Astronomy
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Why does the full moon appear reddish sometimes?

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2009-01-19 04:31:27

The question asks why does the full moon appear red

sometimes. The moon appears red when it is near the horizon.

This answer addresses that aspect of moon reddening. Rayleigh

Scattering The moon appears red sometimes for the same reason

the sky appears blue. It is due to a phenomenon called Rayleigh

scattering, named after Lord Rayleigh (1842-1919). Rayleigh

scattering occurs when very tiny particles in the atmosphere

deflect light in random directions. The likelihood of Rayleigh

scattering is inversely proportional to the wavelength to the

fourth power. That means Rayleigh scattering occurs much more

prominently in shorter wavelengths of light (blue) and much less

noticeably in longer wavelengths of light (red). Consider a beam of

white light, made up of photons, passing through the atmosphere.

Remember that white light is made up of all the colors in the

rainbow. The blue photons are more likely to be deflected in random

directions due to the Rayleigh scattering, whereas the red photons

are less likely to be deflected. This process of scattering the

blue photons creates a filtering effect. Because they are scattered

off the path of the beam of light, fewer blue photons travel with

the beam of light than red photons. So by the time the light

reaches you, some of the blue has been removed, leaving a relative

abundance of red light. The more atmosphere a beam of light travels

through, the more intense the effect of Rayleigh scattering,

meaning the more red (or less blue for you sticklers) the resulting

beam of light becomes. Light that travels low across the horizon

passes through much more atmosphere than light from directly

overhead. Therefore the Rayleigh scattering is more intense when

viewing objects near the horizon compared to viewing objects

overhead. Red Moon That is why the moon looks redder when it

is low on the horizon - the beam of light between the moon and you

travels through much more atmosphere at a low angle of inclination

compared to when the moon is overhead. Because the beam of light

travels through more atmosphere, more Rayleigh scattering occurs,

and therefore there is less blue light in the resulting beam of

light than red light. Our eyes only register the light that strikes

our retina - they do not know what proportion of blue to red light

there was at the beginning of the beam of light's journey. Because

there is less blue than red when the light strikes our retinas, our

eyes record more red than blue, and we therefore see the moon in a

reddish color. That is also why the sunset and sunrise appear red -

the light from the sun travels through more atmosphere at sunrise

and sunset compared to when the sun is overhead. When it is low in

the sky, the air filters out more blue from the sunlight than red,

and therefore we see a relative abundance of red. Blue Sky

Now consider a ray of light passing horizontally high over your

head. You do not see this beam of light, because it is not shining

at you. But as it travels through the atmosphere overhead, Rayleigh

scattering deflects some of the blue photons in random directions.

A small proportion of those deflected blue photons are randomly

directed toward you. Therefore, when you look up in the clear sky,

you see blue. A partial eclipse from the Earth. When the light rays

pass over the edge of the Earth they are filtered into more red

than pure direct sunlight onto the surface of the moon. When there

is a lot of dust or pollution in the atmosphere, a full moon may

appear anywhere from buttery yellow to salmon to vivid orange to

blood red - ''when it is fairly low in the sky''.

The angle at which you view the moon also contributes to it's

apparent color.

You'll notice that as a full moon rises it will lose the reddish

hue and retain its normal color. When the moon is low on the

horizon you are viewing it through a thicker layer of atmosphere

and therefore through more of the particles that happen to be in

the air.

Refraction may also have something to do with this, but without

further research into this I can't be sure.


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