Illustration par défaut

Jupiter and Saturn meet in the sky

21 décembre 2020

This Monday, December 21, 2020 in the early hours of the night, Jupiter and Saturn are very close together. On this rare phenomenon, the Institute of Celestial Mechanics and Ephemeris Calculation of Paris Observatory - PSL gives the explanations.

The whole period around Monday, December 21 is favorable for observing the rapprochement between the two largest planets of the Solar System, even if they are setting earlier and earlier as the days go by and their height on the horizon is quite low (less than 10°). The planets are visible only in the early evening, under clear skies.

Trajectoires de Jupiter et Saturne en décembre 2020
© Observatoire de Paris - PSL / IMCCE

On December 21, this rapprochement is exceptional because of the small distance that remains between the planets : 6 minutes of degree, or about one fifth of the lunar diameter. In other words, to the naked eye, the planets will brush against each other. Not all conjunctions result in such close approximations.

Such conjunctions are rare, even extremely rare, and are part of the astronomical phenomena that gave birth to the star of the Magi, very close Jupiter-Saturn conjunctions that took place in the time before the beginning of our era.

At what time ?

The rapprochement of the two stars takes place at the junction of the constellations of Sagittarius and Capricorn, therefore unfortunately quite low on the horizon.

It is necessary to look for the couple from 5:30 p.m. (in French legal time) in the glow of the setting sun, towards the southwest. The planets setting around 7:00 pm, the best visibility is around 6:00 pm.

The scene promises to be magnificent, as it is uncommon to the naked eye. As long as you have a clear sky.

Le 17 décembre 2020, à 17h15, Saturne (en haut à gauche) et Jupiter (en bas à droite) sont dans le même champ, un phénomène rarissime.
Cliché pris sur un télescope de 25,6 cm depuis Versailles.
© Nicolas Biver / SAF / Observatoire de Paris-PSL
Rencontre de Saturne (en haut de l’image) et de Jupiter (en bas), le 20 décembre 2020, à 18h15.
Cliché réalisé sur un télescope de 25,6 cm depuis Versailles, alors que les planètes, à 8° au-dessus de l’horizon, étaient invisibles à l’œil nu, assombries par les cirrus ; l’exposition a dû être multipliée par 200. (Somme de 60 images à 20000 ISO cumulant près de 2 secondes de pose au travers des nuages - champ de 13,8 x 13,8’).
© Nicolas Biver / SAF / Observatoire de Paris-PSL

The last conjunction between the two giant planets took place in May 2000. But, at that date, the gap in the sky between the planets was a little more than 1°, twice the lunar diameter.

In other words, during this rapprochement on December 21, 2020, Jupiter and Saturn are ten times closer than in 2000. In other words, seen from the Earth, they will touch each other, almost touch each other... This is of course only a visual effect, since Saturn is located far behind Jupiter.

If the scene is already superb to the naked eye, any astronomical instrument to admire it is welcome. A simple pair of binoculars is already very effective, offering the beautiful image of two tiny yellow balls very bright and very close together.

Know more

Translation

Actualités