Friday, December 4
Although the calendar says December, the Summer Triangle of bright stars remains prominent during the early evening. Look for this large asterism in the western sky after darkness falls. More than a third of the way to the zenith lies the Triangle’s brightest member, magnitude 0.0 Vega in the constellation Lyra. Approximately 25° above Vega stands magnitude 1.3 Deneb in Cygnus. And scan 35° to Vega’s left to see magnitude 0.8 Altair in Aquila. Altair sets shortly after 9 p.m. local time, destroying the geometric pattern.
Saturday, December 5
Although people in the Northern Hemisphere won’t experience the shortest day of the year for another two weeks (at the winter solstice December 21), those at 35° north latitude will see the Sun set earlier today than at any other time this year. Tomorrow’s sunset will arrive less than a second later than today’s. The date of earliest sunset depends on latitude — the farther north you live, the closer it occurs to the solstice.
The Moon reaches apogee, the farthest point in its orbit around Earth, at 9:56 a.m. EST. It then lies 251,531 miles (404,799 kilometers) from Earth’s center.
Sunday, December 6
Look for the waning crescent Moon below Mars this morning. From North America, the two objects passed closest to each other yesterday evening (before they rose), but they nevertheless make a pretty pair in the predawn darkness. The Red Planet lies between Venus and Jupiter all week, though it appears significantly fainter than its companions. Still, at magnitude 1.5, it glows brighter than most stars and stands out for its ruddy color. When viewed through a telescope, Mars spans just 5" and shows little detail.
Monday, December 7
The waning crescent Moon hangs lower in the sky this morning, where it makes a spectacular pair with Venus. The two are well clear of the horizon by 4 a.m. local time, and by 30 minutes before sunrise, they have climbed one-third of the way to the zenith. If you keep watching during the predawn hours, you’ll no doubt notice the Moon edging closer to the planet. After sunrise, Luna passes directly in front of (occults) Venus for observers across North America. Binoculars or a telescope will deliver wonderful views of this occultation. The timing of when the planet disappears behind the Moon’s sunlit limb depends of your location. Sites on the West Coast experience the event shortly after sunrise while those on the East Coast see it between noon and 1 p.m. local time.
Tuesday, December 8
Shining at magnitude –2.0, Jupiter stands out among the background stars of western Leo the Lion from the time it rises shortly after midnight local time until twilight is well underway. This week, Jupiter lies more than halfway to the zenith in the south-southeast as morning twilight commences. A telescope reveals the gas giant’s 36"-diameter disk and at least two parallel dark belts in its dynamic atmosphere. And this morning, you should notice a black dot silhouetted on the cloud tops. This is the shadow of Jupiter’s outermost major moon, Callisto. It starts to cross the planet’s disk at 4:17 a.m. EST and doesn’t exit until nearly four hours later.
Wednesday, December 9
The variable star Algol in Perseus reaches minimum brightness at 6:20 a.m. EST. If you start watching the star the evening of December 8, you can see it dim by 70 percent over the course of about five hours. (Astronomers would say its brightness drops from magnitude 2.1 to 3.4.) This eclipsing binary star runs through a cycle from minimum to maximum and back every 2.87 days. Algol appears nearly overhead in late evening and dips low in the northwest as dawn approaches.
Thursday, December 10
A 10th-magnitude asteroid passes close to the bright galaxy M77 in Cetus the Whale this week. Both last night and tonight, 39 Laetitia appears on the fringe of this spiral. At first glance, the solar system object might look like a supernova in the outer arms of this galaxy, which lies nearly 50 million light-years away. You can pick them up through a 4-inch telescope from a dark site or an 8-inch instrument from the suburbs. The pair lies less than 1° east-southeast of the 4th-magnitude star Delta (δ) Ceti.
Friday, December 11
The Geminid meteor shower ramps up nicely this week. Although the shower doesn’t peak until December 14, you should start to see several Geminids during the overnight hours, particularly now that the Moon has left the morning sky. To tell a Geminid meteor from a random dust particle burning up in Earth’s atmosphere, trace the streak of light’s path backward. A Geminid will appear to originate from the constellation Gemini the Twins.
New Moon occurs at 5:29 a.m. EST. At its new phase, the Moon crosses the sky with the Sun and so remains hidden in our star’s glare.
Saturday, December 12
Comet Catalina (C/2013 US10) has brightened to 6th magnitude and appears nice through binoculars and telescopes for those up before the Sun. The comet lies in eastern Virgo, a region that rises shortly after 3 a.m. local time and climbs some 30° high in the southeast by the time twilight starts to paint the sky. This morning, it stands 2° due north of 4th-magnitude Iota (ι) Virginis. It will continue to climb northward for the next several weeks and pass within 1° of Arcturus in early January.
Sunday, December 13
There’s a lot of activity on December mornings. But now that the Moon has passed between the Sun and Earth and has exited the predawn sky, no object can rival Venus. The brilliant planet shines at magnitude –4.1, two full magnitudes brighter than its closest competitor, Jupiter. Venus currently lies among the much fainter background stars of western Libra, having crossed over from Virgo on December 11. If you point a telescope at the planet, you’ll see a 16"-diameter disk that appears nearly three-quarters illuminated.