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.
PLANETARY PILE-UP ON THE ECLIPTIC
The planets visible at sunset on the evening of 25 December, with the line of the ecliptic through which they transit marked. Credit: Borja Tosar / Stellarium
Nightfall on the 25th of December 2022 will present us with one of the most spectacular sky views of the season. It will be a special occasion for stargazers—something doesn’t happen every year—to see at a glance the five planets visible to the naked eye aligned on the ecliptic:
- Just after sunset, looking in a south-westerly direction, we will be able to see the planets Venus and Mercury. Venus is very easy to spot, even in twilight (as only the Sun and Moon are brighter in our sky). Mercury is a bit more elusive and you will need good eyesight to spot it (if you have difficulty, try using a simple pair of binoculars, but only after the Sun has set, as looking directly at our star with binoculars could cause irreparable damage to your eyesight.)
- Moving eastwards (left) and upwards in the sky we will find the Moon in its crescent phase, a barely illuminated sliver. If we continue to scan the sky, we will come across something that looks like a rather dim star, which is actually the planet Saturn.
- Jupiter is easy to spot, just above the south cardinal point at sunset; it is the brightest object you can see in the sky after Venus. With the aid of simple binoculars, four small objects can be easily seen around it, which are its larger moons: Ganymede, Callisto, Io and Europa.
- If we continue our journey to the east, we will find the last visible planet, Mars, which looks like a very bright star and is clearly red in colour.
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. Every December, 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 Geminid meteor shower occurs every year between 4 and 17 December and gives us up to 150 shooting stars per hour, under ideal conditions. In order not to create false expectations, we should clarify that these conditions are difficult to achieve—the radiant of the shower would have to be in the centre of the sky, under total darkness—and that the estimated number of shooting stars is for the whole sky, a 360° dome that human vision cannot cover, no matter how much attention and enthusiasm we put into it. Realistically, from intermediate latitudes and a little further away from cities, between 20 and 40 shooting stars can easily be seen per hour at the peak of the Geminids, around the night of 14 December.
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.
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