Due to exam preparation, this forecast is arriving late. Now, it can finally meet everyone. The night sky in the last month of 2009 has not become quiet due to the arrival of harsh winter. Wonderful astronomical phenomena will follow one after another, and I believe the cold weather cannot stop the enthusiasm of astronomy enthusiasts. Of course, enthusiasm alone is not enough; cold protection measures must be taken. It would be unfortunate to spend a week on an IV drip for the sake of one night of observation.
December 22 is the Winter Solstice, which means the Northern Hemisphere has reached the period of the longest nights, and the observable time has also reached its maximum! Here, I also hope everyone arranges their observation time reasonably, pays attention to rest, and avoids overexertion. I wish everyone can better enjoy the fun of astronomy in this best month, thus perfectly ending this International Year of Astronomy!
December: The Sun moves from Scorpius to Sagittarius
01: Moon occults the Pleiades
06: Phoenicids meteor shower peak (17:30, ZHR=?)
07: Puppid-Velids meteor shower peak
09: Monocerotids meteor shower peak (ZHR=2)
10: Saturn in conjunction with the Moon
12: Spica in conjunction with the Moon
12: \sigma-Hydrids meteor shower peak
14: Geminids meteor shower peak (13:10, ZHR=120)
15: Moon occults Antares
16: Venus in conjunction with the Moon
18: Mercury in conjunction with the Moon
19: Mercury at greatest eastern elongation
21: Neptune and Jupiter in conjunction with the Moon
22: Ursids meteor shower peak (21:30, ZHR=10–50)
24: Uranus in conjunction with the Moon
30: Coma Berenicids meteor shower peak
Lunar Phases:
Full Moon: 02, 15:30
Last Quarter: 09, 08:13
New Moon: 16, 20:02
First Quarter: 25, 01:36
Comet Activity:
06: Comet 100P/Hartley at perihelion
December 14: Geminids Meteor Shower Peak
It is precisely because of the existence of the Geminids that the night sky in December never lacks highlights. Along with the Quadrantids and Perseids, the Geminids are known as one of the three major meteor showers of the Northern Hemisphere. They are not only high in volume but also very stable. At the same time, they have characteristics such as many bright meteors, good observation effects, and a long peak duration. For those of us in the mid-latitudes of the Northern Hemisphere, the radiant point of this meteor shower is visible almost all night during the peak, reaching an altitude of over 70 degrees at midnight, making it one of the best meteor showers for observation throughout the year. In the Southern Hemisphere, the nights are shorter, so the window for good observation is limited. The active period of the Geminids is not very long; this year, we may see meteors from this group on nights between December 7 and 17. According to the International Meteor Organization, the peak this year is expected around 13:00 Beijing time on the 14th, with a ZHR of about 120. Observations over the past 20 years show that the peak can last for more than two hours. For fainter meteors, the high volume can even last for about a day. Therefore, for astronomy enthusiasts across China, the nights of the 13th and 14th are favorable for observation. This year, the peak coincides with the 27th and 28th of the lunar month, so the waning crescent moon, which rises only at dawn, will not affect observations. Based on our analysis of the Geminids two years ago, 2007 had almost no moonlight interference and saw a burst with a ZHR of over 150. Although 2008 had serious moonlight interference, we still observed over a hundred meteors in one night, making this shower very much worth observing.
Next, let’s talk about how to actually observe the Geminids. First, check the weather forecast two or three days in advance and prepare for observation only after confirming clear skies. The observation site should ideally be at a high altitude, far from urban light pollution, which requires attention to safety and cold protection. You can find an open farmhouse courtyard, preferably with heating facilities for resting. If you previously focused on visual observation, you can try taking beautiful meteor photos during the Geminids.
The equipment needed for photographic observation mainly includes a digital SLR camera, a wide-angle lens, a tripod, a shutter release cable, and enough batteries and memory cards. We can start by using bright stars to focus. After setting up the tripod, we frame the shot. Framing is an art; it is best to place the radiant point in the field of view, and including some characteristic landscape is also a good choice. Of course, the landscape should ideally have some slight illumination, otherwise it will appear too dark in the photo. Next, we adjust the shooting parameters. My experience is to stop down the maximum aperture by one stop or not at all, reduce the camera’s highest ISO by two to three stops, set the white balance to daylight, and set the exposure time to 10 to 20 seconds, as long as the background sky light is not too bright. Most importantly, set the camera to continuous shooting mode and use the shutter release cable to lock it. Facing the fleeting meteors, we can only "wait for the rabbit."
December 19: Mercury at Greatest Eastern Elongation
The last greatest elongation of Mercury this year will also take place this month, bringing a successful conclusion to the seven greatest elongations of the year. The greatest eastern elongation on December 19 is already the fourth eastern elongation of the year. On that day, the angular distance between Mercury and the Sun is about 20 degrees, and its declination is slightly lower than the Sun’s. Taking the region around 40 degrees north latitude as an example, Mercury’s altitude at sunset will be about 12 degrees. Whether these conditions are good or not is a matter of opinion. If the weather is clear and the atmospheric transparency is high, and you can find a very open area to the west, seeing Mercury in the twilight should not be a problem since its brightness can reach magnitude -0.5.
It is worth mentioning that although this greatest eastern elongation occurs on the 19th, its declination will gradually increase thereafter, and the maximum altitude at sunset will occur on the 21st. Therefore, we have the opportunity to see Mercury for several consecutive days after the 19th. Observing Mercury during greatest elongation is relatively simple with photography or a telescope, but finding it with the naked eye is more challenging. Opportunity is rare, and luck is also important. To be honest, so far, the author has only seen Mercury with the naked eye three times.
December 22: Ursids Meteor Shower Peak
Compared to the Geminids, the Ursids are much less famous. Naturally, the level of attention a meteor shower receives is proportional to its volume. Normally, the ZHR of the Ursids is only about 10, and even during larger outbursts, it only reaches about 50. In the past 70 years, outbursts occurred only in 1945 and 1986. In 2006 and 2007, the ZHR was only around 35 due to serious moonlight interference.
The active period of the Ursids is from December 17 to 26, with a possible larger volume on the 22nd Beijing time. However, due to low temperatures and low attention, there is very little observational data for the Ursids, making it difficult to define or provide precise forecasts. There are several different forecasts for the peak time this year: the first says the peak will occur between 21:30 and 24:00 on the 22nd Beijing time. If so, it would be excellent for observers in China because the radiant point is near the North Celestial Pole and visible all night, and the moon phase is near the first quarter, meaning the moon will set in the first half of the night. The second forecast, based on the model by scientists Jenniskens and Jeremie, suggests that the perihelion passage of the parent comet 8P/Tuttle in January 2008 has an important impact on this year’s shower. This forecast indicates the peak might occur around 15:00 Beijing time on the 22nd, with a ZHR of about 14, and will be very brief, making it more suitable for observers in North America and the North Pacific.
(Part of the content is from "Astronomy Enthusiasts", Skylook.org, and other online resources; images are from Stellarium; organized by Scientific Space.)
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