is this an optical illusion?

Guy Macon wrote:
brian wrote:

Sometimes, when I see the moon lit by earthshine, against a twilit
sky, it looks as if the unlit part of the moon is actually darker than
the background sky - as if the moon is silhouetted against the sky.
This shouldn't be physically possible.

Has anyone here noticed this? Is it an optical illusion, and can
anyone here explain it?


Next time you see that, grab a camera and take a picture.
If it's not the same as what you see, it's an optical illusion.

If it *is* the same as what you see, then I would really like
someone to explain the effect to me. It seems impossible.

I haven't personally noticed that phenomena. It might be possible if
the Moon is positioned in line with one of the brighter parts of the
Milky Way Galaxy (in the general area of Sagittarius) and the area of
reflection of sunlight off of the Earth toward the Moon is from a land
mass less reflective than an ocean. Sometimes "Earth shine" is rather
weak.

It might show up best in late twilight when the sky glow from the Sun
illuminating the upper atmosphere is less bright than the light from the
Milky Way but the angle between the Sun and Moon is small so only a thin
crescent is illuminated on the Moon. Zodiacal light could also add to
the background illumination with the right alignment.

It might be a fun project to try to photograph the phenomena. I'm not
saying it happens, just offering a possible explanation if it does.

Lou Boyd
Director - Fairborn Observatory