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Circumhorizontal Arc

Photo of the Week.. Not a rainbow, but a "circumhorizontal arc," caused by refraction of sunlight through floating ice crystals, graces the sky below the Sun, which is well off the top of the picture.

Astronomy news for the short week starting Friday, December 20, 2002.

A good holiday season to all.

Our Moon spends nearly the entire week waning in the gibbous phase, not reaching third quarter until the evening of Thursday, December 26, well before Moonrise in the Americas. The night of Sunday the 22nd, it will make a fine sight with rising Jupiter, the Moon up and a bit to the west of the giant planet, which is now rising just a bit after 8 PM. The following night, the Moon will be to the east of Jupiter and firmly ensconced within the Sickle of Leo, the backward question mark that makes the forefront of the celestial Lion.

At the same time, be sure to admire Saturn, which, in eastern Taurus, is much higher than Jupiter in early evening, and is now rising before sunset. If you observe early enough in the evening, in settling twilight, you might also catch Mercury over in the southwest, as the little planet passes greatest eastern elongation to the Sun (when it is farthest in angle to the east of the Sun) on Christmas night. If you manage to stay up late enough, Mercury's inner partner Venus will climb above the eastern horizon about 3:30 AM, this closest planet to the Earth a glorious sight before dawn.

The big event of the week involves ourselves, as the Sun passes the Winter Solstice in Sagittarius at 7:14 PM Central Time (8:14 PM EST, 5:15 PST) the night of Saturday, the 21st, when astronomical winter begins in the northern hemisphere, summer in the southern hemisphere. At that time, the northern pole of the Earth's axis will be tilted as far as possible away from the Sun, the Sun will pass overhead at the Tropic of Capricorn (at a latitude of 23.4 degrees south), and will be as high in the sky as possible at the Earth's South Pole in Antarctica, where the weather may be a balmy freezing. The North Pole of course will not have seen the Sun for three months, and will not see it again for three months hence, until the Sun reaches the Vernal Equinox in Pisces.

Orion is now grandly climbing the eastern sky in the evening, and sits close to the meridian to the south at midnight. Follow his brilliant three-star belt up and to the right, and it passes rather close to Aldebaran and the Hyades cluster in Taurus, and then on up to the remarkable Pleiades, or Seven Sisters, star cluster. Most people can see perhaps six stars within the group, but a few with acute vision might see eight or even more. Oddly, few will see seven, as if the seventh is visible, so will be the eighth, the cluster a memorial to the Sisters of ancient mythology, the daughters of Atlas. How many can you see? The cluster provides us with an amazing illusion. Seeming small to the eye, it is actually twice the angular diameter of the full Moon, which occasionally passes in front of it.
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