[Chaos-l] What's wrong with this picture?

[D Gary Grady]

NASA's latest Astronomy Picture of the Day is a bit surprising:

http://apod.nasa.gov/apod/ap080423.html

It appears to be a full moon setting next to Venus, which if you think 
about it isn't possible.

The caption says the photo was shot in "early December of 2005," and per 
http://aa.usno.navy.mil/data/docs/MoonPhase.php the new moon was 
December 1, 2005, so one would expect the phase to be waxing, somewhere 
in the neighborhood of the first quarter. Hence there's nothing at all 
implausible about Venus being near the moon (and if I were feeling 
better I might try figuring out the exact date using planetarium software).

So the real mystery is why the moon in the photo looks like a round 
disk. I think the explanation is that in exposing enough to pick up the 
Milky Way, we're seeing the dark side of the moon illuminated by enough 
reflected earthshine to make it appear white. The part of the moon 
illuminated by sunlight is of course far brighter, but in photography, 
past a point white is white and adding more light doesn't make the image 
any whiter.


-- 
D Gary Grady
Durham NC USA
dgary at mindspring.com