The following screen captures from Stellarium show the coming together of Jupiter and Saturn throughout the month of December. The Great Conjunction will occur on December 21 2020.
This is a very exciting conjunction because it is the closest one since 1623 which was just shortly after the telescope had been turned to the night sky for the first time by Thomas Harriot in 1609. This was just 3 months before Galileo Galilei who has been widely accredited as being the first to use and even invent the telescope.
Jupiter and Saturn begin the month 2 degrees apart but on December 21 they will come to within 0.1 degrees of each other and look pretty much like one large object to the unaided eye. However, you will be able to separate them in binoculars but they will appear in the same field of view. The next time they will be this close will be in 2080.
Although very close Great Conjunctions are rare, conjunctions of Jupiter and Saturn occur every 20 years. Before this is explained you may want to know what a conjunction is.
Conjunction is when two or more objects appear to meet each other from our line of site on Earth. Their orbits appear to line up. Technically speaking they have the same right ascension (that is an astronomical coordinate similar to longitude) on the celestial sphere. Practically speaking, objects in conjunction will likely be visible near each other for some days.
The word conjunction comes from Latin, meaning to join together.
Conjunctions involve either two or more objects in the Solar System or one object in the Solar System and a more distant object, such as a star. A conjunction is an apparent phenomenon caused by the observer's perspective: the objects involved are not actually close to one another in space they just appear to be from our line of sight. Conjunctions between two bright objects close to the ecliptic, such as the two bright planets Jupiter and Saturn, can be seen with the naked eye.
So why do they occur at all? You have to imagine you are looking down on the solar system. Inner planets such as Mercury don't have as far to travel as the outer planets to complete one full orbit. They also travel faster too. In other words the further out from the Sun the planet is, the larger the orbit and the longer it takes the planet to orbit the Sun. They also travel slower the further out they are. Take Jupiter and Saturn. It takes Jupiter nearly 12 years (11.9 years) to complete one orbit around the Sun travelling at an average 13.07 km/s and it takes Saturn nearly 30 years (29.5 years) travelling at an average 9.68 km/s. This means that Jupiter actually catches up with Saturn and makes its rendezvous every 20 years before undertaking Saturn again and continuing on its orbit around the Sun. So let’s do the maths. Jupiter completes about 30 degrees of its orbit for every one Earth year; Saturn completes about 12 degrees. 30 – 12 = 18 degrees. Jupiter ‘catches up’ to Saturn by 18 degrees every one Earth year. 18 x 20 = 360 degrees so every 20 years there is a conjunction of Jupiter and Saturn. Because the numbers are not exact then the closeness of the conjunction will differ every time.
You will need a fairly flat horizon because both planets are quite low down in the sky (see the images below) and you will need to look as soon as it gets dark enough for the planets to be visible because you don’t have long! Here in Herstmonceux the Sun will be setting at 3.53pm and the planets will set at 6.46pm. Look south/south west. It is also the winter solstice on the 21st December so the shortest day and the longest night. The phase of the Moon is first quarter so will already be high in the sky and pretty bright but it shouldn’t spoil the conjunction.
Fingers crossed for a clear sky so we can all enjoy this fabulous phenomenon.