As 2021 draws to a close, Venus, Jupiter and Saturn are seen together soon after sunset in the south to south-western sky. This attractive and enticing spectacle is embellished when a waxing crescent Moon comes a-calling in early December. Try to make the most of this observing and astrophotography opportunity, as Venus is lost to the twilight from the last week of the year on its way to inferior conjunction early in 2022 and the gas-giant pairing inexorably slide toward the twilit mire.
Venus brilliantly beckons as an ‘evening star’
Venus can be seen as an ‘evening star’ for the first three weeks or so of December, dazzling in its magnitude –4.7 brilliance, especially when its viewed through a pair of binoculars (astronomers have calculated the planet hits its greatest brilliancy on 4 December at 07h UT). The Solar System’s brightest planet by far is a pearl beyond price for observers when it pulls sufficiently far from the Sun’s glare.
As December opens, from the south of England Venus lies slightly under 13 degrees high above the south-south-western horizon at sunset, which from London occurs at about 4pm GMT. In Edinburgh, Venus lies at an altitude of around nine degrees at sunset (~3.45pm GMT). Town and city dwellers will need to seek out a view free from obstructing building and trees to watch Venus materialise out of the diminishing twilight as it descends towards the horizon as early evening wears on.
By the end of civil twilight, the brightest period of the three phases of twilight which ends when the centre of the Sun lies more than six degrees below the horizon, at about 40 minutes after sunset, Venus has slipped to an altitude of 10 degrees from London. A small telescope can reveal Venus’ 28.2-per cent- illuminated disc spanning nearly 40 arcseconds.
Venus maintains its elevation for the first three weeks or so of December, but by the start of the last week of the year it sinks rapidly towards the horizon. Venus’ phase shrinks and its disc swells as its elongation from the Sun decreases. On Christmas Day, Venus sports a seven-per cent-illuminated disc spanning nearly a degree.
Jupiter and Saturn near to the end of their visibility
Saturn and Jupiter both are early-evening objects throughout December, though you’ll want to seek them out as early in the month as possible, especially the more westerly-lying Saturn.
Saturn lies among the stars of Capricornus 18 degrees east of Venus and, shining at magnitude +0.7, it’s more than a hundred times fainter. At the beginning of December, Saturn has not long since culminated (peaked in altitude due south) by the end of civil twilight; from London, Saturn sits around 19 degree high, with observers in Scotland around 4 degrees worse off. The ringed wonder is unquestionably the jewel in the Solar System’s crown and the planet all newcomers to observational astronomy want to see through a telescope; its marvellous system of rings are accessible to a small telescope.
By the end of the year Saturn is deeply mired in the horizon mirk by the end of civil twilight, just about signalling the end of its 2021 apparition.
Mighty Jupiter also is located in Capricornus, some 16 degrees east of Saturn. It’s an unmistakable object at its brightness, shining brilliantly at magnitude -2.3. At the beginning of December, from the south of England Jupiter transits the southern meridian (culminates) at about 5.10pm GMT at an elevations of nearly 25 degrees. In Edinburgh, Jupiter peaks with an altitude of 20 degrees when it culminates at about 5.20pm.
From London, there’s about a 90-minute-observing window before Jupiter dips below 20 degrees in altitude, though the planet is probably already beyond meaningful observation. However, a pair of 10 x 50 binoculars can show Jupiter’s four bright Galilean moons, while a small telescope reveals the planet’s major dark belts and bright zones in any moments of steady seeing.
Jupiter’s easterly motion of five degrees during December takes into into Aquarius’ territory by mid-month. By the end of the year its observing window has lessened, as from London Jupiter sits 25 degrees up in the south-south-west by the end of civil twilight.
Follow the dance of the Galilean moons
Take a look at Jupiter as soon as it’s dark on 18 December. Its four bright moons (in order, Callisto, Ganymede, Io and Europa, all lie to the west of Jupiter prior to Callisto, seen just off Jupiter’s preceding limb, moves behind the giant planet at about 5pm GMT. Then, on 29 December the Galilean moons again all lie to the west of Jupiter by about 7pm GMT, when Ganymede ends a transit across its parent’s face and appears adjacent to Jupiter’s preceding limb. Farther out lie Io, Europa and finally Callisto, lying at a distance of around ten Jupiter discs.