A Great Conjunction is when Jupiter and Saturn reach the closest point in their orbit, and appear mere degrees apart, as one bright star from Earth. 21 December 2020 will be the first Great Conjunction in 400 years and the closest since 1226. The next will be in 2080.
Jupiter and Saturn are the two largest planets in our solar system. Both are gas giants over nine times the diameter of Earth and the furthest planets from Earth we can see with the naked eye.
Seeing Uranus, Neptune or Pluto requires a telescope. Jupiter takes 12 earth-years to orbit the sun, Saturn takes 30. During the Great Conjunction, they appear 1° apart with a naked eye and 5° with a telescope.
The Great Conjunction of 2020 will take place on the Winter/Summer Solstice and be best visible in the Northern Hemisphere. The planets will appear in the southwest, with Saturn being above/left of Jupiter in the Northern Hemisphere and below/right in the Southern. With binoculars, you can see Jupiter’s four Galilean moons – Callisto, Europa, Ganymede and Io. With a telescope, you can catch all 75 of its moons and the ‘Great Red Spot’. The conjunction will be visible one hour after sunset.
We know of older conjunctions from astronomical records left by the Babylonians, Greeks, Romans and early modern Europeans. Many, however, were either too shrouded or close to the sun to be visible from Earth.
Largest visible recorded Great Conjunctions:
- 1st March 1793 BC – 0.02°, the closest recorded conjunction.
- 6 March 372 – 0.03°
- 6 March 372, 0.1°
- 13 September 709 – 0.1°
- 4 March 1226 – 0.03°
- 25 August 1523 – 0.1°
- 16 July 1623 – 0.08°
- 21 December 2020 – 0.01°
Polish astronomers observed the Great Conjunction of 1523 from the Krakow Academy and used it to prove Copernicus’s heliocentric model.
The last Great Conjunction was in 1623, in the early days of the telescope. Jupiter and Saturn appeared 0.08° apart. 2020’s conjunction will be even closer, and the near since 1226 when the hordes of Genghis Khan were ravaging Asia and Saint Francis was in the last year of his life.
Johannes Keppler, the famed German astronomer, suggested in 1614 that the (minor) conjunction of 7 BC was the Star of Bethlehem attested in the Bible. Modern scholarship suggests it is most likely the ‘Star’ was an eclipse of Jupiter by the Moon around 14 April 6 BC, which would have appeared in the west of Judah and thus led the Three Wise Men from the east.
Sources: Astrtonomy, News Scientist, Wikipedia