Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.
It's the end of the year already - where did it go? December, as ever, brings those of us in the northern hemisphere to midwinter, where the Sun is to be found in its most southerly point in the Ecliptic. The Winter Solstice occurs this year on December 21st. This date will herald the shortest northern hemisphere day and its longest night - a bonus for us astronomers, if no-one else.
The Sun at transit point, 21st December, in western Sagittarius - the lowest point in the ecliptic as seen from the northern hemisphere. Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.
At the same point, those in the southern hemisphere will be experiencing the Summer Solstice - the longest day and shortest night, with the Sun riding high, at its most extreme elevation in the sky.
Wherever you find yourself, we wish you a peaceful end to the year, clear skies and all the best for 2022.
The Solar System
The Moon begins December as a 10% illuminated Waning Crescent in Virgo. Rising at just before 3.45am (GMT) and setting around 11 hours later.
New Moon occurs on the 4th, making the early and latter parts of December are prime for deep sky observations. The Moon becomes New joining the Sun in the non-zodiacal constellation of Ophiuchus, after which it becomes an evening object.
The evenings of the 7th and 8th finds the slim crescent of the Moon between the very bright Venus and Saturn and then Saturn and Jupiter on respective evenings. You’ll need reasonable S/SW horizons to have a good view of the first evening’s event, but less so the 8th’s coming together with Jupiter and Saturn, which occurs a little higher in the sky.
First Quarter occurs on the 11th, in Aquarius. On this evening (and the prior 10th), the Moon can be found near to Neptune, providing a guide to its location in the sky. However, light scatter from our much brighter natural satellite can make detection of the far fainter planet tricky.
The Moon at First Quarter, Dec 2021. Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.
The Moon becomes Full on 19th in Taurus, rising at just after 4 pm GMT and setting at a little before nine the following morning. This is the “highest” Full Moon of the year for those observers in the northern hemisphere. The Moon will appear to be nearly 64 1/2° elevation above the horizon at transit point (from 51° north). This will roughly coincide with the moons apogee (its farthest point from earth), meaning that this particular Full Moon will be the exact opposite of a so-called “Supermoon” and that this, coupled with its elevation above the horizon at transit, will cause the Moon to appear somewhat smaller than average at this time.
The moon will then appear to “roll down” the other side of the ecliptic plane as seen from the northern hemisphere, passing through Gemini, Cancer and Leo until it reaches last quarter in Virgo on December 27th. Luna libration at present causes this part of the month to be a particularly good one for those interested in observing the extreme western limb of our natural satellite, with features such as the Mare Orientale being more apparent than average.
We end 2021 with the moon in Scorpius, at a very Old Crescent phase, showing just 8.9% illumination in the morning sky. It will be just a couple of days into the New Year and the moon will become new again.
Mercury is emerging from superior conjunction, which occurred in late November and as such will be unobservable for the first part of December. However as we are fond of pointing out nothing stays the same for very long as far as the innermost planet is concerned.
By the latter part of the month Mercury will have re-emerged into the evening sky and will be observable very low in the Southwest as the Sun sets. On December 31 Mercury will be found at sunset at around 8 1/2° elevation above the horizon (from 51° north), shining at steady -0.7 magnitude and displaying a disk just under six arc seconds in diameter. At this point in time Mercury will be found in the same part of the sky as the much brighter Venus, which sits around 6° to the west. Venus will act as a convenient signpost for the fainter Mercury, making it easier to find from the naked eye perspective, or using binoculars or telescopes.
Mercury at sunset, 31st December. Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.
We begin December with Venus at practically maximum brightness of -4.7 magnitude. At this point in the month, the planets will appear as a 28% illuminated crescent in binoculars and telescopes, displaying a 39 arc second diameter. The planet will be unmistakable in the Southwest at sunset, as it is brighter than any other object in the sky (bar the Sun and the Moon). However, Venus, while bright, does not appear very high in the sky from a northern hemisphere perspective. The planet attains a reasonable altitude of just over 13° in the SSW at sunset on the 1st of December, but those in built-up areas will need reasonable horizons to be able to see it well.
Venus, sunset 1st December, flanked by Jupiter and Saturn. Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.
Venus reached maximum elongation from the Sun in November, and is now headed back towards our parent star from our perspective here on Earth. On the 1st December, Venus is separated from the Sun by just over 41°. By mid-December this separation will have diminished to just over 32°, though as Venus has now gone past its lowest point in the ecliptic plane (from the northern hemisphere perspective) its altitude from temperate northern areas at sunset increases a little, to over 14° (from 51° north). By this point in the month Venus will have decreased in brightness fractionally to -4.6 magnitude.
The end of December finds Venus having faded a little further to a brightness of -4.3 magnitude, now showing a very large arc minute-sized disc, but with a sliver-thin crescent phase of just 2.6% illumination. The planet will sit around 9 1/2° above the horizon at sunset (from 51° north). At this point Venus will be separated from the Sun by just over 13 1/2°, and will be headed towards imminent inferior conjunction on January 8, 2022.
The red planet is a morning object in December though at +1.6 magnitude and just 3.8 seconds diameter on the first, will appear a very disappointing site to any observer. Nothing changes very much during the month as far as Miles is concerned. We end 2021 with Mars having brightened fractionally to +1.5 magnitude, now displaying a still-tiny 4 arc second disk. It is joined by the very Old Crescent Moon in the same area of sky at sunrise on the 31st.
Mars, alongside the Old Cresent Moon, sunrise, 31st December. Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.
Jupiter is to be found in Capricornus on 1st, shining at a bright -2.3 magnitude, displaying a 38 arc second diameter disk. Reaching transit point at a little over an hour after sunset, Jupiter stands at an altitude of just under 25 1/2° (from 51° north). The giant planet’s separation from the Sun is just under 76°, at this point in time. Jupiter will set at a little after 10:15 pm on the 1st December.
By mid-month, Jupiter has faded a little to-2.2 magnitude and is now displaying a 36.8 arc second diameter disk. It will transit at 4:35 pm - a little under half an hour after sunset. Jupiter’s separation from the Sun on the 15th is now just under 64 degrees and the planet sets at just after 9:30pm (GMT).
Jupiter ends the year in the constellation of Aquarius, at a visual magnitude of -2.1. It will now show a 35 arc second diameter disk. The planet will set at around a 8:45 in the evening (GMT).
Notable Jovian transit events during December include a Europa transit around 5 pm on 3rd December; a very slim window of mutual Great Red Spot and Io transit, just before 6 pm on the 7th; Another Europa transit at just before 7 pm on the 10th; a nice mutual Great Red Spot, Io and Io shadow transit at just before 5 pm on the 16th - an event which re-occurs at just before 6pm on the 23rd. Further events include a mutual Great Red Spot and Europa transit starting at a little before 5 pm on December 28th, which is followed by a Ganymede transit at around the same time the following day. There’s another mutual Great Red Spot and Io transit to observe, which starts just before 7 pm on December 30th. All times GMT.
Jupiter, Great Red Spot and Io and Io shadow Transit, 5pm, 16th December. Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.
The ringed planet is to be found some 16 1/2° to the west of Jupiter on the 1st. Being over 15° to the west of Jupiter in the sky, means Saturn will set just over an hour before its neighbouring world. At +0.7 magnitude and displaying 16 arc second diameter disc, Saturn is by no means as bright nor as prominent as Jupiter, though is easy enough to find, as it appears about equidistant between it and the very bright Venus.
Saturn is just over 59° separation from the Sun on the 1st and attains a height of just over 20 1/2° above the horizon (from 51° north) at transit point, which it reaches at 4:17 pm (GMT). Although some way past opposition, Saturn is always a lovely sight in a telescope and those with a facility to observe it during the early evening in the earlier parts of December will doubtless be rewarded with a decent view. However, as we have pointed out in previous sky guides Saturn is still pretty low in the south from a temperate northern hemisphere perspective, so observers with telescopes are probably best advised to use modest medical magnification when it comes to Saturnian observation at this point in time. The observational window for Saturn is reasonably short, as it sets at a little after 8:40 pm (GMT) is on the 1st.
By mid month, Saturn will have shrunk a little to just under 16 arc seconds diameter, but will still display the same brightness as it did at the months beginning. By this point in time Saturn will set out a little before 8 pm (GMT).
Saturn and Moons, sunset, 15th December. Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.
As we fast forward to the end of December, Saturn will appear much the same as it did at the beginning of the month, standing a little lower in altitude, at 17.4° (from 51° north. However, the planet now sets at 7 pm, meaning that the window for evening observation of Saturn is surely closing.
Uranus and Neptune
The outer gas giants are both well-placed for observation in the evenings. Neptune being further west in the ecliptic in the constellation of Aquarius is always the more challenging of the two to observe: at +7.9 magnitude and 2.3 arc seconds diameter is now some way past opposition, but still close to its best for this year (as we’ve noted before, neither outer gas giant changes dramatically in brightness or diameter, even when fairly far from a opposition). Requiring a telescope to see any sign of its diminutive disc, Neptune can never be seen with the naked eye. However, those with larger binoculars can easily make it out amongst the background stars of Aquarius and even a relatively small telescope will show its rather vibrant blue colour. In the middle of the month, Neptune transits at around astronomical dusk (a little past 6pm GMT), at which point the planet stands just over 34 1/2° above the horizon (from 51° north).
Uranus and Neptune relative positions 15th December. Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.
Further east in the ecliptic in the constellation of Aries, Uranus at +5.7 magnitude and 3.7 arc seconds diameter, is a much easier target. Technically, a naked eye object for those with good eyesight and exceptional sky conditions, Uranus can be found readily with binoculars and a small telescope will easily show its tiny green-grey disc. Around mid month Uranus transits at a little after 9 pm. At transit point, the planet will stand just over 54° in the altitude (from 51° north).
Comet C/2021 A1 (Leonard) continues to show some progress though at time of writing is still some significant margin below naked eye visibility. The comet is to be found in the eastern part of Canes Venatici at the months beginning though swiftly makes the transition over the border into the constellation of Bootes, the herdsman, passing its principal star, the bright Arcturus, on the 6th December. It will be best seen in the morning sky in the early part of the month, but rapidly heads Sunward, significantly decreasing its separation from the horizon as it does. This should go hand-in-hand with a steady increase in brightness as the comments is predicted to reach a peak in mid December. However, as it reaches peak brightness the comet will be very close to the Sun from our perspective here on Earth and the latter part of the month will see it skirt the horizon during the night making it a difficult target to observe. The comet's closest approach to the Sun as seen from our perspective here on Earth occurs on the 14th to the 15th of December at which point the comment will be just under 15° from the Sun. This will make observation of it more difficult, even if brightness is significant at this point.
Comet Leonard's path during the first part of December. Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.
Maximum brightness estimates for C/2021 A1 vary from around +5 magnitude to +3.5 magnitude. Emphatically, this will not be a show-stopper, like the recent Comet Neowise was - but should hopefully be able to be located in binoculars, at its best. Though during the latter part of the month it will be interesting to see how the comet appears in the evening sky, especially as it passes close (5 1/2° south) of the planet Venus on the 17th and 18th of December, making its location relatively easy to find on these dates.
The comet is predicted to fade quite rapidly during late December, which means the window for “easy” observational opportunities is reasonably narrow. C/2021 A1 will appear to loop back on itself in early January before diving south, disappearing from view from the northern hemisphere, by which time it will have faded even further.
As ever, as far as comets go, we always remind people not to get too overexcited - but it will certainly be interesting to see what sort of a show C/2021 A1 puts on in December. While we never know what’s really around the corner for us, cometary-wise, C/2021 A1 seems to be our best hope for some time to come.
The annual spectacle of the Geminid Meteors, which peak on the night of the 13th-14th December, are always worth looking forward to. Peaking at anything up to 100 meteors per hours (not all of which will be visible from any given location), the Geminids are arguably the most reliable shower of the year, fed by the mysterious "rock comet" asteroid 3200 Phaethon. The shower is expected to be visible from 4th/5th to the 17th December this year.
A Geminid meteor captured over SW London. Image credit: Kerin Smith
The Geminids radiate from an area inside the constellation of Gemini and are usually very well seen from the northern hemisphere. 2021’s shower is not a best case scenario in terms of the influence of moonlight, with the 10 day old Waxing Gibbous Moon lurking in Pisces. However, those who are willing to brave the small hours, post-moonset (just after 3am GMT on the morning of the 14th) will be presented with a much darker sky and significantly better conditions for observing and photography of the shower. The Geminids present great opportunities for astrophotographic record - all you need is a solidly mounted camera, capable of timed exposures, with a reasonably wide field lens. Once set up - even in a fairly light polluted environment - you will be unlucky not to capture a couple of brighter meteors, given an hour-or-so’s multiple exposures. The brightest of the Geminids will cut through even the worst influence of light pollution.