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08/13/2014

Wx 101: Why Does the Moon Appear to be Distorted?

Cosmonaut Oleg Artemyev on the ISS tweeted this image of the 'supermoon' setting behind the Earth.

Sqooshed moonCosmonaut Oleg Artemyev on the ISS captured this image of the 'supermoon' setting behind Earth on August 13, 2014.

Question: Why does the moon appear to be distorted?   

Below is a series of images taken by astronaut Don Pettit from the International Space Station, showing the full moon setting on April 16, 2003.  Notice how the shape of the moon changes from a perfect circle to something that more closely resembles an elongated elipse as it moves towards the horizon.  

SpacemoonsetThis is a composite image containing a series of photos taken of the setting full moon from April 16, 2003 aboard the International Space Station by astronaut Don Pettit.

Answer:  The apparent elongation of the moon near the horizon is due to an optical phenomenon known as refraction.

You see, when light is moving through a uniform medium such as our atmosphere at a constant pressure or outerspace (with a constant zero pressure), light travels at a constant speed in a straight-line motion.  

However, when it passes from one medium to another, such as when it travels from space into the atmosphere or vise-versa, its velocity will change and when it does, the light is "refracted" or bent at the boundary between the two medium.

This bending of the light causes the moon to look distorted when it gets near the horizon.  Because the amount of atmosphere that the light has to pass through gets larger the closer to the horizon it gets, the more dramatic this bending or refracting of light becomes.

The same process holds true with the setting sun.  Have you ever noticed how the sun looks larger or fatter when it is sitting on the horizon?  

Sunset-atmospheric-refraction-and-miragesThe photo above showing a gorgeously hued and misshaped sun was taken from Alexandroupoli, Greece. When the sun is on the horizon or very near it, refraction in the lower atmosphere appears to flatten the solar disk.  Photographer: Athanasios Sismanis

Because of this refraction, or bending of light, when you are watching the moon or sun set, it is possible to see the sun AFTER it has actually set below the horizon!

Sounds confusing right?  The below diagram shows how this is possible with the actual position of the sun in relation to its apparent position in the sky due to refraction at sunset.

Sunset

This is a type of optical illusion known as an "inferior mirage", which simply means the true position of the object has been displaced below the refracted image.  

 

Meteorologist Jeremy Kappell

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