As 2020 draws to a close we can look forward to a once in 20 year astronomical event, the conjunction of Jupiter and Saturn, on the 21st of December.
What is a Conjunction?
Occasionally two or more objects meet up with each other in our sky. Astronomers use the word conjunction to describe these meetings. Technically speaking, objects are said to be in conjunction in that instant when they have the same right ascension on our sky’s dome. 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. In language, conjunctions relate to clauses brought together in sentences with words like and. In astronomy, conjunctions relate to two or more objects brought together in the sky as we see them.
An inferior conjunction is when an object passes between us and the sun. Any object in space that orbits the sun closer than Earth’s orbit might pass through inferior conjunction from time to time, assuming its orbit lies more or less close to the ecliptic. Usually, though, when you hear the words inferior conjunction, astronomers are speaking of the planets Venus and Mercury, which orbit the sun inside Earth’s orbit. Astronomers sometimes refer to Venus and Mercury as inferior planets. When they’re at or near inferior conjunction, we can’t see them. They’re hidden in the sun’s glare. Occasionally, though, Venus or Mercury can be seen to transit across the sun’s disk at inferior conjunction. Consider also the moon. It passes between the Earth and sun at new moon once each month. Therefore you could say that the Moon is at inferior conjunction when it’s a New Moon.
A superior conjunction is when an object passes behind the sun from our point of view. Think of Venus or Mercury again. Half of their conjunctions with the sun – when they are brought together with the sun on our sky’s dome – are inferior conjunctions, and half are superior conjunctions. It’s kind of fun to imagine them on an endless cycle of passing in front of the sun, as seen from Earth, then behind it, and back again, like watching squirrels running around a tree. Meanwhile, the superior planets – or planets farther from the sun than Earth such as Mars, Jupiter, Saturn, Uranus and Neptune – can never be at inferior conjunction.
However there are many more conjunctions that occur which fall into neither of these categories. They occur when planets, or a planet and the moon, appear close together in the sky. The conjunction between Jupiter and Saturn is one of these and is often called a Great Conjunction.
How often do they happen?
As a result of their long orbits, Jupiter and Saturn meet in the sky only once every 20 years. In this period of time, Saturn completes two-thirds of its 30-year orbit (since 20 is two-thirds of 30). In the same period, Jupiter completes one 12-year orbit, plus, in the remaining 8 years, two-thirds of its next orbit (since 8 is two-thirds of 12). In other words, 20 years is the time it takes Jupiter to catch up and pass Saturn again as they circle the Sun.
The last conjunction of Jupiter and Saturn occurred on 28 May 2000, but was almost impossible to view because it occurred while the two planets were just 14.9° west of the Sun from our point of view. So the spectacle was largely lost in the Sun’s glare.
After 2020, the next great conjunctions will occur on November 2, 2040 and April 7, 2060. On both these occasions, the minimum separation of Jupiter and Saturn will be 1.1 degrees—which means they will appear 11 times further apart from each other than on December 21, 2020.
In fact, the 2020 great conjunction of Jupiter and Saturn is exceptionally close. Over a period of one thousand years, from 1600 to 2599, there are only six great conjunctions where the minimum separation between Jupiter and Saturn is less than 0.2 degrees: 1623, 1683, 2020, 2080, 2417, and 2477.
Though each successive ‘great conjunction’ takes place around 117º apart in the sky, the planets’ orbital resonance is such that each conjunction returns to the same part of the sky roughly every 800 years.
How and when to see it
The conjunction actually occurs at 13:30 GMT – that’s when the right ascension of the two planets will be the same, but the Sun will still be high in the sky at that point. When the Sun sets at 16:05 (according to Heavens Above website) the two planets will only be 14 degrees above the south-western horizon and set less than 2hrs 30mins later.
But don’t just rely on clear skies on the night. The two planets are already nice and close together as we look at them so just in case the great British weather lets us down on the 21st make sure you take a look in the evenings leading up to the big night.