First Quarter Moon
Published on Sep 30, 2006 at 7:20 pm.
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Filed under moon, skywatching.
This is the First Quarter Moon. That means that half of the Moon is lit up. For an observer in the Northern Hemisphere, that means that the Moon wil be low in the southern sky at sunset, with the right hand side lit up. For observers in the southern hemisphere south of the Tropic of Capricorn, 
the Moon will be high in the northern sky, with the left hand side lit up at sunset. Why the difference? Well, it has to do with orientation of the observer and the Moon. The side towards the Sun will be the side that is lit up. So, for the First Quarter Moon, that means that the western side will be lit (as viewed from Earth).
The Moon follows a path in the sky near the ecliptic. Right now the Sun is near the Autumnal equinox, so that means that the First Quarter Moon will be a quarter of the way around the sky from there (hence the name, instead of calling it the First Half since half is lit up!). But, a quarter of the around the sky from the Autumnal Equinox is the southernmost point along the ecliptic, so the Moon will appear about as low as it can get in the southern sky for Northern Hemisphere observers, and about as high as possible for Southern hemisphere observers. The First Quarter Moon, being right angles to the Sun, is highest in the sky at sunset. It sets at about local midnight.
As I said, this is about as far south as the fFirst Quarter Moon can get. A First Quarter Moon near the time of the Vernal Equinox will be the farthest north that a First Quarter Moon can be. At either of the solstices, the angle of the First Quarter Moon from the zenith (the point directly over your head) will be within a few degrees of your latitude. All this is due to the motion of the Moon along a path close to the ecliptic.
So, go out and look at the Moon tonight!
-Astroprof
