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Rainbow Moon

Photo of the Week. A curving rainbow greets a waxing gibbous Moon.

Astronomy news for the week starting Friday, August 6, 2010.

The Moon begins as a waning crescent, passing its new phase on Monday, August 9th. It then pops up in twilight as an extremely thin crescent the evening of Wednesday the 11th below Mercury, the pairing a difficult sight at best even though the little planet has just passed greatest eastern elongation with the Sun. The next night, that of Thursday the 12th, will be much better, with the Moon lying below Venus, Mars, and Saturn, with a frosting of fainter Porrima (Gamma Virginis) on top. Only 15 hours after the Moon goes through new, it passes perigee, where it is closest to Earth, the combination bringing especially high tides to the coasts, ameliorated only by the Earth still being near its farthest point (aphelion) from the Sun.

It's conjunction time. Passages between the moving planets are always a treat to see even in light-polluted skies. Last week, it was between Mars and Saturn, while this week belongs to Venus and Saturn, the "bright one" passing three degrees south of the "ringed one" on Monday the 9th (Venus far outshining Saturn). At the same time, Mars will be up and to the left of Venus, the three planets making an unusual triangle, the trio offering a sight well worth going outdoors for. But look early, as all this action happens in twilight, as by dark they are all setting and quite impossible to admire.

But then, just as we seem to have lost them all, up comes Jupiter. Though alone (except for dim Uranus to the west of it), the giant planet still -- wonderfully so -- makes its mark. Rising as twilight ends, around 10 PM Daylight Time, Jupiter (still in western Pisces), is with us the rest of the night, transiting the meridian to the south just before dawn begins to light the eastern sky.

With the Moon near new, the skies darken, allowing a fine display of the August meteors, the Perseids, which, though they range across the sky, appear to emanate from the constellation Perseus. This year look to morning of Friday the 13th for the best show, with as many as 100 meteors an hour (the previous morning perhaps pretty good too). The Perseids are the flaked leavings of periodic comet Swift-Tuttle, which last passed us in the 1990s.

As bright Vega climbs the eastern sky in early evening, look about 15 degrees to the northwest to find a pair of modestly bright stars (Eltanin and Rastaban, Gamma and Beta Draconis) that represent the head of Draco, the Dragon, which then proceeds to wind its way between the Big and Little Dippers, finally ending about a third of the way from the Big Dipper's Bowl to the North Star, Polaris
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