The sky in December and January

Natural History, Dec, 2004 by Joe Rao

Between December 16 and January 5 all five "naked eye" planets are visible in the early morning sky. Intriguingly, through December 28, about an hour before sunrise, they are stretched out across the sky in the "right" order: Mercury, Venus, Mars, Jupiter, and Saturn.

Mercury is invisible in the glare of the Sun for the first half of December. It is easiest to see between the 22nd and January 3. To an observer at forty degrees north latitude, the planet is at least ten degrees above the southeastern horizon a half hour before sunrise; farther south, the view gets even better. On the 29th Mercury shines at magnitude -0.3 as it reaches its greatest elongation west of the Sun.

In the first week of January Mercury continues to shine near Venus. Thereafter, Mercury rises later with each passing morning and becomes obscured by the twilight.

Throughout December Venus is low in the southeast as dawn brightens.

In January Venus doesn't even rise until morning twilight is under way, but it is still bright enough to find easily, low in the east-southeast before sunrise. The planet becomes harder to see as the month progresses, and by month's end Venus doesn't rise until mid-twilight.

Mars is in the constellation Libra, the scales, as December begins, escorting Venus across the morning sky. By month's end, Mars rises more than two-and-a-half hours before the Sun.

In January Mars races eastward from the head of the constellation Scorpius, the scorpion, into the constellation Ophiuchus, the serpent-holder, by the 5th. The planet passes nearly five degrees north of the star Antares, which the ancients considered Mars's rival, on the 8th: the red star outshines the red planet by more than half a magnitude.

Jupiter, in the constellation Virgo, the virgin, rises due east a little after 2 A.M. at the beginning of December and about 100 minutes earlier by New Year's Eve.

The waning Moon occults, or passes in front of, Jupiter and its four Galilean satellites on the 7th. The event, a rare and striking one to watch, is visible in the United States and southern Canada east of a line extending from roughly Edmonton, Alberta, south to Tucson, Arizona. A swath roughly thirty-three miles wide that runs east from Marfa to Matagorda, Texas, defines the southern limit of the occultation's zone of visibility within the U.S. Within that region the Moon partly occults Jupiter, though the extent of occultation diminishes as one's vantage moves south; south of that swath the Moon misses Jupiter entirely. You may be able to watch Jupiter disappear--an event known as immersion--without optical aid, though binoculars help. The spectacular reappearance of the planet--emersion--should be plainly visible to the unaided eye [see "When Will I See You Again?" by Peter Brown, page 64]. The timetable below gives details for a few cities. The Web site www.lunaroccultations.com/iota/1207jupiter.htm lists specific times for more than 200 cities.

In January Jupiter rises soon after 12:30 A.M. at the start of the month and about two hours earlier by month's end. The best time to observe the planet is is before dawn, when it is high in the south.

Saturn is in the constellation Gemini, the twins, in December. It rises from the east-northeastern horizon around 7:45 P.M. at the start of the month and more than two hours earlier by month's end.

In January Saturn reaches opposition to the Sun on the 13th; it rises at sunset and sets at dawn. The famous rings are tipped more than twenty-two degrees to our line of sight and are certain to delight anyone lucky enough to get a telescope as a holiday gift.

In December the Moon wanes to last quarter on the 4th at 7:53 P.M. and becomes new on the 11th at 8:29 P.M. It waxes to first quarter on the 18th at 11:40 A.M. and to full on the 26th at 10:06 A.M.

In January the Moon wanes to last 0quarter on the 3rd at 12:46 P.M. and becomes new on the 10th at 7:03 A.M. It waxes to first quarter on the 17th at 1:57 A.M. and to full on the 25th at 5:32 A.M.

The Geminid meteors make their annual December appearance, and this year promises a fine display, with no Moon to spoil the view. The meteors can be seen all night long for a week or so, but activity should peak on the night of December 13-14, particularly in the predawn hours.

A newly discovered comet, comet Machholz, appears early this winter. Comet Machholz could become as bright as fourth magnitude and remain so for about a month beginning right after Christmas. At that brightness the comet should be at least dimly visible to the naked eye in dark skies and more easily seen with binoculars or a telescope. Look for it as it tracks from the southern end of the constellation Taurus, the bull, on up into the constellation Perseus by mid- to late January. The comet's closest approach to Earth comes on the night of January 5-6, when it is just 32 million miles away.

The solstice occurs at 7:42 A.M. on December 21. Winter begins in the Northern Hemisphere; summer begins in the Southern Hemisphere.

All precise times given an eastern standard time.