The Sky Above You, December 2024
by Duncan Lunan
The Moon is New on December 1st and 30th, and Full on December 15th. The crescent Moon is near Venus on the early evening of December 4th and 5th, below the Pleiades on the 13th and above Jupiter on the 14th, and between Mars and Castor and Pollux in Gemini on the 17th. The winter solstice is on December 21st.
The planet Mercury is at inferior conjunction with the Sun on December 6th, after which it becomes visible mid-month, at its greatest separation from the Sun on the 25th, and three days later on the 28th it will be to the left of the morning crescent Moon, with Antares in Scorpius between them.
Venus is now brilliant against a dark sky in early evening, setting at 8 p.m. and near the crescent Moon on the 4th and 5th.
Mars rises in Cancer about 7 p.m., 2 degrees above the cluster Praesepe at the beginning of December, and is passed by the Moon on the night of the 17th/18th, after which it will be occulted by the Moon in daylight. From December 9th Mars will appear to move westward against the stars, doubling in brightness as it’s overtaken by the Earth, to which it will be closest on January 12th, and at opposition, directly opposite to the Sun, on January 16th.
Jupiter is very bright, visible all night long, in Taurus, to the left of the Hyades open cluster and the red giant star Aldebaran, moving back towards them as it's passed by the Earth. It’s at its closest to us on the 6th, at opposition, due south at midnight, and the Moon passes it on the 14th. The volcanic moon Io and its shadow will cross the face of Jupiter on the 10th, grazing the Great Red Spot at 11.30 p.m..
The Europa Clipper spacecraft, now 13 million miles from Earth, has deployed its magnetometer boom and several of its radar booms, the first two of its suite of scientific instruments. Seven more will be commissioned during December and January. The thermal imager and radar will be tested during the Mars flyby on March 1st 2025, and the magnetometer will be calibrated during its final Earth flyby in 2026.
Saturn in Aquarius sets at 11.00 p.m., above and left of Venus, passed by the Moon on the 8th, and occulted by it on the 9th, as seen from the Far East.
Uranus, between Aries and Taurus, sets at 5.30 a.m., and is passed by the Moon on the 13th.
Neptune, between Aquarius and Pisces sets at midnight, and is near the Moon on the 9th.
One of the surprises from the Voyager 2 flyby of Uranus in 1986 and Neptune in 1989 was that neither planet had a dipole magnetic field, unlike Earth, Jupiter and Saturn. One of the possible explanations, though not widely accepted, was that Uranus had a huge liquid water ocean under its visible cloud layers, as Jupiter was believed to have (probably frozen) but turned out not to, in the spacecraft flybys of the 1970s. (Byron Preiss, ed., The Planets, Vintage, 1985; Pioneering the Space Frontier, The Report of the National Commission on Space, Bantam, 1986).
Burkhard Militzer, a planetary scientist at the University of California, Berkeley, has spent 10 years experimenting with mixtures of carbon, oxygen, nitrogen and hydrogen, in the proportions found in the early Solar System, and found that in certain conditions of high temperature and pressure, they can form separate layers, with water above, and heavier nitrogen and hydrocarbons below, preventing interior convection and the formation of a dipole magnetic field. So far, these simulations apply only to the interior of Uranus, and more information on the composition of Neptune is needed to test the process there. The resulting model predicts that the gaseous outer layer of Uranus is 3000 miles thick, above a liquid water layer 5000 miles deep, and a similar hydrocarbon layer below that, overlying a rock and metal core the size of Mercury, which was detected in the Voyager 2 flyby. (Conor Feehly, 'Neptune and Uranus have a magnetic mystery — but the case may finally be cracked', Space.com, online, 25th November 2024.)
In a water ocean of that depth, water would be compressed into forms unknown on Earth, and its properties in those conditions are largely unknown, because they can be generated in the laboratory only in minute quantities. It's possible that these exist within some of the exoplanets which have been discovered, either on 'super-Earths' much larger than ours, or in 'water worlds' whose low density suggests that they are almost entirely composed of it. In the huge quantities postulated within Uranus, they could well generate magnetic fields by processes currently unknown or guessed at. A first guess is that they might help to explain why Uranus's magnetic field is much further displaced from its rotational axis that Earth's or Jupiter's, for example. At the very least, the case for a return mission to Uranus and its moons c.2030 is considerably strengthened.
The Geminid meteors peak on the night of 14th/15th December, but will be spoiled by moonlight. Comet C/2024 S1 (ATLAS), aka Comet S1, discovered on September 27th, might have become as bright as the planet Venus and visible in daylight, if it survived solar passage on October 28th – it didn’t, but that’s comets for you.
Duncan Lunan’s recent books are available through Amazon; details are on Duncan’s website, www.duncanlunan.com.
The Sky Above You
By Duncan Lunan
About this Column
I began writing this column in early 1983 at the suggestion of the late Chris Boyce. At that time the Post Office would allow 1000 free mailings to start a new business, just under the number of small press newspapers in the UK at the time. I printed a flyer with the help of John Braithwaite (of Braithwaite Telescopes) offering a three-part column for £5, with the sky this month, a series of articles for beginners, and a monthly news feature. The column ran from May 1983 to May 1993 in various newspapers and magazines, but never in more than five outlets at a time, although every one of those 1000-plus papers would have included an astrology column. Since then it’s appeared sporadically in a range of publications including The Southsider in Glasgow and the Dalyan Courier in Turkey, but most often, normally three times per year, in Jeff Hawke’s Cosmos from the first issue in March 2003 until the last in January 2018, with a last piece in “Jeff Hawke, The Epilogue” (Jeff Hawke Club, 2020). It continues to appear monthly in Troon's Going Out and Orkney News, with an expanded version broadcast monthly on Arransound Radio since August 2023
The monthly maps for the column were drawn for me by Jim Barker, based on similar, uncredited ones in Dr. Leon Hausman’s “Astronomy Handbook” (Fawcett Publications, 1956). Jim had to redraw or elongate several of them because they were drawn for mid-US latitudes, about 40 degrees North, making them usable over most of the northern hemisphere. The biggest change needed was in November when only Dubhe, Merak and Megrez of the Big Dipper, as the US version called it, were visible at that latitude. In the UK, all the stars of the Plough are circumpolar, always above the horizon. We decided to keep an insert in the January map showing the position of M42, the Great Nebula in the Sword of Orion, and for that reason, to stick with the set time of 9 p.m., (10 p.m. BST in summer), although in Scotland the sky isn’t dark then during June and July.
To use the maps in theory you should hold them overhead, aligning the North edge to true north, marked by Polaris and indicated by Dubhe and Merak, the Pointers. It’s more practical to hold the map in front of you when looking south and then rotate it as you face east, south and west. Some readers are confused because east is on the left, opposite to terrestrial maps, but that’s because they’re the other way up. When you’re facing south and looking at the sky, east is on your left.
The star patterns are the same for each month of each year, and only the positions of the planets change. (“Astronomy Handbook” accidentally shows Saturn in Virgo during May, showing that the maps weren’t originally drawn for the Hausman book.) Consequently regular readers for a year will by then have built up a complete set of twelve.
©DuncanLunan2013, updated monthly since then.