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Thursday, September 23, 2010

The Moon and Jupiter, September 23, 2010


The Moon is full tonight, or as near to full as to make no difference.  Most people assume that this is the best possible time to observe the Moon, but in fact it is the worst.  Because the light that we are seeing is coming directly at the surface of the Moon from behind our heads, the shadows of craters and mountains that make the Moon a wonder to behold through a telescope are absent.  Also, the extreme brightness of the full Moon creates terrible eyestrain if you view it through binoculars or a telescope.  As a bonus, scattered light from the Moon washes out surrounding stars and wrecks your night vision.  Observers dread when a full Moon (or the Moon at any phase) is in the sky during a meteor shower, as it drowns out all but the brightest meteors.

One thing the full Moon can't drown out is Jupiter, currently making its best and brightest appearance in some time.  Even in a sky full of moisture and haze and clouds, the King of the Planets shines through.  Jupiter is extra-bright right now not just because it is at its closest point to Earth in some decades, but also because it has lost (or at least misplaced) its South Equatorial Belt, one of the reddish bands of clouds that are semi-permanent features in the giant planet's atmosphere.  The belt isn't really gone, or at least not gone gone; it appears to be temporarily obscured by a layer of lighter-colored clouds.  But while it is at least temporarily absent from view, Jupiter will appear that much brighter.

If you go outside tonight and see the full Moon, take a look at the bright "star" to the south of it, and realize that this is no star at all, but rather the largest planet in our solar system!

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