Rare 'great conjunction' of Jupiter and Saturn wows skywatchers around the world

For the last several days, skywatchers have been captivated by the Great Conjunction between Jupiter and Saturn. The closest pass took place on Monday, December 21, but the spectacle itself began days earlier — and will last until at least Christmas Day. 

What’s great conjunction?

A pairing between any pair of planets in conjunction. Jupiter and Saturn are the two largest planets visible to the naked eye, hence the expression ‘Great Conjunction’. These two align roughly every 20 years, which is relatively rare compared to the alignments of planets closer to the Sun (and which consequently have shorter orbits). 

Jupiter orbits the Sun once in 12 years, and Saturn once in 30. High school arithmetic tells us that in 60 more years (the LCM of 12 and 30), i.e. in 2080, the two planets will align at roughly the same place where stargazers watched them on December 21, 2020. In these 60 years, Jupiter will have orbited the Sun five times, while Saturn will have done so twice.

But they will have met twice more during this period, though at different places in the sky. In 12 years more, Jupiter will return to its current place; in the next 8 years, it will complete 2/3rds of another 12-year cycle around the Sun. In the same 20 years, Saturn will have completed 2/3rds of its 30-year cycle. In other words, the two planets will meet again in 2040. And yet again in 2060. 

So, why is this conjunction special?

It’s the alignment. We measure the position of a planet in terms of the angle it makes on the Earth’s orbital plane, with a given reference direction. When we say two planets have aligned in a conjunction, it suggests they are casting the same angle with that reference direction.

In fact, this is almost never the case. Planets in a conjunction are typically above or below each other, because their orbits are slightly tilted with respect to each other.

This time, Jupiter and Saturn are a tenth of a degree apart viewed from Earth. From some views, that might give them the appearance of converging into one, but viewers around the world have found them distinct enough to tell them apart.

Also, the position of Earth matters. Not every alignment provides a clear viewing.

And how rare is this conjunction?

The last Great Conjunction happened in 1623. For context, Galileo had discovered four of Jupiter’s moons with his telescope a few years previously — but scientists today believe Galileo would not have found it easy to see the conjunction, because the planets were aligned too close to the Sun from Earth’s perspective. From an Indian context, Jahangir was ruling the Mughal empire at the time, and the Maratha warrior king Chhatrapati Shivaji was yet to be bor

The last time the two planets were close enough to be viewed in the night sky was in 1226. This was just a year before the death of the Mongol ruler Genghis Khan.