MadSci Network: Astronomy

Re: Why does the moon look yellow sometimes and white somtimes?

Date: Tue Feb 9 16:22:46 1999
Posted By: Nick Hoffman, Oil and Gas Exploration Geophysics - Melbourne, Australia
Area of science: Astronomy
ID: 918580200.As

Hi, Tina,

before I answer your question, I'll ask you another. "Why is the sky blue"

At the end, I'll show that it is actually the same question, and that one 
answer covers both!

We know that the sun is bright in the day (far too bright to look at) and 
it's a white sort of colour. At sunrise and sunset it goes orange and red. 
So too with the Moon. It isn't so bright because we see it by reflected 
sunlight, but it's still silvery white when it's high in the sky and 
dull yellow orange when near the horizon.

The reason for this is due to the Earth's atmosphere. The gasses in this 
are clear and colourless, but air contains many tiny specks of dust. We 
don't normally notice these, but when the sun or Moon are near the horizon, 
the light has to pass through a very wide path through the atmosphere. The 
nature of the dust particles causes blue light to be scattered away 
sideways, and lets the red light get through, so we see a more Orangey-red 

At noon the Sun is high in the sky, and the light comes straight doen 
through the relatively thin layer of atmosphere and not much scattering 
occurs so the Sun is bright white. But, the blue light scattering sideways 
is what gives the sky its blue colour.

When the moon is high, it appears silvery white. We don't notice the faint 
blue light in the sky, but Astronomers prefer to photograph the stars near 
new moon, or when it has set becuse that faint light fogs their film a bit.

So, the yellow of the moon/sun and the blue of the sky have the same cause.

Now my final question to you. Why does the moon look so big at Moonrise 
compared to when it is high in the sky? This is a different cause, it's 
nothing to do with light, and is just an illusion. It's part of how our 
brains interpret the world around us. See if you can find out something 
about it!


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