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Astronomy Picture of the Day

Mars in Sagittarius

Photo of the Week.. Mars glides through Sagittarius.

Astronomy news for the week starting Friday, December 21, 2001.

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Prepared by Jim Kaler.

The lunar phase period is 29.5 days, just short of a month, which is no coincidence: the interval of the month actually came from the Moon's period, as did the word "month" itself. As a result, the quarters of the lunar phases closely match the week (which again is no coincidence, the week a combination of the interval of the lunar quarters and of the "seven moving bodies of the sky," the five planets known since ancient times plus the Moon and the Sun). We are now in brief time when Skylights' week closely begins with a new phase, this week with the first quarter, which formally takes place on Saturday, the 22nd. The Moon will thereafter wax toward full, which it will reach next week.

As the Moon moves, the lunar disk will once again occult, or pass across, Saturn. The event, which will be visible throughout the US, southern Canada, and Mexico, will take place the morning of Friday the 28th beginning on the west coast about midnight PST, in the mid-section of North America shortly after 2:30 CST, and on the east coast just before 4 AM EST. The best part is that since the Moon is in its waxing phase, the leading lunar edge will still be in night, so Saturn will appear to wink out before the bright Moon gets to it. The bad part is that the Moon will be very bright, rendering Saturn much harder to see.

Sunlight illuminates the Moon, and also heats and illuminates Earth. When the Sun is high we have a warm season, when low a cold one. At 1:21 PM Central Time (2:21 EST, 11:21 AM PST) on Friday, the 21st, the northern axis of the Earth leans directly away from the Sun, the Sun crosses the winter solstice in Sagittarius, winter begins in the northern hemisphere, and summer in the southern. Northerners will see the Sun as far south and as low in the sky at noon as it can get. The Sun will then gradually creep northward, but at first so slowly that the days will continue to chill as winter takes its firm grip.

With Mars hanging out in the evening southwest and Jupiter rising just after sunset, the northern winter stars are rapidly approaching the early evening sky. If very far south, below 30 degrees north latitude, you can see the bright star Achernar hitting the meridian about 8 PM, the star representing the end of Eridanus, the River. This amazingly long constellation begins at Chara near Orion, then winds back and forth through a large set of fainter stars to the west, where for those in mid-northern latitudes it drops below the horizon, Achernar one of those fabled stars that is never seen. Orion of course helps make up for it, as the great hunter of the ancient Greeks now rises just about the time of sunset. A couple hours afterward, watch for the southwestern rising of the luminary of the skies, brilliant Sirius in Canis Major, which shows that winter really is upon us.
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