Christmas is undoubtedly one of the most widespread celebrations in the world, helped along by religion, cinema and major marketing campaigns. The streets of innumerable cities and their shops and houses are filled with typical Christmas decorations, which serve to remind us of the cold night on which the birth of the baby Jesus in Nazareth is commemorated. However, the roots of this now global festival are more astronomical than religious. Let’s take a moment to remember that origin and celebrate it with a guide to observing the December sky, one of the most spectacular of the year.
Between the 21st and the 22nd of December every year, the winter solstice takes place, when the nights stop getting longer and begin to wane, and the longest night and the shortest day of the year occur. Nowadays, a few hours more or less of light or the seasonal change in temperatures have become almost trivial, thanks to electricity and central heating, but for people living in the intermediate latitudes of the northern hemisphere—where this variation of hours is maximum—it was an important issue. It was the end of a natural cycle, in which the darkness stopped growing and the hours of light began to increase; a triumph to celebrate in a battle between light and darkness.
Long before Christmas was celebrated, when daylight began to lengthen after the winter solstice, the Romans observed the festival of Sol Invictus (Unconquered Sun) where the god Helios (Greek), or Sol (Roman), triumphed over the darkness, The Germans and Scandinavians celebrated the birth of Frem, the Norse god of the rising sun. The Aztecs celebrated the arrival of the sun god Huitzilopochtli and the Incas marked the rebirth of Inti, their sun god. The relationship between these celebrations and the solstice seems clear.
The most spectacular constellations
However, the solstice is not the only astronomical protagonist of this period of the year. These are not only the longest nights of the year in the northern hemisphere, but also the coldest ones. This favors the inversion, a weather phenomena that allows the stars to be seen more clearly. And it’s also a pleasant coincidence that on these days of the year—on the southern horizon in the northern hemisphere and the northern horizon in the southern hemisphere—you can see right from the beginning of the night some of the most spectacular constellations in the sky:
At first glance, what captures the attention is the constellation of Orion, the hunter, with its brightest star, the red-coloured Betelgeuse, as well as the three stars of the belt. Under the belt, the naked eye can just make out a blur, but with some binoculars or a small telescope a recognizable shape appears: the great Orion nebula (M42, according to the nomenclature of Messier objects), which is like a cosmic nursery of newborn stars.
To the north of Orion you can spot the constellation of Taurus, with another red star as its badge: Aldebaran. In the same area, it’s quite easy to recognise a small cluster of stars, M45 or the Pleiades cluster. South of Orion we can easily find the brightest star that can be seen in the sky throughout the year: Sirius, in the constellation Canis Major.
If we put Betelgeuse in the centre and join Aldebaran (the brightest star in Taurus) with Rigel (the foot of Orion), continue on to Sirius in Canis Major and add Procyon in Canis Minor, Castor and Pollux in Gemini and Capella in Auriga, we will have a hexagon that will help us remember this area of the sky.
As the evening passes while we enjoy the stars, a planet will mark the end of the night. Venus, the brightest body in the sky after the Sun and the Moon, warning us of the imminent end of the night, appearing just a couple of hours before sunrise in the same place where the Sun appears.
Now in the dawn twilight, just as the Sun begins to rise, the most seasoned observers can try to locate tiny Mercury in the sky just above Jupiter, when there are no stars left in the area due to the light of dawn.
An approach between the Moon and Mars
In addition to all that, in 2018 we have an updated star of Bethlehem, which heralds the arrival of Christmas: Comet 46P/Wirtanen, which may become visible to the naked eye, from both hemispheres, in mid-December. On the nights from the 14th to the 18th of December it will pass very close to the Pleiades star cluster, in the constellation of Taurus, which will be the best time to spot it, both for its brightness and because its proximity to an easily locatable object means it’ll be easier to find.
Those same nights will also produce one of the most spectacular shooting star spectacles of the year: the Geminids, with a maximum activity on the night of the 14th of about 100 meteors per hour under ideal conditions. The light from the crescent moon may diminish observations early in the evening, but it’ll set in a few hours allowing the meteor shower to be viewed in all its glory at the best time of night.
In addition to the comet and shooting stars, this year it’s worth highlighting, on the night of December 14, the conjunction of the crescent moon with the planet Mars, an apparent approach between these two heavenly bodies that will be a real treat—visible at the beginning of the night.
The next few weeks we’re going to have the opportunity to enjoy lots of lights over Christmas. Sure, we can relax and drink eggnog under the coloured light bulbs of our Christmas decorations, which aren’t so bad, or we can venture out a little, away from the lights of the cities, searching for a dark sky, and enjoy the real stars of Christmas: those of the sky, which nowadays we have almost forgotten about.