Five thousand years of Jupiter–Saturn conjunctions


Not all close approaches of two planets are conjunctions. A classical conjunction occurs when a planet has the same right ascension as another celestial object, which may be a star, another planet, a satellite, etc. In other words, the two objects pass each other crossing the north–south line. As the ecliptic is inclined with respect to the celestial equator, it is possible for two objects to have the same ecliptic longitude without ever crossing the north–south line: such approximations in ecliptic longitude are not classed as conjunctions. Here, we only treat the former case.

The 2020 conjunction of Jupiter and Saturn (Figure 1) attracted a great deal of interest. Part of this interest was down to the minimum separation between the two planets being rather small (six arcminutes). Part was due to the fact that the conjunction occurred only a few days before Christmas. Another part was explained by the excellent placement of the two planets, low in the evening sky, which made observation easy (albeit less so from northern latitudes). However, the biggest single factor was possibly the parallel with the 7 BC triple conjunction that occurred around the time of the Nativity. This event is often linked with the Star of Bethlehem and the Nativity story, by authors such as Hughes (1976).1

Figure 1. The 2020 Dec 21 conjunction, observed from the seafront in Puerto de la Cruz (Tenerife). 1/17s exposure at ƒ/1.7; equivalent ISO 320 and 4mm focal length with a hand-held Samsung SM-A705FN smartphone. (Photo by Mark Kidger)
Figure 1. The 2020 Dec 21 conjunction, observed from the seafront in Puerto de la Cruz (Tenerife). 1/17s exposure at ƒ/1.7; equivalent ISO 320 and 4mm focal length with a hand-held Samsung SM-A705FN smartphone. (Photo by Mark Kidger)

Much has been written about Jupiter–Saturn conjunctions and, in particular, those of 7 BC and AD 2020, but the author is unaware of any studies of Jupiter–Saturn conjunctions as a whole that would put these two events in context. This has been the motivation for this study.

To give context, it should be stated that, although planetary conjunctions are common, Jupiter–Saturn events are much rarer. Sinnott (1968) pointed out that there were almost 200 conjunctions of the five brightest planets in the 18 years from 12 BC to AD 7 and 20 massings of three or more planets,2 with them all located within a circle of diameter three degrees. However, most of these events took place close to solar conjunction and would not have been observable, or would have been difficult to observe, being very close to the Sun in the sky. Others were relatively unspectacular, with a wide separation between the planets – these Sinnott rejected as ‘too commonplace’ (on this criterion, the 7 BC Jupiter–Saturn triple conjunction was rejected by him as a candidate for the Star of Bethlehem).

In the end, Sinnott found six two-planet conjunctions that took place at least 20 degrees from the Sun in the sky and with a minimum separation of 12 arcminutes or less. Of these, by far the most spectacular were the conjunctions of Venus and Jupiter in 3 BC and 2 BC. Unfortunately, these two events are strongly in conflict with the dating of the death of King Herod, which Josephus (c. 75 AD) appears to state happened in the month after the 4 BC March partial lunar eclipse.3 Josephus also gives a second line of dating, stating that the Battle of Actium occurred in the seventh year of Herod’s reign and that he died 37 years after the Roman Senate voted to make him king of Judea. The vote by the Senate happened in 41 BC, also giving a 4 BC date for his death.

Interestingly, the 7 BC triple conjunction is one of the few astronomical events for which we have surviving Babylonian records. Tablet BM 35429, in the British Museum, is an astronomical almanac for Seleucid Era 305 and describes the three conjunctions.4

Five millennia of Jupiter–Saturn conjunctions

The calculation

A search for conjunctions of Jupiter and Saturn was made for the interval 1000 BC to AD 4000, using Dance of the Planets (hereinafter ‘Dance’) software (v. 2.71, QED edition, ARC Software), using the DOSBox emulator. Despite its relative age, the software has excellent accuracy over several millennia. The calculation took approximately seven and a half hours with a Dell Ultrabook laptop.

The Dance software distinguishes in its results between genuine, classical conjunctions and approaches in which the ecliptic longitude is equal, but no physical conjunction occurs. No approaches of the latter class were found in this study.

(Login or click above to view the full illustrated article in PDF format)

The British Astronomical Association supports amateur astronomers around the UK and the rest of the world. Find out more about the BAA or join us.