Great conjunction


A great conjunction is a conjunction of the planets Jupiter and Saturn. Great conjunctions occur regularly, every 19.6 years, due to the combined effect of Jupiter's approximately 11.86-year orbital period and Saturn's 29.5-year orbital period.
When studying the Great Conjunction of 1603, Johannes Kepler thought that the Star of Bethlehem might be the occurrence of a Great Conjunction. He calculated that a triple conjunction of Jupiter and Saturn occurred in 7BC.
The most recent great conjunction occurred on 31 May 2000; the next one occurs December 21, 2020. At this time Jupiter is 0.1 degree south of Saturn. The 2020 conjunction is the closest since 1623.
The 2000 conjunction fell within mere weeks after both planets had passed their conjunctions with the Sun; hence, the event was difficult to observe without visual aid because the pair rose only 30–45 minutes before sunrise, depending on the location of the observer.
Conjunctions occur in at least two separate coordinate systems. The conjunctions in Right Ascension occur in a coordinate system measured by a set of coordinates based on the celestial equator. This great circle is a projection of the earth's equator into the sky. The second system is based on the ecliptic, the plane of the solar system. When measure along the ecliptic, the separations are usually smaller. Additionally, it is important note that the exact moment of a conjunction cannot be seen by every observer because the two planets are not in the sky for everybody. So the observer's location must be taken into account. So this third system takes in the closest point of an observer. This is usually very close to the calculated date and time in the ecliptic coordinate system. See the charts below for the differences in time.
A Triple 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. The most recent occurred in 1981.
In the chart that follows, the elongation indicates the separation of Saturn from the sun. When angles are small, then the conjunction is difficult to see because of the brilliant sun's proximity.

In the years 1800 to 2100

DateTime
Angular distance
from Jupiter to Saturn		Elongation
from Saturn to the Sun		Constellation
21 July 180203:22:0042' South37.9° EastLeo
25 June 182100:05:091°15' North67.5° WestPisces
22 November 182123:49:551°20' North140.2° EastPisces
23 December 182109:28:491°22' North108.5° EastPisces
25 January 184222:22:3132' South26.8° WestSagittarius
25 October 186115:11:2052' South43.1° WestLeo
22 April 188111:58:201°18' North1.0° EastAries
21 December 202013:226' South30.3° EastCapricornus
5 November 204013:19:461°14' South24.8° WestVirgo
10 April 206009:01:251°09' North39.8° EastTaurus
15 March 208008:29:246' North43.8° WestCapricornus
24 September 210001:40:381°18' South25.1° EastVirgo

DateTime
Angular distance
from Jupiter to Saturn		Elongation
from Saturn to the Sun		Constellation
17 July 180222:57:0039' South40.6° EastLeo
19 June 182116:56:571°10' North63.3° WestPisces
26 January 184206:16:5332' South27.1° WestSagittarius
21 October 186112:27:0248' South39.7° WestLeo
18 April 188113:35:591°13' North3.1° EastAries
28 November 190116:37:3326' South38.2° EastSagittarius
10 September 192104:13:0357' South9.7° EastLeo
8 August 194001:13:201°11' North90.9° WestAries
20 October 194004:42:141°14' North164.0° WestAries
15 February 194106:36:251°17' North72.9° EastAries
19 February 196100:07:1814' South34.9° WestSagittarius
31 December 198021:17:241°03' South90.9° WestVirgo
4 March 198119:14:361°03' South155.9° WestVirgo
24 July 198104:13:351°06' South63.8° EastVirgo
28 May 200015:56:271°09' North14.9° WestAries
21 December 202018:37:316' South30.1° EastCapricornus
31 October 204012:02:471°08' South20.8° WestLibra
7 April 206022:36:241°07' North41.9° EastTaurus
15 March 208001:49:556' North43.5° WestSagittarius
18 September 210022:50:401°13' South29.4° EastVirgo

As omens

Great conjunctions have attracted considerable attention as celestial omens. As noted above, Johannes Kepler thought that the "Star of Bethlehem" was a great conjunction that occurred c. 7 BC. During the late Middle Ages and the Renaissance, great conjunctions were a 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 and 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 to 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 triangular pattern 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. Since each 'element' consists of 3 signs it takes 800 × 3 = 2400 years for the whole process to start anew.
Originally a trigon was thought to last 240 years, and the full cycle 960 years; but later more correct estimations 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.