[chris (OP)]

When the sun sets the horizon becomes very red. The argument goes that the sunlight is following a longer path through the atmosphere and hence more light at the blue end of the spectrum (480nm ish) is scattered, making the remaining light more red dominated.

So why does the moon look white on the horizon then ?

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[tweener]

I've always thought the moon was quite reddish while it's within about one moon-width above the horizon.  Maybe it's just my eyes, or all the pollution where I live.

----
John - The Eternal Pessimist.

[OldMan]

Agreed I always thought the moon was very yellow or close to a reddish colour when close to the horizon.

Tim

[chris (OP)]

Fair enough. Probably my imagination that it was white than it really is.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[gsmollin]

Well, like Chris, I have noticed that the moon does not appear as red as the sun does when it is at the horizon, except at the full moon when the sun and moon are both at opposite horizons simultaneously. On those occasions, the moon is so red it looks like a fire on the horizon. A rising moon in the middle of the night casts a far whiter appearance. This phenomenon notwithstanding, I shall have to spend some time observing a moonrise at first or last quarter, where the sun is well below the horizon, to see if the moon appears red as it clears the horizon. My memory is of a whiter moon than sun, for sure.

[chris (OP)]

So if that is the case, why ?

C

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[gisburnuk]

Interesting Question. During  sunset the atmospheric dust modifies the radiation, ie it reduces and changes the luminosity of the suns light, and not the moon.(perhaps because the moon absorbs most of the solar radiation etc), and reflects visible light that we see as the visible part of the lunar phases. Don't take my word though, take further advice amongst other Members.[]

M Theory for String Theory, stay focused....

[gisburnuk]

The visible red colour that we observe as being cosmic dust is made up of graphite and silicates which are coated in frozen ammonia and dry ice (carbon dioxide that is solid in structure). These elements absorb and diffuse star radiation, ie from the blue part of the visible spectrum. The presence of this dust therefore makes the star appear more red than they really are, similar in the way that we observe our sun today during sunset and sunrise. []

M Theory for String Theory, stay focused....

[Predator_X]

i onli know tat y the moon is white is bcuz of reflected light frm the sun.

[Gneiss]

quote:Originally posted by gsmollin

My memory is of a whiter moon than sun, for sure.

You memory is fine...

The problem is your eyes []

Light from the Moon will be refracted and diffused by the atmosphere in exactly the same way as that from the Sun. The difference is that the sun (even at sunset/sunrise) is many hundreds of times brighter than the moon.

Below certain light levels our eyes stop detecting colour completely and are particulary insensitive to red light.

In the case under discussion, simply the less light there is available to the eye the less red the object appears.

Theory is when you know everything and nothing is working. Organization is when nothing is working and everyone knows why. Practice is when everything is working and no one knows why.

[gsmollin]

Good point, perception can be everything. I still remember a red moonrise. It was a clear sunset in the woods. I was walking west, and I saw a red glow through the trees. I panicked, thinking it was a fire! Them I realized it was a full-moon-rise

[chris (OP)]

Thanks Gneiss, well explained.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[Gneiss]

quote:Originally posted by gsmollin

Good point, perception can be everything. I still remember a red moonrise. It was a clear sunset in the woods. I was walking west, and I saw a red glow through the trees. I panicked, thinking it was a fire! Them I realized it was a full-moon-rise

Photographically the moon is distinctly red at moonrise, but the eye only detects this around the time of the full moon.

Photographic exposure times for the moon at various phases help demonstrate why…

Using ISO100 film @ f/16

Full 1/60
Gibbous 1/30
1st quarter 1/15
Thick Cresent 1/8
Thin Crescent 1/4

The thing to bear in mind is that these variations in exposure time are not simply down to the fact that we see less of the moon and therefore receive less light. Instead they reflect a genuine drop off in surface brightness as seen from the earth.


Theory is when you know everything and nothing is working. Organization is when nothing is working and everyone knows why. Practice is when everything is working and no one knows why.