In the Sky This Month – December 2009

This feature highlights a number of meteor showers, comets and asteroids which are visible during the month of December 2009. Jupiter continues its reign as not only king of the planets but king of the evening sky. Though not visible until about 10pm or so, Mars will start to contest Jupiter’s reign this month. December also brings the Geminids which are one of the best meteor showers of the year.

Note: If anyone has pictures or observations of these objects/events and want to share them, send me a comment and I’ll post them on the blog.

Planets

Mercury – Mercury will be an evening object this month. Though not a great apparition for northern observers, it should be easy to see if you have an unobstructed view of the southwestern horizon. The best time to see Mercury will be when the Moon passes near it on during the evenings of Dec 17 and 18. At that time Mercury will be magnitude -0.5. By the end of the month, it will be rapidly fading and getting closer to the Sun.

Dec 18 - Moon within 1.3° of Mercury

Jupiter – Jupiter continues to dominate the early evening sky. At magnitude -2.2, Jupiter is ~10 times brighter than the brightest stars in the sky this month. Of all the planets, only Venus, and on very rare occasions Mars, are brighter.

Jupiter is located high in the southwestern sky by the end of dusk for easy observation. As has been the case all year, it is slowly moving through the southern constellation of Capricornus.

Dec 21 - Moon passes 4° from Jupiter
Dec 21 - Jupiter passes 0.5° from Neptune

Neptune – For those with a telescope or binoculars and a dark sky, Neptune is located within a few degrees of Jupiter. This month Jupiter and Neptune will experience the 3rd of this year’s triple conjunction as they pass ~0.5° from each other on the night of Dec 20/21. For most of the month, the 2 planets will be very close to each other and within the same field of view of most small telescopes and binoculars. This is a great opportunity to use the easy-to-find Jupiter to help locate the usually hard-to-find Neptune. Jupiter will be a bright magnitude -2.2 while Neptune will be a faint +8.0. Even Jupiter’s 4 large Galilean moons are about a dozen times brighter than Neptune even though they are much smaller. The big reason for the faintness of Neptune is its distance from both the Earth and Sun. It is roughly 6 times further away from us and the Sun as Jupiter. The distance also explains its apparent small size of 2.3″. A good sized telescope will be required to see Neptune as anything other than a faint star.

Dec 21 - Jupiter passes 0.5° from Neptune

Uranus – Uranus is located in western Pisces and is bright enough to be seen in small binoculars at magnitude +5.9 but will still require a telescope in order to see it as anything other than a star (it’s disk is only 3.5″ across).

Mars – Mars can be seen rising in the eastern sky late in the evening (~9:30 pm at the start of the month and ~8 pm at the end of the month). Mars is rapidly brightening and will double in brightness this month as it rises from +0.0 to -0.7 magnitude. By the end of the month, Mars will be brighter than all stars expect Sirius and far southern Canopus. Mars will continue to brighten to a max of magnitude -1.3 at its opposition on Jan 29 of next year. This month Mars will appear roughly stationary as it starts its retrograde motion near the Leo/Cancer border.

Dec 6 - Moon passes close (5°) to Mars

Saturn – Saturn is easy to observe during the last few hours of the night. Located in Virgo at magnitude +0.9, Virgo only rises right before the start of dawn. Telescope users should note that Saturn’s rings are still close to edge-on.

Dec 10 - Moon and Saturn within 7° of each other

Venus – Venus will be too close to the Sun this month for most observers. Some with unobstructed views of the southeastern horizon may catch a glimpse of Venus rising 30-50 minutes or so before sunrise.

Dec 15 - Venus and Moon within 3° of each other

Meteors

December hosts one of the best showers of the year in the Geminids. In addition, the background rate of meteors is still rather high. The year is usually split in 2 with January through June having low rates with few major showers while July through December (really through the 1st week of January) have high rates with many major showers.

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, 10 or so Sporadic meteors can be observed per hour from a dark moonless sky.

Major Meteor Showers

Geminids (GEM)

Along with the Perseids of August, the Geminids are one of the best meteor showers delivering great displays year after year. This year’s Geminids are perfectly timed as the Moon will be nearly New and will not spoil the show.

According to Sirko Molau’s analysis of video data, the Geminids are already observable at the beginning of the month though their rates are very low. The peak is predicted for the night of December 13/14 though numerous meteors should be visible for a day or two on either side of the peak. With a radiant near the star Castor in the constellation of Gemini, the Geminids are one of the rare major showers that are observable before midnight and can be observed as early as 8:00 pm though rates are usually best after 10:00 pm. Under a dark rural moon-less sky, the Geminids can produce as many as 100+ meteors per hour. Observers under suburban skies will see lower rates.

The Geminids are the result of the break-up and subsequent activity of the “asteroid” Phaethon. Why asteroid in quotes? Most meteor showers come from comets yet Phaethon is on a very non-cometary orbit and has never shown any cometary activity. There is still much scientific discussion about what exactly Phaethon is.

More details on the Geminids and their parent “asteroid” Phaethon will be posted as we get closer to its peak.

Minor Meteor Showers

Minor showers produce so few meteors that they are hard to notice above the background of regular meteors. Starting this month, info on most of the minor showers will be provided on a weekly basis by Robert Lunsford’s Meteor Activity Outlook.

Ursids (URS)

The Ursids will produce up to 10 meteors per hour at their peak on December 22-23. That rate makes it a borderline major/minor shower though the Ursids have experienced a number of outbursts in the past. With a radiant near the “bowl” of Ursa Minor (the “Little Dipper”), this shower is also observable all night long though the best time to observe it is during the last hours of the night.

Additional information on these showers and other minor showers not included here can be found at the following sites: Wayne Hally’s and Mark Davis’s NAMN Notes, and the International Meteor Organization’s 2008 Meteor Shower Calendar.

Comets

Naked Eye Comets (V < 6.0)

None

Binocular Comets (V = 6.0 – 8.0)

None

Small Telescope Comets (V = 8.0 – 10.0)

Comet C/2007 Q3 (Siding Spring)

This long-period was first seen on 2007 August 25 by Donna Barton of the Siding Spring Survey in Australia. This past Oct. 7th the comet reached a rather distant perihelion at 2.25 AU from the Sun. Unfortunately, the comet and Earth are located on opposite sides of the Sun so the comet is rather far from Earth. Still the comet is observable in the early morning hours as a ~9.0 to 9.5 magnitude comet in Coma Berenices. At mid-month the comet is 2.38 AU from the Sun and 2.43 AU from Earth.

A finder chart for Comet Siding Spring can be found at Comet Chasing and Aktuelle Kometen (in German).

A nice collection of images can be found at the VdS-Fachgruppe Kometen (Comet Section of Germany) and Seiichi Yoshida’s Comet Homepage.

Comet 88P/Howell

P/Howell is an evening comet and currently the brightest in the sky. Howell is a short-period comet and takes only 5.49 years to orbit the Sun. Ellen Howell was a student at Caltech when she found the comet on photographic plates taken on 1981 August 29 with the 48″ Palomar schmidt.

In 1981 the comet was on an orbit that never brought it closer to the Sun than 1.62 AU (perihelion distance). As a result, it never got very bright. A relatively close approach to Jupiter in 1990 changed its perihelion distance to 1.40 AU. Further perturbations by Jupiter have decreased its perihelion distance to 1.36 AU. The closer perihelion distance allows the comet to get bright enough for small backyard telescopes. In addition, the comet seems to be running about ~2 magnitude brighter than usual. No obvious reason for the additional brightening has been observed yet.

This year perihelion occurred on Oct 12 so the comet is currently moving away from the Sun and should be fading. A day after perihelion I observed the comet from Tucson with a 12″ dobsonian. The comet was very difficult to observe from the city. At the time, I estimated its brightness at magnitude 8.5. The comet should be a fainter (95 mag or fainter) this month. The comet is located low in the southwest sky after dusk and will spend the monthcrossing Capricornus. At mid-month P/Howell will be 1.53 AU from the Sun and 2.02 AU from Earth.

A finder chart for Comet Howell can be found at Comet Chasing and Aktuelle Kometen (in German).

A nice collection of images can be found at the VdS-Fachgruppe Kometen (Comet Section of Germany) and Seiichi Yoshida’s Comet Homepage.

Asteroids

Binocular and Small Telescope Asteroids (V < 9.0)

(4) Vesta

Though not as large as Ceres, Vesta is more reflective making it the brightest asteroid in the Main Belt. Vesta is peculiar in that it appears to have evidence of volcanism on its surface. Similar to the Moon, Vesta may be covered with large expanses of frozen lava flows. It is classified as a V-type asteroid and is the only large asteroid with this classification. Many of the smaller V-type asteroids are chips of Vesta blasted off it by past asteroid and comet impacts. Vesta is similar in size to Pallas with dimensions of 347x336x275 miles or 578×560×458 km. Vesta will also be visited by NASA’s Dawn spacecraft which will arrive in 2010.

Vesta is once again observable in the morning sky before the start of dawn. It is brightening from magnitude 7.7 to 7.2 as it travels just north of Regulus in Leo.

A finder chart (needs to be flipped upside down for Northern Hemisphere observers) can be found at the Royal Astronomical Society of New Zealand. Finder chart for Vesta from Heavens Above.