Invisible Moon

Got a beginners' question? No matter how elementary, our friendly forum community and magazine writers will answer it.

Invisible Moon

Postby Stuart.Hamilton.140191 » Sun Mar 22, 2015 9:17 pm

When there's a new moon, I understand that the sun lights the side that we can't see.

Why isn't there a black disc in the sky where the sunlight doesn't get through? Why can I see the sky behind the moon?

Sorry, I know this is a silly question.
Stuart.Hamilton.140191
 
Posts: 1
Joined: Sun Mar 22, 2015 9:12 pm

Re: Invisible Moon

Postby david48 » Tue Mar 24, 2015 4:40 pm

Stuart.Hamilton.140191 wrote:When there's a new moon, I understand that the sun lights the side that we can't see.

Why isn't there a black disc in the sky where the sunlight doesn't get through? Why can I see the sky behind the moon?

Sorry, I know this is a silly question.


It's actually a very good question! The answer seems to be, that in the night-sky, you can't actually see the sky "behind" the moon. The moon's disc hides all the stars behind its disc. But as there are so many stars in the night-sky, we don't notice the small loss of stars hidden by the moon. That seems OK.

But what has always puzzled me is this:

Suppose we see the moon during daytime, when it's in "half-moon" phase, seen high in the blue sky, in the afternoon. We see a white half-moon, which is explained by one half getting illuminated by the sun. OK again

But surely the other half, the unilluminated half, ought to look dark against the sky. But it doesn't - it's just invisible.

It doesn't appear as a dark half-disc in the blue sky, it's just not there. Why is that?
david48
 
Posts: 173
Joined: Mon Feb 13, 2012 7:13 pm

Re: Invisible Moon

Postby dave.b » Wed Mar 25, 2015 8:32 am

Because the sky is illuminated by the sun and what we see as the moon is extra light reflected off the moon. Hence we don't see the part of the moon that is in the sun's shadow because there's no reflected light from those regions.

Dave B.
dave.b
 
Posts: 803
Joined: Mon Feb 13, 2012 6:35 pm
Location: Staffordshire

Re: Invisible Moon

Postby uea74 » Thu Mar 26, 2015 8:36 pm

The moon is actually quite small, so what is hidden is a very small section of the sky and your eye simply does not register anything missing.

The pleiades are bigger then the moon, they are about 1x1 degree, the moon is 0.5x0.5 so in effect 1/4 the area. And as it is black on black you do not see an empty area.
uea74
 
Posts: 459
Joined: Mon Feb 13, 2012 6:54 pm

Re: Invisible Moon

Postby david48 » Mon Mar 30, 2015 4:39 pm

dave.b wrote:Because the sky is illuminated by the sun and what we see as the moon is extra light reflected off the moon. Hence we don't see the part of the moon that is in the sun's shadow because there's no reflected light from those regions.

Dave B.


But if there's no reflected light from these lunar regions, why do they look blue in daytime.

Shouldn't they be dark - or at least, a bit darker than the general background of the blue sky?
david48
 
Posts: 173
Joined: Mon Feb 13, 2012 7:13 pm

Re: Invisible Moon

Postby dave.b » Tue Mar 31, 2015 9:26 pm

When the moon is in a direct line of sight between us and the sun then we will see a solar eclipse. However, that rarely happens and for the most part the moon is off to one side of that line of sight. So what we see is the sun's light reflected off the moon.
A new moon is in front of our line of sight with the sun and so there is no reflected light from the moon's illuminated face. Whereas a full moon is directly behind our line of sight with sun and so we can see all of its illuminated face. In between these two states we see only the part of the illuminated face that is facing us. We can't see the unilluminated face that is facing us for the simple reason that there is no reflected light to reach our eyes. (Actually, sometimes we can due to something came Earthshine. That's when the sun's light is reflected off the Earth to the moon and back again.)

Is that any clearer?

Dave B.
dave.b
 
Posts: 803
Joined: Mon Feb 13, 2012 6:35 pm
Location: Staffordshire

Re: Invisible Moon

Postby david48 » Wed Apr 08, 2015 7:08 pm

Yes, thanks dave.b, you explained it a bit. The Earth's atmosphere scatters the sun's light rays, and makes things in the sky look blue. Except for one side of the Moon, which always stays white. Not blue. Sometimes yellowish, but never blue!

Isn't this worse than Quantum Theory! :shock:
david48
 
Posts: 173
Joined: Mon Feb 13, 2012 7:13 pm

Re: Invisible Moon

Postby The Man with the Corrugated Iron Roof » Tue Apr 28, 2015 9:52 pm

The Moon CAN turn blue when there is the right type and amount of smoke and/or volcanic ash in the atmosphere. It only happens every 70 years or so.

A few years ago, I posted a spoof here about the Moon turning blue as we were travelling through a dense part of the interstellar medium. Some people forgot the date (April 1st) and got caught. I simply adjusted the colour of one of my images to blue!
How can I be one with the universe when we don't know what 96% of it is.

About Me: https://www.amazon.com/Philip-Pugh/e/B0034NTCJK

My blog: http://sungazer127mak.blogspot.com/

Photos: https://www.flickr.com/photos/philippughastronomer/
The Man with the Corrugated Iron Roof
 
Posts: 552
Joined: Sat Nov 23, 2013 1:53 pm
Location: Wiltshire but can be just about anywhere up to 41 000 feet


Return to Ask a silly (astronomy) question

Who is online

Users browsing this forum: No registered users and 0 guests