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On December 21, Jupiter and Saturn will be practically together in the sky.

There has been no such close and visible conjunction between the two giant planets since the Middle Ages.

These types of conjunctions are sometimes evoked as a possible astronomical explanation for the star of Bethlehem.

On the same day the 21st, at 11:22 a.m. (peninsular time), winter will begin in the northern hemisphere.

The Great Conjunction

For months we have watched Jupiter and Saturn dominate the night sky in a brilliant pair.

Over the days, the apparent positions of the giant planets have come closer and closer to each other.

The process culminates on the 21st, when Jupiter and Saturn will be seen on the same line of sight, so close in the sky that, with the naked eye, we can barely distinguish one from the other.

It is the Great Conjunction, as the closest approximation of these two gas giants, the largest planets in the solar system, is often called.

Because their periods are very different (11.86 years for Jupiter and 29.46 years for Saturn), only one major conjunction occurs every 20 years or so.

But not all conjunctions are the same.

Depending on the relative position of the Earth with respect to the two planets, in each conjunction they are more or less aligned on the line of sight.

Recall that the two stars are at very different distances: Jupiter is now one 890 million kilometers from Earth, while Saturn is almost twice as distant, at 1.62 billion kilometers.

The conjunction of December 21 is exceptionally close.

The planets will be separated by only 6 arc minutes, this is one fifth of the diameter of the full moon.

Being able to witness an almost perfect alignment, over such long distances in space, is something extraordinary.

How to observe it

There is no waiting for the night to start.

It is convenient to start observing the conjunction, at the latest, one hour after sunset, around 7:00 p.m., looking to the southwest.

The scene will take place on the border between the constellations Capricorn and Sagittarius.

You have to find a place with a clear horizon, since the planets will then be about 10 degrees above the horizon.

After half an hour they will be lower still, but then the sky will be darker, thus favoring the observation with greater contrast.

The Great Conjunction of December 21 as seen with a telescopeStellarium / RB

With the naked eye we must observe, depending on visual acuity, a kind of double planet.

But since the brightness of Jupiter is much greater than that of Saturn, it is very possible that it is difficult to separate the images of both stars and that most of us only see a slightly elongated structure.

The lucky ones who can look out through a telescope will enjoy the image, in the same field of the eyepiece, of the two giant planets surrounded by their processions of moons.

The Galilean satellites of Jupiter, Io, Europa, Ganymede and Callisto will stand out, as well as Titan, the largest moon of Saturn.

With a telescope of about 50 to 100 magnification it will be possible to distinguish the cloud bands of Jupiter and the always spectacular rings of Saturn.

Again, do not neglect the time, because as the planets get closer to the horizon, atmospheric turbulence will make the image more unstable.

A very rare phenomenon

To find such a close Great Conjunction in the past, one must go back to April 15, 1623. But, on that occasion, the conjunction was not visible, since the planets were only 13 degrees away from the Sun. The bright sky did not allow observation with the naked eye and the observational optical means were still very scarce (the first telescope had been pointed at the sky by Galileo only 14 years before).

We must go back to March 4, 1223 to date a similar conjunction that is just as well visible as that of December 21.

On that occasion the planetary separation was only 2 minutes of arc.

We will have to wait until March 15, 2080 to have an opportunity similar to this year, because then the planets will also be separated by 6 arc minutes.

And the next one, with similar characteristics, will take place in the year 2417.

Beginning of winter

Winter will begin in the northern hemisphere on December 21 at 11:22 a.m. (Peninsular time), at which time summer will begin in the southern hemisphere.

The night skies of winter will be dominated by Mars that will be seen much of the night.

Jupiter and Saturn will see each other at dusk until mid-January, when they will disappear to return, a month later, at sunrise.

Venus will remain in the sunrises until February and Mercury will have occasional appearances.

This season will not offer us any eclipse.

Winter will last 88 days and 23 hours, ending with the arrival of spring on March 20, 2021.

Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.

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