For the last several days, sky watchers 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 a great conjunction?
A pairing between any pair of planets is a 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 born.
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.
How long will the spectacle continue?
Jupiter has been catching up with Saturn since the beginning of December as the two planets move along their orbital paths around the Sun. The date the world celebrated, the night of December 21, was when Jupiter “overtook” Saturn (from Earth’s perspective). But even after December 21, the planets will still appear very close together for the next few days. Between December 16 and 25, the distance between the two planets in the sky will appear to a viewer from Earth to be less than the diameter of a full moon.
This does not mean, of course, they are really that close — they are currently over 700 million kilometres apart. Yet their separation during the current conjunction is smaller than they usually ever get during most other conjunctions.
What if some watchers miss it?
For the very young, there is still an opportunity. The next two conjunctions are in 2040 and 2060, although they will not be as easy to view or as prominent as this one. It’s the big one after that to look out for — on March 15, 2080. That will have almost exactly the same separation as this one does, and will be much easier to see, according to Rice University. That’s 60 years from today, something for younger sky watchers to be optimistic about.
Inputs from ENS Delhi