Triple planetary conjunction: an ideal opportunity to enjoy the night sky

This December, due to the alignment of the Earth, Jupiter and Saturn, we will be able to see the giant planets of the Solar System very close in the sky. Many activities are being organized to enjoy the event




This month the conjunction of three planets of the Solar System takes place: Jupiter, Saturn and Earth will be located approximately in a straight line. The Earth being the closest to the Sun, that means that we can observ Jupiter and Saturn very close in the sky. The separation between the two will be just 0.1 degrees, which is roughly one-fifth of the apparent diameter of the full Moon. This apparent closeness between Jupiter and Saturn has not occurred for 400 years, and will not occur again until March 2080. These two giant planets are two of the brightest objects in the night sky and having them so close to each other (apparently) will be an excellent occasion to enjoy the winter sky.

The planetary conjunction is a good excuse to promote the observation of the night sky and the beautiful natural spectacles that it offers us, and different international and national organizations have organized to communicate their enthusiasm for astronomical observation to the public. At an international level and from the Europlanet Society, Ricardo Hueso (University of the Basque Country), and Miguel Angel López Valverde (Institute of Astrophysics of Andalusia, IAA-CSIC), want to take this opportunity to promote the dissemination of planetary sciences and put in contact amateur and professional astronomers.

“The conjunction between Jupiter and Saturn will take place at dusk in mid-December and will culminate in closest approach on the night of December 21 with Jupiter and Saturn low above the winter horizon. Most of the professional telescopes have difficulties to point to objects of low elevation and therefore the observation with small, more versatile telescopes, promises to offer the best images of this event”, point out the researchers.

Iñaki Ordóñez and Ana Ulla, from the Spanish Astronomy Society, and Amelia Ortiz, from the International Astronomical Union, add that “although from a scientific point of view it is not a special occasion that allows a discovery, for amateurs and the general public it will be a magnificent occasion to admire these two planets, since both, their satellites and their rings will be visible simultaneously when they are observed through a telescope no matter how modest their size is”.

From the Federation of Astronomical Associations of Spain (FAAE), Blanca Troughton encourages all associations and groups of fans to enjoy and photograph the event. Although the maximum will be on the 21st, the approach can already be observed in the sky, looking at the horizon in a western direction just after sunset.

From Astronomers Without Borders (ASF), Victoriano Canales points out that this is an observable event from almost anywhere on our planet. “This is not a one-off event; we will be able to appreciate how the planets are getting closer to each other, although the distances between us and Jupiter and Saturn are, respectively, 900 and 1600 million kilometers. We expect to receive images from observatories and amateurs around the world around the days of closest approach, between December 17 and 26. We encourage everyone to send us theirs".

The five organizations have developed a web page coordinated by Joaquín Álvaro from the Cuenca Astronomical Association. It explains what a conjunction is, offers advice on observation and astrophotography of planets in general and this conjunction in particular. They will also announce the activities that are being programmed in various planetariums and centers of the country, including live broadcasts of the event. Also, questions will be answered and images will be shared.


Those with a small telescope have a more powerful tool than Galileo used 400 years ago, when he pointed his rudimentary telescope at Jupiter and made one of the key discoveries in the history of astronomy: four points very close to Jupiter revolved around it. He had just discovered the Galilean satellites, Ganymede, Callisto, Europa, and Io. This discovery marked the launch of the scientific revolution and the demonstration that the universe does not revolve around the Earth. Today about 80 satellites are known around Jupiter and as many around Saturn.

Throughout this month, with the help of a small telescope, Galileo's work can be repeated, that is, following the movement of the four Galilean satellites, observing them continuously day after day to try to determine their orbital period, or duration of its rotation around Jupiter.




Instituto de Astrofísica de Andalucía (IAA-CSIC)
Unidad de Divulgación y Comunicación
Silbia López de Lacalle - sll[arroba] - 958230676