MadSci Network: Astronomy

Re: why can I sometimes see the moon during daylight hours?

Date: Thu Feb 8 16:22:00 2001
Posted By: Lew Gramer, MIT S.B. Math (Theoretical)
Area of science: Astronomy
ID: 981531035.As

You asked a really good question, Jackie! I recall as a kid the
first time I noticed the moon in the daytime sky: I was really stunned!
The moon was a "night time" thing, and it had never occured to me to even
look for it in the blue skies of daytime.

The answer to your question has two parts: why is the moon VISIBLE during
the day, and why is it even ABOVE THE HORIZON during daylight hours. The
answer to the first question is easy: you can see the moon even against
the blue backdrop of the daytime sky, because the moon is very BRIGHT!
It isn't anywhere near as bright as the Sun of course. After all, it only
shines by reflecting light from the Sun anyway - and not that well. But
it is brighter than any OTHER natural object in the sky.

(You can confirm this for yourself by going out into the country on a
moonlit night, and noticing how much brighter the sky and the landscape
both look than compared with a time when the moon is NOT up. There's a
reason why our ancestors called the Autumn full moon the "Harvest Moon":
it was bright enough to keep harvesting their crops all night long!)

In fact, there is one other object which is just BARELY bright enough to
be visible in the daytime, besides the moon and sun: that's the planet
Venus - the third brightest natural light source in the sky. If you know
EXACTLY where to look in the daytime sky, and Venus isn't too close to
the Sun, so you are able to put yourself in the shadow of a convenient
building to block out the Sun's glare, Venus looks like a pale little
"star" shining amid the blue heavens - very pretty, in fact!

Now for the second tougher question: How can the moon ever be above the
horizon when the sun is up? The truth is that the moon goes ALL the way
around the sky roughly once every 29 days. For part of that time (around
Full Moon), the moon is in the part of the sky OPPOSITE the Sun, and is
generally just rising as the Sun sets. But right after Full Moon, as the
moon gradually drifts Eastward each night, it gets closer and closer to
the Sun in the early morning sky - finally appearing to pass quite near
the Sun in the sky around New Moon time. Then it gradually continues its
path Eastward, getting farther and farther from the Sun in the sky each
night, until it gets back to Full Moon!

(And in fact, every once in a great while, the moon actually passes right
OVER the Sun, partially or wholly eclipsing it for a few minutes to hours.
This doesn't happen at every New Moon, of course! Normally, the moon will
pass just north or south of the Sun at each New Moon, as seen from Earth.)

I hope this answers your question - and more importantly, makes you a bit
more curious about the moon, its motion and its phases in our night (and
daytime!) sky. Clear skies, and keep looking up,

Lew Gramer

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