Many explanations have been proposed for the Star of Bethlehem, the star that guided the Magi to Jerusalem. One of the most popular is that of the conjunction of Jupiter and Saturn that took place during 7BC. Although it is not exactly the same event that is repeating this Christmas, sky watchers can see an event quite similar to one observed before the Nativity and can see for themselves if it would have been impressive enough to launch the Magi on a journey of around 2000km to Jerusalem. By a curious coincidence, this Christmas conjunction is very similar to one observed by the astronomer Johannes Kepler, in 1603, that first led him to draw attention to the 7BC event.

The question of what the Star of Bethlehem was has occupied the minds of scientists, philosophers and theologians for almost two thousand years. Almost certainly we will never know what it was or, if it even existed. Many biblical scholars believe that the Star never existed. To them, the reason why Matthew is the only Gospel to mention it is that he added it to his account of the Nativity to emphasise its miraculous nature. The more fundamental Christians, on the other hand, say that the Star was a miraculous event and needs no explanation. For a scientist, though, all the mysteries of the Universe have a rational explanation: you just need enough data to find it.

Some of the explanations offered for the Star of Bethlehem are more plausible than others. Some theories are only possible if the accepted biblical chronology is out by a number of years. So, if we accept that King Herod indeed died in late March or early April 4BC, there are four basic theories that all widely accepted:

• A triple conjunction of Jupiter and Saturn in 7BC.
• An occultation of Jupiter by the Moon in 6BC.
• A nova in 5BC.
• An unrecorded bright comet.

Each has its defenders. One of the issues that complicate matters is the fact that Chinese records are quite incomplete around the time of the Nativity. Just four events are recorded between 12BC and 1BC, two of which are duplicates. When these duplicates are eliminated, we are left with just two:

• The return of Comet Halley in 12BC, which was well-observed.
• More fragmentary records of a probable nova in southern Aquila or northern Capricorn in March/April 5BC, although some people interpret this event as possibly a comet.

Although not recorded in Chinese chronicles (in contrast, it was by the Babylonians) an event that happened shortly before the Nativity has intrigued astronomers since the time of Johannes Kepler at the start of the 17^th Century is the 7BC triple conjunction of Jupiter and Saturn.

The combined effect of Jupiter orbiting the Sun every 11.8 years and Saturn doing so every 29.5 years is that approximately every twenty years Jupiter catches and passes Saturn in its orbit. To be precise, Jupiter overtakes Saturn, on average, every 19.6 years. When this happens, the two planets align. Usually, the two pass each other in the sky a degree or two apart; sometimes this happens too close to the Sun in the sky to observe (for example, the last conjunction, in May 2000 occurred too close to the Sun in the sky to be visible easily) but, more usually, we see the conjunction as an interesting, if not striking pairing. More rarely, the Earth, Jupiter and Saturn align in such a way that we see not one, but three conjunctions over a period of approximately seven months: what is called a Triple Conjunction. The average is about seven triple conjunctions per millennium, although the interval between them can be as little as 40 years (as it was between 1941/42 and 1981) and as long as three centuries.

Johannes Kepler was a brilliant astronomer and mathematician, but also an astrologer. He had calculated that a conjunction of Jupiter and Saturn would take place in the constellation of Sagittarius between December 16^th and 18^th 1603. This event excited him because it was the third in a series of conjunctions that had taken place successively in Leo, Aries and now, Sagittarius, the three constellations associated with fire (each constellation of the zodiac was associated with one of the elements: earth, water, air and fire). For an astrologer, this triplet of conjunctions over forty years in fire signs constituted a fiery trigon. Then, Mars joined Jupiter and Saturn in Sagittarius , forming a triangle in Sagittarius, thus  forming a “fiery trigon within a fiery trigon”. Excitement in Germany reached fever-pitch in expectation of calamities and the appearance of a comet. What followed was a brilliant new star (now known as Kepler’s Supernova), which appeared between Jupiter and Saturn on October 10^th 1604. Kepler calculated that a similar conjunction of Jupiter and Saturn would have occurred in 7 BC, before the Nativity and speculated that, like the 1603 conjunction, it was followed by a bright new star – the Star of Bethlehem.

We can calculate the exact circumstances of the 1603 conjunction. In reality, it occurred in the constellation of Ophiuchus although, for astrologers, this is the sign of Sagittarius. Closest approach of the two planets was 60 arcminutes (twice the diameter of the Moon) at 05:13UT on December 18^th 1603, very low in the dawn sky. The two planets were just 18 degrees from the Sun and although Kepler could see Jupiter, seeing the much fainter Saturn was not possible until some nights later.

What Kepler did not know is that the next conjunction of Jupiter and Saturn, that of 1623, would be far closer and more spectacular. In fact, the 2020 conjunction, which takes place on the Sagittarius-Capricorn border, is the closest since 1623. Having spent thirteen months in Sagittarius, Jupiter moves into Capricorn on December 18^th, just before the conjunction of the two planets. Closest approach between the two planets, to just 6 arcminutes, is in daylight from Europe, at 13:30UT. Jupiter and Saturn will be between the stars Beta Capricornii and Rho Sagittarii.

It will not be until March 2080 that Jupiter and Saturn will be this close again, low in the dawn sky in Sagittarius.

The two planets have been quite close together all through 2020. In mid-May they closed to under 5⁰ before separating again.

Since the start of September the two have got steadily and obviously closer, initially only very slowly and then much faster. Compare these two images from the end of October and from early December:

We can see in an expanded plot of separation against date how close the two planets will get to each other. For two weeks they will be less than a degree apart in the sky and for several nights around December 21^st they will be difficult to separate with the naked eye, as Jupiter is about ten times as bright as Saturn and will overpower its fainter neighbour in its glare.

Although this will not be a spectacular event to the casual observer, it is an interesting and very unusual one that provides us with a link to an astronomer/astrologer of four centuries ago, his speculations about the Star of Bethlehem and an event that the Magi would have seen with their own eyes. And, we will see it in the Christmas sky. Miss it and you will have to wait until March 2080 to see a similar event.