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All That You Wanted To Know About Sun Dogs (Parhelion) and Moon Dogs (Paraselene)

written by: Sushma•edited by: RC Davison•updated: 11/29/2009

People are often confused about the terms sun dogs and moon dogs. What are they? Where do they appear? And why do they appear at all? Let us find out the answers to all these questions.

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    Atmospheric Optical Phenomena

    Many types of atmospheric optical phenomenon are a result of scattering of light. This can happen due to various reasons. Scattering of light can occur while passing through an aerosol or air particles of the atmosphere. Light can be reflected by larger entities like rain drops and ice crystal . It can also get refracted (bent) while passing through these crystals (forming rainbows or halos) or while entering from one medium to another.

    Formation of the sun dogs and the moon dogs is the result of one such atmospheric optical phenomenon - refraction. Let us look into the details of these formations and find out why and how are they formed.

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    The Sun Dogs

    Sun Dog Sun dogs (also spelled as sundogs) (Scientific name: Parhelion), can be defined as a kind of atmospheric phenomenon in which there is a creation of bright spots of light that can be seen in the sky. These spots of light, or coloured patches willSundog  form within a halo or a ring and can be seen on either side of the sun.

    They appear almost anywhere in the world . Sometimes, you will also find that you are not able to see them clearly due to the fact that the sun dogs are not always bright and the reflection and glare from the sun can make it quite impossible to notice this phenomenon. The best time to check for the sun dogs would be when the sun is low in the sky.

    {Image Source (Left): Sun dog: http://www.webportfolio.com/blog/_photos/sundogs_1597.jpg}

    {Image Source (Right): Credit: Rauber: Defining Sun dog: http://ww2010.atmos.uiuc.edu/guides/mtr/opt/ice/gifs/sd1.gif}

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    Formation of Sundogs

    Sun dogs are formed when the sun's rays are refracted by hexagonal shaped plates of ice crystals, in cold cirrus clouds at about 30,000ft above sea level. Generally the size of these plates is larger than 30 µm in diameter. The sun's rays enter these crystals, which act like a prism and the light rays are refracted. When the ice crystals are uniformly oriented the sundogs appear, if they are randomly oriented then a halo will be visible around the sun.Formation of sun dog 

    They are visible on either side of the sun when the light rays are bent by 22 degrees before reaching the observer's eyes.

    A sun dog is also called a mock sun. Often, two sun dogs can be seen on either side of the sun. At times, one can also see a second faint halo surrounding the inner bright halo.

    {Image Source (Right): Formation of Sun dog: http://ww2010.atmos.uiuc.edu/guides/mtr/opt/ice/gifs/sd2.gif}

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    What Are Moon Dogs?

    Moon Dog Moon dogs (or Paraselene) can be defined as a very rare but bright spots that can be seen around the moon. The moon dogs are normally caused by the refraction of moonlight, which is caused by ice crystals in cirrus clouds and these ice crystals are usually shaped in a hexagonal plate form.

    Just like sun dogs, moon dogs also appear 22° to the moon on its right and left sides. The difference between the moon dogs and the sun dogs is that the former appears only when the moon is bright, typically when it is near the full moon. The light emitted by them is very low but they are visible to the naked eye but moon dogs show little red and blue colors.

    {Image: (Left) Moon dog: Source: http://pixdaus.com/pics/1241529397uPIRzE2.jpg}

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    Where Are Moon Dogs And Sun Dogs Seen?

    The occurrence of sundogs and moondogs is dependent on ice crystals in the atmosphere and one would expect to see them all around the world if the weather conditions are right. However, in most of the developing countries people might see them but do not keep records of how many times they appear. Therefore, most of the cases of moon dogs and sun dogs are reported from the United States and Europe.

    The fact remains that if you closely watch the sky, you will be able to see such phenomena irrespective of where you are.

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    References

    en.wikipedia.org/wiki/Sun_dog

    en.wikipedia.org/wiki/Moon_dog

    www.astrophys-assist.com/wilobs/weathwin/sundog.htm

    http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/opt/ice/sd.rxml

    http://www.sci.ccny.cuny.edu/~stan/opt_ccny.pdf