Kepler and the Great Conjunctions Jupiter/Saturn



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The conjunction of Jupiter and Saturn in the constellation of Capricorn of December 21, 2020 will be especially interesting both for astronomical and historical reasons.
With an apparent distance of only 6’ between the two giant planets, it will be the closest since the year 1623, when Johannes Kepler himself wrote a short treatise about the event (Discourse about the Great Conjunction, 1623).


Medieval astronomers, especially Perso-Arabic authors such as Masha’allah and Albumasar, had been particularly attracted by the so-called Great Conjunctions, and had developed a complex system of classification and astrological interpretation that divided conjunctions into small (recurring every 20 years), medium (every 240 years with change of “elemental triplicity”) and greatest (every 960 years), attributing to them power over human affairs and an influence on the history of the world.


Johannes Kepler, wishing to verify and reform these theories, observed some of these conjunctions in the wake of the invention of the telescope, updating the data and correcting the calculations. The 1603/1604 conjunction, in particular, saw the contemporaneous appearance of a new star (today known as SN1604 or Kepler’s Nova) that was described in De stella nova in pede Serpentarii, helping Kepler to challenge the traditional Aristotelian view of a perfect and immutable cosmos, where nothing new can ever happen.
That discovery also suggested to him the possibility that a similar occasion (a conjunction of the superior planets with the apparition of a comet, a nova, or some other unexpected astronomical phenomenon) had happened a little before the birth of Christ, signalling the “miraculous” event and letting him calculate a more precise date for it.


An awareness of the importance attributed to the conjunctions Jupiter/Saturn in history, and the peculiarity of the coniunctio media of 2020, then, will add new value to the observation of the sky on the 21st of December.