As their 2020 apparition draws to a close, the planets Jupiter and Saturn will make a close approach on 21 December, passing within 6.1 arcminutes of each other. This will be the closest approach of the two planets since the year 1623, and offers a once-in-a-lifetime chance to see the two planets within the same telescopic field of view.

Close approaches of the two planets are called great conjunctions because they are the rarest of all conjunctions between the planets that are visible to the naked eye. The last time such a conjunction was observable was in 1980. The most recent great conjunction, in 2000, was not observable since it took place while the two planets were a mere 3° away from the Sun.

The rarity of great conjunctions is due to the slow motion of Jupiter and Saturn across the sky. Among the planets that are visible to the naked eye, they are the two most distant from the Sun, taking 11.86 years and 29.5 years respectively to orbit it. As the two planets gradually move through the constellations at different speeds, they follow almost the same path across the sky, called the ecliptic. Periodically, Jupiter catches up with Saturn and overtakes it, resulting in a great conjunction, on average once every 19.6 years.

Not all of these great conjunctions are equally dramatic. Sometimes they happen when the planets are too close to the Sun to be observable, as happened in 2000. And at other times they may not pass any closer than five degrees (the width of ten full moons) apart. The 2020 great conjunction will be the closest approach of the two planets since 1623, and they will not come so close again until their 2080 great conjunction.

The two planets will lie deep in the southern sky at the time of this year's conjunction, at a declination of 20°S, and so the best views will be had from the southern hemisphere. But they will nonetheless still be accessible to any northern observers who are willing to travel to find a clear southern-western horizon. Since the pair will be several months past opposition, they will only be visible in the early evening sky and will be well on their way to setting at sunset. The best time to see the pair will be soon after sunset on the night of 21 December.

Observing from Cambridge

From Cambridge , the pair will be difficult to observe as they will appear no higher than 14° above the horizon. They will become visible at around 16:52 (EST), 14° above your south-western horizon, as dusk fades to darkness. They will then sink towards the horizon, setting 2 hours and 20 minutes after the Sun at 18:34.

Jupiter will be at mag -2.0; and Saturn will be at mag 0.5. Both objects will lie in the constellation Capricornus.

They will be close enough to fit within the field of view of a telescope, but will also be visible to the naked eye or through a pair of binoculars.

At around the same time, the pair will also share the same right ascension – called a conjunction.

A graph of the angular separation between Jupiter and Saturn around the time of closest approach is available here.

The positions of the pair at the moment of closest approach will be as follows:

The coordinates above are given in J2000.0. The pair will be at an angular separation of 30° from the Sun, which is in Sagittarius at this time of year.

The sky on 21 Dec 2020

Source

The circumstances of this event were computed using the DE430 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

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Image credit

The Moon in conjunction with Venus and Jupiter, with the Very Large Telescope in the foreground. Image © Y. Beletsky, ESO, 2009.

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