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.
THE GREAT PLANETARY CONJUNCTION OF 2020
Exactly on the night of the solstice, December 21st, there will be one of those astronomical events that only happens once in a lifetime. The planets Jupiter and Saturn will make their closest approach to each other in the last four centuries, something that will not be repeated until 2080. This phenomenon is called an astronomical conjunction, and this one in particular will be one of the most spectacular that we will be privileged to observe in our lives.
To observe the conjunction it is important not to lose track of the time since the observation window is just a couple of hours. The best time to see it in the northern hemisphere is just after sunset; as the hours pass the planets will approach the horizon, making it more difficult to observe the event. However, in the southern hemisphere the opposite occurs; the conjunction will be seen shortly before sunrise. In both cases, the best observation plan is to locate a site with the south-west horizon as clear as possible and start observing at the right time. It shouldn’t be hard to spot as it will be the brightest thing in the sky after the Moon.
Saturn and Jupiter will appear so close together, just 0.5 arcminutes apart, that it will be hard to distinguish them separately with the naked eye, and for many it will look like a single point of light. Sharp-eyed observers may be able to make out the two planets separately, but it will certainly be a test of one’s visual acuity. In any case, the use of binoculars or a small telescope will be useful to get a better look at the two gas giants. These do not need to be astronomical instruments or of particularly high quality; any optical instrument such as a basic pair of old binoculars forgotten in a drawer will do.
THE BEST SHOOTING STAR SHOWER OF THE YEAR: THE GEMINIDS
Although the Perseids meteor shower, which occurs in August, is the most famous, it is not considered by many astronomers to be the best. The Geminids tend to be more active and deliver more spectacular shooting stars, so many judge this to be the best meteor shower of the year. This year from December 4th to 17th you will be able to see more meteors (shooting stars) than usual. The Earth passes through the cloud of dust left behind by the asteroid Phaeton and each of these small particles that collide with the atmosphere at a speed of kilometres per second, heats up until they volatilize, and from the ground we can see their brightness and speed as a shooting star.
The best night to observe the meteor shower will be December 14th at 12:48am UTC. This is only a forecast, as the exact activity cannot be known with complete certainty. Under ideal conditions, about 120 shooting stars per hour are expected.
These figures are calculated for the ideal situation of having no light pollution and that the radiant is at the zenith, and they count the number of meteors across the whole sky. Of course, one set of eyes is only able to cover a limited part of the sky. From a fairly dark place, at an intermediate latitude, an observer can easily spot between 25 and 50 shooting stars per hour. This year the conditions for observation are very good as the peak of the Geminids coincides with the new moon, so our natural satellite will not trouble us with its light in the sky, one of the most important factors to take into account when observing shooting stars.
In order to observe the greatest number of meteors, we must look towards the eastern quadrant and observe between the horizon and the middle of the sky. The meteors seem to arise from the constellation of Gemini, so looking around that constellation, not directly at Gemini but at the sides, is the best strategy for star hunting. Staying put until shortly before sunrise is also a good strategy, as just before the Sun rises the best configuration between the movement of the Earth and the dust cloud occurs, so that’s when you usually see the most spectacular ones 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.