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06 May 2013 @ 09:37 pm
AKICILJ: Astronomy question  
It's possible and easy to see the waxing moon in daylight. It's apparently not possible, or anyway very difficult to see the waning moon in daylight, according to a friend.

He writes, "Corollary to that request: I understand that the last few days of the waning crescent phase have the (minimally lit) moon too close to the sun to be distinguishable; it's the earlier days, particularly while the moon is still in waning gibbous, that confuse me as to my inability to find it in the daytime sky during any of the daylight hours prior to moonset (which times I verify with the US Naval Observatory website)."

Why would this be the case?

crossposted to LJ and FaceBook
 
 
 
Jim Hetley: Powersjhetley on May 7th, 2013 02:12 am (UTC)
No answer, except that I saw the waning crescent a couple of days ago. Maybe he has a brighter sky -- more haze.
Buddha Buckblaisepascal on May 7th, 2013 02:29 am (UTC)
Some thoughts.

A waxing moon is always west of the sun in the sky (it rises after, and sets after, the sun). A waning moon is always east of the sun in the sky (it rises before, and sets before, the sun).

This means that a waxing gibbous moon will always rise in the afternoon, between noon (at first quarter) and sundown (at full), and a waning gibbous moon will always rise in the evening, between sundown (at full) and midnight (at third quarter).

So a waxing gibbous is in the daytime sky in the afternoon, while a waning gibbous is in the daytime sky in the morning. I would tend to think that for casual observances, it is more likely that folks would be looking up at the sky in the afternoon rather than the morning, especially looking westward.

Of course, that wouldn't account for the difficulty of seeing it when one is looking for it. I wonder if there is a difference in clarity between the morning sky and the afternoon sky.
Bill the bold bosthoonwcg on May 7th, 2013 05:18 pm (UTC)
It's just that the afternoon sky is usually brighter, but the Moon is still there. Our ability to see anything in the sky is a matter of the signal to noise ratio, where the noise component is the scattered light in the sky. That scattered light is sunlight, and the scattering is caused by molecules and dust grains in the atmosphere. While the number of molecules is pretty constant, the dust content varies by orders of magnitude, with more dust typically in the afternoon sky than in the morning sky.
Peter Alway: Astronomypeteralway on May 7th, 2013 10:21 pm (UTC)
(sent here by a friend)

Aside from the fact that people are too bleary-eyed to think to look for a waning moon in the morning, the two are pretty symmetrical over the year, but not on a given date. The inclination of the ecliptic to the celestial equator creates a seasonal effect.

In the spring, the waxing crescent moon a is higher in the sky at or near sunset, and the waning crescent moon is lower in the sky at or near sunrise. So for a given number of days before or after new moon, it may be easier to spot the waxing moon.

In the fall, the waxing crescent is lower at sunset and the waning moon is higher at sunrise. So in the fall, the waning crescent will be easier to spot than the waxing crescent the same number of days from new.

One way to think of this is that the waxing crescent moon is close to where the sun will be in a month or two. The waning crescent moon is close to where the sun *was* a month or two ago. In spring, the sun will be higher, and *was* lower. In the fall, the sun will be lower and *was* higher.