moon and sun

[julieh]

why does the moon and sun appear in the same sky in the morning

[jake-the-peg]

The moon and the sun appear in different areas of the sky depending on how their orbits fall.

If you think about it they occasionally appear at exactly the same point and you get an eclipse of the sun and sometimes they are directly opposite and you get an eclipse of the moon (when the moon goes through the earth's shadow)

[NetSquirrel]

Julieh, do you mean "Why is the Moon visible in the morning and evening, but not during the daytime?"?  If so, this has to do with the brightness of the sky, which in turn has to do with Rayleigh scattering.

The Moon is often visible at night because (1) it's dark at night, (2) the atmosphere is basically see-through, and (3) when it catches the sun, the Moon is very bright.

In the daytime, you might expect to still see the Moon (it's in the visible portion of sky about 50% of the time, after all) and yet it's seldom seen.  In fact, the Moon shines just as brightly during the day as it does at night.  In the daytime, however, the atmosphere is "lit up" bright blue by the Sun - so bright that the Moon is insignificantly faint by comparison, and our eyes can hardly detect it.

[Continued]

[NetSquirrel]

Why does the sky light up in this manner - if air is virtually transparent then why doesn't the sunlight pass straight through?  This is best explained with a diagram (as usual I spent far too long on this - someone had better look at it!).  Light from the Sun enters the atmosphere, and indeed most passes straight through, but some light is scattered by the atmospheric gases, and bounces from molecule to molecule, in every direction, all over the atmopshere.  The effect is known as Rayleigh scattering and mostly disperses blue light, while other wavelengths remain largely unaffected.  I won't go into detail as more information can readily be found with Google, but basically because blue light is the most scattered, there is an abundance of blue light all over the sky (and of course, of the light that passes straight through the atmosphere, there is a deficiency of blue light, giving it a red/orange tinge).

At point A on my diagram it is early morning.  Sunlight reaching point A has had to travel through a lot of atmosphere, losing much of its blue light to scattering in the process, and so you see a red/orange sunrise.  Because there is little blue light reaching point A, minimal Rayleigh scattering occurs, so the sky itself appears dark (dark enough to see the Moon, and even stars).  At point B it is noon - clearly maximum scattering takes place and as a result the sky glows too brightly to see such dim objects as the Moon or stars through it.

http://img469.imageshack.us/img469/3623/netsquirrelsunmoonanswerbank6h.jpg

 

[Dakota]

Wow Netsqurrel, I think that's the most informative post I have ever read on this site!!!  Thank you so much for taking the time out to go into such details about it, if juleih doesn't appreciate it and thank you, then I do 100 times over!

**********

[julieh]

That is exactly what I meant NetSquirrel. Thank you so much for your v. detailed response, absolutely brilliant.

[NetSquirrel]

Dakota and julieh, it is very kind of you to say so.