December sky at night — click on the image below to see a larger version

Some of the year’s best observing can be found under the dark skies and early nights of December, a month marked this year by a series of conjunctions and the return of Saturn to full prominence. Any clear night brings a chance to glimpse the winter sky, but look particularly for those days when the sky is a dark, deep blue. This is a sign of good “seeing”, the steady, haze-free atmosphere that astronomers crave.

The first, and most spectacular, of the conjunctions takes place this evening, when the Moon, Jupiter and Venus put on a celestial show. The Moon will be a crescent low down in the south west, with Jupiter just two degrees above it. The real action starts, though, at 15.46 (in London; allow ten minutes either way elsewhere in the country) when Venus slips behind the Moon.

The event therefore starts just before sunset, but the Moon should be obvious and Venus a naked-eye object just off the Moon’s dark limb. Jupiter, at magnitude -1.8 compared with Venus’s -4.0 (as with golf handicaps, the brightest stars have the lowest numbers), will be harder to see, but should still be visible in binoculars. If you do use an optical aid, be very careful to keep the Sun out of the field of view; even the weak December sun is very dangerous.

The occultation ends with Venus emerging from the bright side of the Moon at 17.16 (in London), by which time the sky should be dark. If clouds intervene this evening, you can still enjoy the sight of the Moon’s close approach to Jupiter and faint, fleeting Mercury very low down on the 29th at about 17.00, or to Venus much higher in the sky at the same time on New Year’s Eve.

Among the other planets, Mars is in conjunction with the Sun on the 5th, and is thus invisible. Saturn, on the other hand, continues its slow march through the hind legs of Leo, rising as early as 22.30 by the end of the month. To find Leo, start with the familiar “saucepan” shape of the Plough. Take the two stars that form the inside edge of the pan, and follow the line they make down until you hit Regulus, the brightest star in Leo.

The brighter star to the left and below Regulus is Saturn, but as throughout the planet’s current apparition, Saturn’s rings appear almost edge on as seen from Earth, and so will only be visible through a large telescope. Instead, those with small telescopes should look for the movements of Saturn’s moons along and across the disc, and hope for the appearance of a white spot. These disturbances on Saturn are rare, but they do appear from time to time. The most famous was discovered, in 1933, by the the comedian Will Hay, not only the star of Oh, Mr Porter! but also an accomplished amateur astronomer.

Turning to the stars, the sky in the late evening is still dominated by Orion in the south east, with Taurus, Gemini and Cancer the most prominent of the zodiacal constellations. One star in particular is worth finding, though, because this month is your only chance in the next 11 months to see it with the naked eye.

Mira — the name is usually translated as “wonderful” or “astonishing” — was one of the first variable stars to be discovered when its strange behaviour was noted by David Fabricus, a theologian in 16th-century Germany. Fabricus was a fascinating man, and seems to have, in collaboration with his son, beaten Galileo to the discovery of sunspots and hence the rotation of the Sun.

To follow his footsteps and find Mira, first look for the head of Cetus, the sea monster. The easiest route is probably to start from Orion’s belt, move upwards to Aldebaran in Taurus, and then look back along the body of the bull until you reach the ring of stars that is the most prominent part of Cetus. From there it is easy (particularly with binoculars) to hop to Mira, which is plotted on our map. Exactly how bright it will become is always a matter of guesswork, but it will reach maximum brightness on the 30th, an appropriate end to a year’s celestial events.