Close Encounter of Jupiter and Saturn in December 2020
Around December 21 and 22, Jupiter and Saturn, two largest planets of our Solar System will appear to have a close encounter in the evening sky. The last close encounter of the two gas giants occurred in May 2000. This time however, the apparent separation between Jupiter and Saturn will be about one-fifth the size of the full moon when they are closest. It is the first time in 397 years that they have such a close separation. Miss this one, and you will have to wait for another opportunity in October 2040, but an encounter as close as this year, will not occur until March 2080.
Astronomers Without Borders calls for participation in the observing campaign of this close encounter.
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 17, and become the smallest around December 21, about one fifths of the apparent size of the full moon. Thereafter the separation will increase. Around December 26, 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 16 to 17, as an extra observing treat you will find a thin Moon near Jupiter and Saturn. You can enjoy changes in the arrangement of Jupiter, Saturn, and the Moon.
How can you 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 low-magnification telescope for the next 20 years!
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 the camera shaking.
In our solar system, eight major planets and innumerable small objects orbit around the Sun. These eight planets orbit in almost the same plane, including the Sun. When viewed from one of those planets, the Earth, the other neighbouring planets rarely appear to overlap in our sky because they are not in the same plane, however on special occasions the planets can appear to be quite close - which are called ‘conjunctions’. Also, the angular separation in our sky between any one of these close encounters varies from one to the next.
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 Earth, but the interval at the time of the closest approach differs depending on how far the orbits are separated, as seen from 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 21 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 15, 2080, about 59 years later, so it is a phenomenon which can be seen once or never in a lifetime.
Saturn's orbital period is about 30 years, and the cycle of Jupiter and Saturn's close encounter is about 20 years, so similar close encounters occur every 60 years. The close encounters of 20-year cycle from 1900 to 2100 seen from Greenwich calculated with a simulation software Astroarts StellaNavigator 11 are shown in the following table:
* denote encounters of the 60-year cycle
Accompanying encounters whose angular separations exceed 80 arcminutes are omitted. Around 1940 and 1980, there are three encounters, where two of them occur when Jupiter and Saturn move prograde in the sky while another occur when they move retrograde.