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A Great Conjunction is a conjunction of the planets Jupiter and Saturn. The last Great Conjunction took place on May 31, 2000, while the next one will be in late December 2020. Great Conjunctions take place regularly, every 18–20 years, as a result of the combined ~12-year orbital period of Jupiter around the Sun, and Saturn's ~30-year orbital period. The 2000 conjunction fell within mere weeks after both had passed conjunction with the Sun, and it was very difficult to observe without visual aid because the two planets rose only 30–45 minutes before sunrise, depending upon the location of the observer.
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Greatest conjunction
Greatest conjunction is a conjunction of Jupiter and Saturn at or near their opposition to the Sun. In this scenario, Jupiter and Saturn will occupy the same position in right ascension on three separate occasions over a period of a few months. Such tripled occurrences are actually known as triple conjunctions.
The so-called "Star of Bethlehem" – thought to have appeared c. 7 BC – was theorized to be a greatest conjunction; and some went so far as to assert that it was an occultation of Saturn by Jupiter, with the two planets appearing to merge into a single object as seen from Earth. However such an event did not take place at historic times. At the greatest conjunction in 7 BC, which is said to be the "Star of Bethlehem", the minimum distance between Jupiter and Saturn was around 1 degree, this is twice the Moon's diameter. The next occultation of Saturn by Jupiter will take place in 7541.
Great Conjunctions and history
Great conjunctions are less spectacular than eclipses or comets but nevertheless they have also attracted considerable attention as celestial omens. During the late Middle age and the Renaissance they became a rather popular topic broached by most astronomers of the period up to the times of Tycho Brahe and Kepler, by scholastic thinkers as Roger Bacon or Pierre d'Ailly, and they are mentioned in popular and literary writing by authors such as Dante or Shakespeare. This interest is traced back in Europe to the translations from Arabian sources, most notably Albumasar's book on conjunction.
As successive great conjunctions occur nearly 120° apart, their appearances form a triangular pattern. In a series every fourth conjunction returns after some 60 years in the vicinity of the first. These returns are observed to be shifted by some 7-8°, so no more than four of them occur in the same zodiacal sign. To each sign astrologers have ascribed one from the series of four elements and thus four triplicities or 'trigons' are formed. Particular importance has been accorded to the occurrence of a great conjunction in a new trigon, which is bound to happen after some 200 years at most. Even greater importance was attributed to the beginning of a new cycle after all fours trigons had been visited, something which happens in about 800 years.
Originally a trigon was thought to last 240 years and the full cycle 960 years but later more correct estimation were provided by the Alphonsine tables. Despite the inaccuracies and some disagreement about the beginning of the cycle the belief in the significance of such events generated a stream of publications which grew steadily up to the end of the 16th century. As the great conjunction of 1583 was the last in the watery trigon it was widely supposed to herald apocalyptic changes; a papal bull against divinations was issued in 1586 and as nothing really significant had happened by 1603 with the advent of a new trigon, the public interest rapidly died.