A great conjunction is a conjunction of the planets Jupiter and Saturn, when the two planets appear closest together in the sky. Great conjunctions, named "great" for being the rarest and one of the brightest and closest on average of the conjunctions between "naked eye" planets (i.e. not counting the rarer conjunctions involving the ice giants as they were too dim to be discovered until after the invention of the telescope), occur approximately every 20 years when Jupiter "overtakes" Saturn in its orbit.
Celestial mechanics
On average, great conjunction seasons occur once every 19.859 Julian years (365.2500 days). Using the periods of Jupiter and Saturn in days, this number can be calculated by the formula 1/(1/4332.59−1/10759.22) = c. 7253.46 days, which is the average frequency of Jupiter "overtaking" Saturn as seen from the Sun due to the combined effect of Jupiter's approximately 11.9-year orbital period and Saturn's c. 29.5-year orbital period.[1] (In practice Earth's orbit size can cause great conjunctions to occur up to some months away from the average every 19.859 years time or the time they happen on the Sun.) Occasionally there is more than one great conjunction in a season when they occur close enough to opposition, this is a triple conjunction (which is not exclusive to great conjunctions). The most recent great conjunction occurred on 31 May 2000, and the next will occur on 21 December 2020. During the 2020 great conjunction, the two planets will be separated in the sky by a minimum of 6 arcminutes, the closest separation between the two planets since 1623.[2] The closeness is the result of one of the three approximately equally spaced longitude zones where great conjunctions occur shifting into the vicinity of one of the two longitudes where the two orbits appear to intersect when viewed from the Sun (which has a point of view similar to Earth). The great conjunction zones revolve in the same direction as the planets at the rate of approximately one-sixth of a revolution per four centuries thus creating especially close conjunctions on an approximately four century cycle. More precisely, the location in the sky of each conjunction in a series should increase in longitude by 16.3 degrees on average, making one full cycle relative to the stars on average once every 2,634 years. If instead we use the convention of measuring longitude eastward from the March equinox we have to keep in mind that the March equinox circulates once every c. 25,772 years so longitudes measured that way increase slightly faster and those numbers become 17.95 degrees and 2,390 years. The longitudes of close great conjunctions are currently about 307.4 and 127.4 degrees, in the constellations of Capricornus and Cancer respectively. Earth's orbit can make the planets appear up to about 10 degrees ahead or behind when they are at the optimal point, which also is true for any other part of their orbits.[1]
Saturn's orbit plane is inclined 2.485 degrees relative to Earth's, and Jupiter's is 1.303 degrees. It is interesting that the ascending nodes of both planets are similar, 100.6 degrees for Jupiter and 113.7 degrees for Saturn, so that if Saturn is above or below Earth's orbital plane Jupiter usually is too (this is partly caused by Earth's orbit being tilted relative to all the large planets). Because the orbit inclination directions of Jupiter and Saturn align reasonably well it would be expected that no closest approach will ever be much worse than Saturn's orbit tilt (2.485°) minus Jupiter's (1.303). Indeed, between the year 1 and 3000 the maximum conjunction distances were 1.3 degrees in 1306 and 1940. Conjunctions in both years occurred when the planets were tilted most out of the plane: longitude 206 degrees (therefore above the plane) in 1306, and longitude 39 degrees (therefore below the plane) in 1940.[1]
List of great conjunctions (1200 to 2400 AD)
The following chart[1] displays Earth's great conjunctions.
Longitude is counterclockwiseness from where the March equinox point was on (for this chart) 2000.0 AD. This non-rotating coordinate system doesn't move with Earth's axis wobble thus being suited for stars and physics calculations. (In astrometry latitude and longitude is based on the ecliptic which is Earth's orbit extended sunward and anti-sunward indefinitely.) The other common conjunction coordinate system measures counterclockwiseness in right ascension from the March equinox point and is based on Earth's equator and the meridian of the equinox point both extended upwards indefinitely (ecliptic separations are usually smaller).
Distance is planet closeness in sixtieths of a degree (minutes of arc) and elongation is angular distance from the Sun in degrees (morning sky is a negative number here).
Elongation between c. –20 and 20 degrees indicates that the Sun is close enough to the conjunction to make it difficult or impossible to see, sometimes more difficult at some geographic latitudes and less difficult elsewhere. Note that the exact moment of conjunction cannot be seen everywhere as it below the horizon or daytime in some places but place on Earth affects minimum separation less than it would if an inner planet was involved.
The digit after elongation is the great conjunction series (roughly analogous to the Saros series). Each great conjunction occurs about 119.16 years before or after the next or previous one of the same number. The reason it is every second conjunction in the same constellational area instead of every one is because adjacent ones are less similar than ones two cycles apart, this is because 119.16 years is closer to a whole number of years than 119.16÷2 is (all series will have progressions where conjunctions aligned with the Sun gradually move to the morning sky, pass midnight and then the evening sky before aligning once again with the Sun. The time for a series to make this full cycle should be about (360/(57.9-16.3))*119.16 = c. 1,360 years). The character after the digit is Y if it is easy to see, N if it is not and question mark if that depends on observer latitude. Finally there is a T if the conjunction is a member of a triple conjunction and a blank otherwise. (In a triple conjunction the series does not advance by one each event as the constellation and year is the same or close to it, this is the only time great conjunctions can be less than c. 2 decades apart.)[1]
Date
| Date (m/d/y) |
longitude | distance | elongation | |
|---|---|---|---|---|
| 04/16/1206 | 066.8 | 65.3 | 023.0 | 2? |
| 03/04/1226 | 313.8 | 02.1 | -048.6 | 3Y |
| 09/21/1246 | 209.6 | 62.3 | 013.5 | 4N |
| 07/23/1265 | 079.9 | 57.3 | -058.5 | 5Y |
| 12/31/1285 | 318.0 | 10.6 | 019.8 | 6? |
| 12/24/1305 | 220.4 | 71.5 | -070.0 | 1YT |
| 04/20/1306 | 217.8 | 75.5 | 170.7 | 1YT |
| 07/19/1306 | 215.7 | 78.6 | 082.5 | 1YT |
| 06/01/1325 | 087.2 | 49.2 | -000.4 | 2N |
| 03/24/1345 | 328.2 | 21.2 | -052.5 | 3Y |
| 10/25/1365 | 226.0 | 72.6 | -003.7 | 4N |
| 04/08/1385 | 094.4 | 43.2 | 058.8 | 5Y |
| 01/16/1405 | 332.1 | 29.3 | 018.1 | 6N |
| 02/10/1425 | 235.2 | 70.7 | 104.1 | 1YT |
| 03/19/1425 | 234.4 | 72.4 | -141.6 | 1YT |
| 08/24/1425 | 230.6 | 76.3 | 062.6 | 1YT |
| 07/13/1444 | 106.9 | 28.5 | -015.9 | 2N |
| 04/07/1464 | 342.1 | 38.2 | -052.6 | 3Y |
| 11/17/1484 | 240.2 | 68.3 | -012.3 | 4N |
| 05/25/1504 | 113.4 | 18.7 | 033.5 | 5? |
| 01/30/1524 | 345.8 | 46.1 | 019.1 | 6N |
| 09/17/1544 | 245.1 | 69.2 | 053.4 | 1Y |
| 08/25/1563 | 125.3 | 06.8 | -042.1 | 2Y |
| 05/02/1583 | 355.9 | 52.9 | -051.2 | 3Y |
| 12/17/1603 | 253.8 | 59.0 | -017.6 | 4N |
| 07/16/1623 | 131.9 | 05.2 | 012.9 | 5N |
| 02/24/1643 | 000.1 | 59.3 | 018.8 | 6N |
| 10/17/1663 | 254.8 | 59.2 | 048.7 | 1Y |
| 10/23/1682 | 143.5 | 15.4 | -071.8 | 2YT |
| 02/08/1683 | 141.1 | 11.6 | 175.8 | 2YT |
| 05/17/1683 | 138.9 | 15.8 | 077.5 | 2YT |
| 05/21/1702 | 010.8 | 63.4 | -053.5 | 3Y |
| 01/05/1723 | 265.1 | 47.7 | -023.8 | 4? |
| 08/30/1742 | 150.8 | 27.8 | -010.3 | 5N |
| 03/18/1762 | 015.6 | 69.4 | 014.5 | 6N |
| 11/05/1782 | 271.1 | 44.6 | 044.9 | 1Y |
| 07/16/1802 | 157.7 | 39.5 | 041.3 | 2Y |
| 06/18/1821 | 027.1 | 72.9 | -062.9 | 3Y |
| 01/26/1842 | 281.1 | 32.3 | -027.1 | 4? |
| 10/20/1861 | 170.2 | 47.4 | -039.5 | 5Y |
| 04/17/1881 | 033.0 | 74.5 | 003.8 | 6N |
| 11/28/1901 | 285.4 | 26.5 | 038.3 | 1Y |
| 09/08/1921 | 177.3 | 58.3 | 011.1 | 2N |
| 08/06/1940 | 045.2 | 71.4 | -089.8 | 3YT |
| 10/21/1940 | 041.1 | 74.1 | -165.7 | 3YT |
| 02/14/1940 | 039.9 | 77.4 | 073.3 | 3YT |
| 02/18/1961 | 295.7 | 13.8 | -034.5 | 4? |
| 01/01/1981 | 189.8 | 63.7 | -091.4 | 5YT |
| 03/06/1981 | 188.3 | 63.3 | -155.9 | 5YT |
| 07/25/1981 | 185.3 | 67.6 | 062.7 | 5YT |
| 05/28/2000 | 052.6 | 68.9 | -014.6 | 6N |
| 12/21/2020 | 300.3 | 06.1 | 030.2 | 1? |
| 11/04/2040 | 197.8 | 72.8 | -024.6 | 2? |
| 04/08/2060 | 059.6 | 67.5 | 041.7 | 3Y |
| 03/15/2080 | 310.8 | 06.0 | -043.7 | 4Y |
| 09/18/2100 | 204.1 | 62.5 | 029.5 | 5? |
| 07/15/2119 | 073.2 | 57.5 | -037.8 | 6Y |
| 01/14/2140 | 315.1 | 14.5 | 022.7 | 1? |
| 02/20/2159 | 215.3 | 71.2 | -050.3 | 2Y |
| 05/28/2179 | 080.6 | 49.5 | 016.1 | 3N |
| 04/08/2199 | 325.6 | 25.2 | -050.0 | 4Y |
| 11/01/2219 | 221.7 | 63.1 | 006.8 | 5N |
| 09/06/2238 | 093.2 | 39.3 | -067.6 | 6YT |
| 01/12/2239 | 090.2 | 47.5 | 161.3 | 6YT |
| 03/22/2239 | 088.4 | 45.3 | 089.9 | 6YT |
| 02/02/2259 | 329.6 | 33.3 | 019.6 | 1? |
| 02/05/2279 | 231.9 | 69.9 | -080.3 | 2YT |
| 05/07/2279 | 229.9 | 73.8 | -172.6 | 2YT |
| 08/31/2279 | 227.2 | 74.9 | 073.3 | 2YT |
| 07/12/2298 | 100.6 | 28.3 | -006.0 | 3N |
| 04/26/2318 | 339.8 | 41.8 | -051.8 | 4Y |
| 12/01/2338 | 237.3 | 66.3 | -007.4 | 5N |
| 05/22/2358 | 107.5 | 18.5 | 050.7 | 6Y |
| 02/18/2378 | 343.7 | 50.5 | 019.4 | 1N |
| 10/02/2398 | 240.7 | 65.9 | 058.2 | 2Y |
Notable great conjunctions
List of close great conjunctions (consisting of — in the first 3 millennia AD — all events under 9.95 arcminutes, plus selected other notable events)[2][1]
| Date | Separation (in arcminutes) | Visibility | Notes |
|---|---|---|---|
| 1 March 1793 BC | 1.3 | Evening | The closest conjunction between prehistoric times and the 46th century AD. Part of a triple conjunction. |
| 28 December 424 BC | 1.5 | Evening, hard to see. | |
| 6 March 372 | 1.9 | Morning | The closest conjunction of the first three millennia AD. |
| 31 December 431 | 6.2 | Evening, hard to see. | |
| 13 September 709 | 8.3 | Morning, part of a triple conjunction. | |
| 22 July 769 | 4.3 | Too close to the Sun to be visible. | |
| 11 December 1166 | 2.1 | Evening, not easy to see. | |
| 4 March 1226 | 2.1 | Morning | |
| 25 August 1563 | 6.8 | Morning | |
| 16 July 1623 | 5.2 | Evening, hard to see, especially from Northern Hemisphere. | |
| 21 December 2020 | 6.1 | Evening, hard to see from high northern latitude, not visible in Antarctic (poor angle, summer sun). | 303+ degree heliocentric longitude close to the ideal 317 degree orbit plane intersection longitude for closeness (J2000) |
| 15 March 2080 | 6.0 | Morning | |
| 24 August 2417 | 5.4 | Morning, not easy to impossible to see from parts of the Southern Hemisphere and Arctic. | |
| 6 July 2477 | 6.3 | Evening, easier to see in the Southern Hemisphere. | |
| 25 December 2874 | 2.3 | Evening, summer sun hinders viewing in Antarctica. | |
| 19 March 2934 | 9.3 | Morning | |
| 8 March 4523 | 1.0 | Morning, not easy to impossible to see from high northern latitudes and South Pole area. | The closest conjunction in an almost 14,400 year period (a gross of centuries) |
7 BC
When studying the great conjunction of 1603, Johannes Kepler thought that the Star of Bethlehem might have been the occurrence of a great conjunction. He calculated that a triple conjunction of Jupiter and Saturn occurred in 7 BC (−6 using astronomical year numbering).[3][4] 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 right ascension on three occasions or same ecliptic longitude on three occasions depending on which definition of "conjunction" one uses (this is due to apparent retrograde motion and happens within months). The most recent triple conjunction occurred in 1980 and 1981[5] while the next will be in 2239.
1563
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). In the Prutenic Tables the astronomers found Jupiter and Saturn so close to each other that Jupiter covered Saturn[6] (actual angular separation was 6.8 minutes on 25 August 1563[1]). The Alfonsine tables suggested that the conjunction should be observed on another day but on the day indicated by the Alfonsine tables the angular separation was a full 141 minutes. The Cracow professors suggested following the more accurate Copernican predictions and between 1578 and 1580 Copernican heliocentism was lectured on three times by Valentin Fontani.[6]
2020
The great conjunction of 2020 will be the closest since 1623[2][1] and eighth closest of the first three millennia AD with a minimum separation between the two planets of 6.1 arcminutes.[1] It will occur seven weeks after the heliocentric conjunction, when Jupiter and Saturn shared the same heliocentric longitude.[7] The closest separation occurs on 21 December at 18:22 UTC,[5] when Jupiter will be 0.1° south of Saturn and 30° east of the Sun. This means both planets will be visible in the same telescopic field of view (though they will be distinguishable from each other without optical aid).[8] From mid-northern latitudes, the planets will be visible at less than 15° in altitude above the southwestern horizon in the constellation of Capricornus, one hour after sunset.[9][10]
7541
As well as being a triple conjunction, the great conjunction of 7541 will feature one transit[note 1] on 16 February, and one occultation[note 2] on 17 June (although the accuracy of planetary positions this far into the future cannot be calculated, and so sources differ as to the exact nature of these occultations[note 3]).[5] This will be the first occultation between the two planets since 6857 BC; superimposition requires a separation of less than approximately 0.4 arcminutes.[1]
In history
See also: Astrological aspect § Great conjunctions
Great conjunctions attracted considerable attention in the past as omens. 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 and Kepler, by scholastic thinkers such as Roger Bacon[11] and Pierre d'Ailly,[12] and they are mentioned in popular and literary works by authors such as Dante[13] and Shakespeare.[14] This interest is traced back in Europe to translations of Arabic texts especially Albumasar's book on conjunctions.[15]
Despite mathematical errors and some disagreement among astrologers about when trigons began, belief in the significance of such events generated a stream of publications which 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 long established astrology as pseudoscience, and planetary alignments were no longer perceived as omens.[16]
In astrology
See also: Astrological aspect § Great conjunctions
Trigon (Kepler's 1606 book De Stella Nova)
As great conjunctions recur c. 120° apart their appearances form a triangle shape. Usually they stay in one of the following triplicities or trigons of signs:
Fire: Aries, Sagittarius and Leo
Earth: Taurus, Capricorn and Virgo
Air: Gemini, Aquarius and Libra
Water: Cancer, Pisces and Scorpio
Every third conjunction is c. 8° east of the original so no more than four can occur in the same sign. A new trigon is considered a major event by believers and a return to the first trigon even more so.
See also
Astronomy portal
Celestial mechanics
Positional astronomy
Triple conjunction
References
Where Jupiter partially obscures the disk of Saturn
Where Jupiter completely obscures the disk of Saturn
Some calculations of planetary positions predict very close conjunctions in 7541 instead of transits and/or occultations
"Jupiter-Saturn Conjunction Series". sparky.rice.edu.
Hunt, Jeffrey L. (20 February 2020). "1623: The Great Conjunction of Jupiter and Saturn". When the Curves Line Up. Retrieved 24 August 2020.
Burke-Gaffney, W. (1937). "Kepler and the Star of Bethlehem". Journal of the Royal Astronomical Society of Canada. 31: 417. Bibcode:1937JRASC..31..417B. Retrieved 27 May 2020.
Molnar, Michael R. (1999). The Star of Bethlehem: The Legacy of the Magi. Rutgers University Press.
Jones, Graham. "The December 2020 Great Conjunction". timeanddate.com. Retrieved 10 December 2020.
Kesten, Hermann (1945). Copernicus and his World. New York: Roy Publishers. p. 320.
Hunt, Jeffrey L. (11 September 2020). "2020, November 2: Jupiter – Saturn Heliocentric Conjunction". When the Curves Line Up. Retrieved 29 October 2020.
"2020: December 21: The Great Conjunction of Jupiter and Saturn". When the Curves Line Up. 11 December 2019. Retrieved 11 August 2020.
Hunt, Jeffrey L. (11 December 2019). "2020: December 21: The Great Conjunction of Jupiter and Saturn". When the Curves Line Up. Retrieved 27 August 2020.
"5 upcoming conjunctions visible in the night sky, and how to see them". Retrieved 17 August 2020.
The Opus Majus of Roger Bacon, ed. J. H. Bridges, Oxford:Clarendon Press, 1897, Vol. I, p. 263.
De concordia astronomice veritatis et narrationis historice (1414) [1]
Woody K., Dante and the Doctrine of the Great Conjunctions, Dante Studies, with the Annual Report of the Dante Society, No. 95 (1977), pp. 119–134
Aston M., The Fiery Trigon Conjunction: An Elizabethan Astrological Prediction, Isis, Vol. 61, No. 2 (Summer, 1970), pp. 158–187
De magnis coniunctionibus was translated in the 12th century, a modern edition-translation by K. Yamamoto and Ch. Burnett, Leiden, 2000
Keith Thomas, Religion and the Decline of Magic: Studies in Popular Beliefs in Sixteenth and Seventeenth Century England (Oxford University Press, 1971) p. 414-415, ISBN 9780195213607
Hellenica World - Scientific Library
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