December 2012 Highlights * Geminid meteor shower peaks on the night of December 14/15 * Jupiter bright in the eastern evening sky * Jupiter and the Moon make a spectacular Christmas pair * Comet C/2012 K5 (LINEAR) brightens to 8th magnitude Note: If anyone has pictures or observations of these objects/events and want to share them with my readers, send them to the Transient Sky at <transientsky1@yahoo.com>.
Planets
Evening Planets
Mars – One needs a very clear southeast horizon to catch Mars in the early evening. Glowing at magnitude +1.2 Mars is currently not as bright as other planets. But it is traveling through a patch of sky with few bright stars so it is rather apparent as the brightest, reddest, “star” in that part of the sky. It spends most of the month against the stars of eastern Sagittarius before approaching Capricornus at the end of the month. The Moon passes near Mars on the evening of the 14th.
Jupiter – Jupiter reaches opposition on December 2nd. At that time it is at its brightest (magnitude -2.7) and closest to Earth (4.07 AU). It spends the month slowly retrograding just north of the stars of the Hyades cluster in Taurus. At the start of the month it rises in the northeast at sundown. By the end of the month, it is well up in the eastern sky by nightfall. On Christmas night, the Moon and Jupiter will make a beautiful pair.
Morning Planets
Saturn – Saturn is an early morning object rising around 4:30-5:00 am at the start of the month. By the end of the month, Saturn rises 2 hours earlier. All month Saturn glows at magnitude +1.3. The Moon passes close to Saturn on the mornings of the 10th.
Venus – Venus rises around 5:00 am on the 1st and 6:00 am on the 31st. In a telescope the planet will appear more than half-illuminated (about 90%). At magnitude -3.8, Venus is by far the brightest ‘star’ in the morning sky. The Moon passes to the south of Venus on the morning of the 11th. Venus will become more and more difficult to observe over the coming months as it approaches conjunction with the Sun.
Mercury – Mercury is the last of the morning planets to rise. On the 1st it is magnitude -0.2 and rises around 5:30 – 6:00 am. It is still observable peaking above the southeast horizon just before day break through the middle of the month.
Meteors
The year is usually split in 2 with January through June having low rates with few major showers while July through December have high rates with many major showers. Meteor activity is still near an annual maximum this month.
Sporadic Meteors
Sporadic meteors are not part of any known meteor shower. They represent the background flux of meteors. Except for the few days per year when a major shower is active, most meteors that are observed are Sporadics. This is especially true for meteors observed during the evening. During December mornings, 10 or so Sporadic meteors can be observed per hour from a dark moonless sky.
Major Meteor Showers
Geminids (GEM) [Max Date = Dec. 13, Max ZHR = ~120 per hour]
The night of December 13/14 will bring the peak the Geminids, one of the year’s better meteor showers. It’s usually a toss up as to which is better, the Perseids of August or the Geminids, though lately the Geminids have been routinely out-producing the Perseids. If the sky is clear it will provide one of the few nights of the year when it’s almost guaranteed that you will be able to observe a meteor after about 10-20 minutes of observing.
From a dark, moon-less sky, the Geminids have been known to consistently produce rates of over 100 meteors per hour at their peak. Unlike most showers that can only be observed in the early hours of the morning, the Geminids can be seen in good numbers as early as 10 pm and are great anytime after midnight. The Geminids will appear to radiate from the constellation of Gemini near the bright star Castor.
According to analysis of meteor video data by Sirko Molau, the Geminids are active for almost an entire month between the dates of November 23 and December 21. Though high rates are only possible within a few days of the peak.
Most meteor showers are produced by comets with orbits that extend out to the orbit of Jupiter or beyond. The Geminids are different. They are on a very short uncomet-like orbit that extends from a very close 0.14 AU from the Sun to a not so far 2.40 AU.
1) Phaethon could be a comet whose original orbit evolved into its current one after many millennia of close approaches with the inner planets. Some models of the formation of the Geminids require the shower particles to be released over many centuries to millennia. This is consistent with the behavior of a comet.
2) Phaethon may be a Main-Belt comet. Main-Belt comets are objects that originate in the outer Asteroid, or Main, Belt. Since they contain a sizable fraction of volatile material (water, carbon monoxide, etc.), they can occasionally exhibit cometary activity. Four of these objects have been observed to display cometary activity in the Main Belt. Since they start on asteroid orbits, it is not too difficult for one of them to find itself on an orbit similar to Phaethon.
3) Phaethon is an asteroid that broke up in the past. There is evidence to suggest that Phaethon is just the largest piece of a past break-up. In fact, two additional asteroids that may once have been a part of Phaethon have been found, (155140) 2005 UD and 1999 YC. According to Peter Jennisken’s book “Meteor Showers and Their Parent Comets”, the Geminids can be explained by the break-up of Phaethon just after perihelion many orbits ago. Since Phaethon gets to within 0.14 AU (14% of the Earth-Sun distance), perhaps it split under the intense solar heat. BTW, this scenario does not rule out Phaethon as a ice-rich Main-Belt comet.
The recent discovery of additional asteroids related to Phaethon points to scenario 3 as the most likely origin of the Geminids. If true, the Geminids were not the result of long-term cometary activity like most meteor showers but were created in a single event when Phaethon split or shed a smaller piece. The Daytime Sextentids and perhaps the very minor Canis Minorids were created by even older break-up events.
The video below contains 159 meteors detected by my SALSA3 camera during the 2010 Geminids peak over Tucson. It starts in the evening where you can catch a short glimpse of the Moon as it quickly moves out of the frame. Note the top of one of my trees illuminated by Christmas lights for the first few meteors. Though not all of the meteors are Geminids most are and it is fun to watch them radiate from the twin stars of Castor and Pollux.
Minor Meteor Showers
Minor showers produce so few meteors that they are hard to notice above the background of regular meteors. Info on many minor showers are provided on a weekly basis by Robert Lunsford’s Meteor Activity Outlook.
Additional information on these showers and other minor showers not included here can be found at the International Meteor Organization’s 2012 Meteor Shower Calendar.
Comets
Naked Eye Comets (V < 6.0)
None this month.
Binocular Comets (V = 6.0 – 8.0)
None this month
Small Telescope Comets (V = 8.0 – 10.0)
C/2012 K5 (LINEAR)
This is a long-period comet discovered by the LINEAR near-Earth asteroid survey program on May 25, 2012. At the time the comet was around magnitude 17-18. Though it passed through perihelion on Nov. 28 at a distance of 1.14 AU, the comet will become brighter this month as it rapidly approaches the Earth. Close approach will occur at the very end of December at a distance of 0.29 AU (27 million miles or 44 million km).
Recent visual observations place the comet around magnitude 10.0 at the end of November. The comet should brighten by another 2 magnitudes by the end of December. This month it is primarily a northern hemisphere object and spends much of the month running the length of the Big Dipper from the handle to the bowl before rocketing southward through the rest of Ursa Major into Lynx. The fact that it spends the first 3 weeks of the month among the familiar stars of the Big Dipper should aid many people in seeing the comet.
The comet starts the month as a small telescope object but by the middle of the month it should be visible in binoculars for observers under a dark sky. For bright sky observers a small telescope may still be needed.
I did observe the comet back on Oct. 21 with the University of Arizona’s Kuiper 1.5-m telescope. Though still over a month from perihelion and located low in the sky, the comet displayed an impressive tail and even some near-nucleus jet activity.
Ephemeris for C/2012 K5 (LINEAR) Date RA DEC Delta r Elong V 2012 Dec 01 14h 03m +45d 41' 0.881 1.142 75 9.9 2012 Dec 11 13h 33m +50d 48' 0.643 1.158 88 9.3 2012 Dec 21 11h 53m +60d 05' 0.418 1.196 110 8.7 2012 Dec 31 06h 48m +49d 48' 0.294 1.253 153 8.0 RA = Right Ascension, DEC = Declination, Delta = distance from Earth in AU r = distance from the Sun in AU, Elong = elongation from Sun in degrees V = Visual magnitude
The Transient Sky - Comets, Asteroids, Meteors 
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