ian Musgrave

in Australia attests, but it seems that the effect is seen most often the farther north you are in the northern hemisphere and the farther south you are in the southern hemisphere.

You never know exactly when you might be lucky enough to see a Moondog, so we recommend looking at the Moon every night! Also, don't forget to

wink at the Moon, too.

For more detailed information and images of Moondogs, Sundogs and other optical effects, Richard Fleet (the photographer who captured the second image in this article) has a great website:

"Glows, Bows and Haloes."

Nancy Atkinson

Nancy Atkinson

Nancy Atkinson is a space journalist and author with a passion for telling the stories of people involved in space exploration and astronomy. She is currently retired from daily writing, but worked at Universe Today for 20 years as a writer and editor. She also contributed articles to The Planetary Society, Ad Astra (National Space Society), New Scientist and many other online outlets.

Her 2019 book, "Eight Years to the Moon: The History of the Apollo Missions,” shares the untold stories of engineers and scientists who worked behind the scenes to make the Apollo program so successful, despite the daunting odds against it. Her first book “Incredible Stories From Space: A Behind-the-Scenes Look at the Missions Changing Our View of the Cosmos” (2016) tells the stories of 37 scientists and engineers that work on several current NASA robotic missions to explore the solar system and beyond.

Nancy is also a NASA/JPL Solar System Ambassador, and through this program, she has the opportunity to share her passion of space and astronomy with children and adults through presentations and programs. Nancy's personal website is nancyatkinson.com