Great conjunction
Great conjunctions occur approximately every 20 years when Jupiter "overtakes" Saturn in its orbit.
They are named "great" for being by far the rarest of the conjunctions between naked-eye planets[1] (i.e. excluding Uranus and Neptune).
During the late Middle Ages and Renaissance they were a topic broached by the pre-scientific and transitional astronomer-astrologers of the period up to the time of Tycho Brahe and Johannes Kepler, by scholastic thinkers such as Roger Bacon[3] and Pierre d'Ailly,[4] and they are mentioned in popular and literary works by authors such as Dante,[5] Lope de Vega,[6] and Shakespeare.
[7] This interest is traced back in Europe to translations of Arabic texts, especially Albumasar's book on conjunctions.
Since the equivalent periods of other naked-eye planet pairs are all under 900 days, this makes great conjunctions the rarest.
These returns are observed to be shifted by some 8° relative to the fixed stars, so no more than four of them occur in the same zodiacal constellation.
[14] Even greater importance was attributed to the beginning of a new cycle after all fours trigons had been visited.
[15] Use of the Alphonsine tables apparently led to the use of precessing signs, and Kepler gave a value of 794 years (40 conjunctions).
[14][5] Despite mathematical errors and some disagreement among astrologers about when trigons began, belief in the significance of such events generated a stream of publications that grew steadily until the end of the 16th century.
As the great conjunction of 1583 was last in the water trigon it was widely supposed to herald apocalyptic changes; a papal bull against divination was issued in 1586 but as nothing significant happened by the feared event of 1603, public interest rapidly died.
By the start of the next trigon, modern scientific consensus had condemned astrology as pseudoscience, and astronomers no longer perceived planetary alignments as omens.
This non-rotating coordinate system doesn't move with the precession of Earth's axes, thus being suited for calculations of the locations of stars.
(In astrometry latitude and longitude are based on the ecliptic which is Earth's orbit extended sunward and anti-sunward indefinitely.)
He calculated that a triple conjunction of Jupiter and Saturn occurred in 7 BC (−6 using astronomical year numbering);[18][19] The astronomers from the Cracow Academy (Jan Muscenius, Stanisław Jakobejusz, Nicolaus Schadeck, Petrus Probosczowicze, and others) observed the great conjunction of 1563 to compare Alfonsine tables (based on a geocentric model) with the Prutenic Tables (based on Copernican heliocentrism).
[23] The closest separation occurred on 21 December at 18:20 UTC,[11] when Jupiter was 0.1° south of Saturn and 30° east of the Sun.
This meant both planets appeared together in the field of view of most small- and medium-sized telescopes (though they were distinguishable from each other without optical aid).
[22] From mid-northern latitudes, the planets were visible one hour after sunset at less than 15° in altitude above the southwestern horizon in the constellation of Capricornus.
