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Lunar Halo

Photo of the Week.. A part of the 22-degree halo, caused by moonlight refracting through ice cyrstal clouds, surrounds the Moon. Saturn shines off to the right.

Astronomy news for the week starting Friday, March 29, 2002.

Prepared by Jim Kaler.

The Moon moves through its waning gibbous phase during most of the week, finally achieving last or third quarter on Thursday, April 4, at which time we see a "half moon," demonstrating one of the many curious nomenclature anomalies in astronomy. The "half moon" refers to the half-illuminated disk, while the "quarter moon" refers to the quartering of the lunar orbit. The "first quarter" occurs when the Moon is 90 degrees around its orbit from the new phase, the third quarter when it is 270 degrees around. Full Moon could just as well be called the "second quarter," though the term is never used. With the Sun just past the vernal equinox in Pisces, this third quarter Moon will be the most southerly of the year and will appear just past the winter solstice in Sagittarius. A couple days before the third quarter, the night of Monday, April 1, for those in North America the Moon will lie just up and to the right of Antares in Scorpius and almost on top of Dschubba (Delta Scorpii). Keep your eye on the star, as it is brightening. Turning into a "B- emission" star, Dschubba is now nearly first magnitude, and perhaps will actually rival Antares. The star is doing what Gamma Cassiopeiae did over 70 years ago, and will probably someday settle back to normal.

Venus is now brilliant and unmistakable in evening twilight. As April begins, this second planet out from the Sun "crosses the line" and sets just as twilight formally ends. Above it is reddish Mars, which sets just before 10 PM. Higher still is Saturn in Taurus. As the month ends, Sunday, March 31, the ringed planet will pass four degrees north of the first magnitude star Aldebaran in Taurus. Saturn, the brighter of the two and a bit yellowish in color as a result of reflection of sunlight from its cloudy surface, makes a fine contrast with coolish Aldebaran's more orange color. Higher still, and not setting until after 1 AM, is Jupiter in Taurus.

As March gives way to April, the winter stars escape to the west. Sirius, which dominates winter with its white sparkle, leads the eye to the south and the ancient constellation Argo, the Ship. Almost immediately below Sirius those who live below about 35 degrees south latitude can see the sky's second brightest star, Canopus, in Carina, Argo's keel. Between, and a bit to the east of, the two stars, find Puppis, the Stern, and more to the east Vela, the Sails. Directly above Sirius is the dim modern constellation Monoceros, the Unicorn, and to the west, Lepus, the Hare, Orion the Hunter's smallish prey.
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