Encounter of Giants

During these days at the end of the year, a unique astronomical event is taking place with the great conjunction of the planets Jupiter and Saturn that will reach a relative distance of just over 6 arc minutes on the night of December 21. This very close planetary conjunction has not occurred with this closeness since 1623 and the next comparable conjunction will be in 2080. Even with a modest telescope it will be possible to see both planets, the rings of Saturn and the main satellites of both planets in the same visual field. As you know, both planets are low in our night skies at a maximum height of less than 30º and visible at the beginning of the night towards the West.

How can I find Jupiter and Saturn? Jupiter is the brightest “star” in the low western sky in the evening of December, so it is quite easy to find Jupiter. Saturn is fainter than Jupiter, but it is as bright as 1st magnitude star, and shines above Jupiter in early December. Because Jupiter and Saturn fall in a short time in the evening, you can enjoy watching these planets only for a limited time. We recommend that you find Jupiter and Saturn in November or early December.

How can we enjoy the encounter? Watch the change of the angular distance between Jupiter and Saturn with the naked eye. With a telescope, you can also see moons of Jupiter and Saturn, Jupiter's bands, and Saturn's rings. Around the closest approach, you will see Jupiter and Saturn in the same field of view. Such a scene will not be seen even with a lowmagnification telescope until 20 years later! --- The simulated figure of the Jupiter and Saturn in the same field of view is inserted here If you have a camera, take pictures, and keep records. You can also take pictures with your smartphone camera. It is likely that auto shooting will not work for Jupiter and Saturn, so familiarize yourself with the manual shooting method of your camera in advance. Focus on infinity and try with different ISO sensitivities and exposure times. If you have a tripod, you can shoot without worrying about camera shake

What will happen to Jupiter and Saturn? The apparent separation between Jupiter and Saturn will gradually decrease during November and early December, becoming smaller than the apparent size of the full moon from around December 17th, and become the smallest around December 21st, about one fifths of the apparent size of the full moon. Then the separation will increase. Around December 26th, it will exceed the apparent size of the full moon. However, the distances of Jupiter and Saturn from the Earth in December 2020 are about 860-900 million kilometers (530-560 million miles), and 1.6 billion kilometers (1.0 billion miles), respectively. Also, from December 16th to 17th, you will find a thin Moon near Jupiter and Saturn. You can enjoy changes in the arrangement of Jupiter, Saturn, and the Moon.

The Spanish-Portuguese node of the Europlanet Society, the Spanish Astronomy Society, the Federation of Astronomical Associations of Spain and Astronomers without Borders are organizing a set of activities to celebrate this astronomical event and the pleasure of night observation. They think that the joint vision of Jupiter and Saturn is an inspiring image, a true meeting of giants capable of raising our gaze towards the sky. We would very much like to have your observation of both planets. Whether you are a fan of deep sky astrophotography, or if you prefer planetary photography or even night landscapes, we invite you to observe these planets during the next few nights and send us your observations to the photo gallery that we are preparing.

The activities of this gathering of giants are avalaible here and include events to disseminate on the internet, retransmission of the conjunction by different telescopes on key days, and a gallery of images that we hope to fill with content thanks to your collaboration.

In our solar system, eight planets and innumerable small objects orbit around the Sun. The eight planets orbit in almost the same plane, including the Sun. When viewed from one of those planets, the Earth, the planets rarely appear to overlap because they are not in the same plane, but sometimes the planets appear to be quite close. Also, the angular distance between the close encounters varies for each encounter. Jupiter's orbit is tilted slightly with respect to Saturn's orbit. Their orbital planes intersect at a slight angle. Seen from Earth, Jupiter's orbit and Saturn's orbit intersect at two points in the sky. Jupiter and Saturn orbit around the Sun in the same direction, but Jupiter has an orbital period of about 12 years and Saturn has that of about 30 years, so Jupiter orbits the Sun in a shorter time than Saturn. As a result, Jupiter catches up with and overtakes Saturn about every 20 years. At this time, Jupiter and Saturn approach each other when viewed from the earth, but the interval at the time of the closest approach differs depending on how far the orbits are separated seen from the earth. In December of this year, Jupiter and Saturn approach each other where the separation between the two orbits is very small.

In fact, on December 21st this year (dates and times are expressed in Universal Time), Jupiter and Saturn will approach each other by 6 arcmin (one-fifth the apparent size of the full moon). It will be the first time in 397 years since July 16th, 1623, that they are this close. The next close encounter to the same level as this time will take place on March 15th, 2080, about 59 years later, so it is a phenomenon which can be seen once or never in a lifetime 

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