By Duncan Steel 20/12/2020


There have been many articles in the mass media about the ‘Great Conjunction’ between Jupiter and Saturn that will occur on December 21st. Some of them have been good, and informative. Many have been fairly poor. Others have been… well, weird.

Some writers have imagined that there is something vitally significant about the conjunction (the close approach to each other in the sky) of the two giant planets occurring on the same day as the solstice. That’s just by chance: there is no astronomical link between the two phenomena. From our perspective in the southern hemisphere the fact that the conjunction occurs at much the same time as the (austral) summer solstice is a bad thing, in that it reduces our opportunity to witness the coming-together of the planets. Here in the antipodes the conjunction takes place near the time of longest daylight, so the Sun spoils our chances to some extent; in the northern hemisphere it’s the shortest day (the winter solstice), meaning that they are better-positioned to watch the approach between Jupiter and Saturn.

Some reports I have read have mixed up the times of the conjunction and the solstice, and as I wrote, these are independent events. Actually, the precise times are not terribly important, but for the record the solstice (when the Sun reaches its southernmost point during the year) is at 23:02 NZDT on December 21st, whilst the smallest angular separation of Jupiter and Saturn will take place at 07:20 on the morning of December 22nd, NZDT again.

What might you see?
If you are fortunate to have clear skies in the west, soon after sunset you will see Jupiter and Saturn being so close to each other in the sky that they may seem almost to be merged. They will be about ten or fifteen degrees above the western horizon, close to where the Sun had recently dipped out of view. They are by far the brightest objects in that part of the sky.

At closest approach (the actual conjunction) they will be about one-tenth of a degree apart. The Moon in the sky appears to be around one-half of a degree in diameter, and so at their closest to each other Jupiter and Saturn will be about one-fifth of the apparent lunar diameter apart.

The diagram at the head of this blog post I will repeat here, in full, to show why this apparent close approach occurs:

The conjunction between Jupiter and Saturn on 2020 December 21st (UTC). In this diagram the planets orbit the Sun in the counter-clockwise direction, and the Earth also spins on its axis in that direction. During the day, the outer planets Jupiter and Saturn are lost in the solar glare, but as Earth rotates and the Sun drops below the horizon at sunset those planets are visible low in the western sky. With white lines I have shown the alignment.

In that solar system map above the alignment is shown in the average orbital plane of the planets, but there is more to it than that. The plane of Earth’s orbit we term the ecliptic, and the other planets mostly circuit the Sun in much the same plane… but not precisely. Therefore the planets appear to follow the extrapolation of the ecliptic into the sky approximately, but not precisely. Sometimes one might appear a few degrees above the ecliptic, and at other times a few degrees below it. Because of this, when most conjunctions (near-alignments) occur, the planets will be several degrees apart. This year, however, is a Great Conjunction: it happens that Jupiter and Saturn will be close to aligned not only in terms of their positions orbiting the Sun (as in the diagram above) but also in terms of the up-and-down dimension, perpendicular to the ecliptic.

If you read this but it is cloudy on Monday night: don’t despair. The two planets have been getting progressively closer over the past few weeks, and although they will be slightly further apart on Tuesday evening, it will still be worth taking a peek. Indeed any time over the next week or so Jupiter and Saturn will be seen in near proximity soon after sunset in the western sky.

Frequency of Jupiter-Saturn conjunctions
Conjunctions between these two bright planets occur about every 20 years. It is easy to see why, with a bit of arithmetic. Jupiter’s takes about 12 years to complete an orbit around the Sun; Saturn, being further out, takes close to 30 years. As a result there are conjunctions about every X years, where (1/X) = (1/12) – (1/30). A few taps on a calculator and one can get the answer that X = 20 years. An old-timer like me, who went to school before electronic calculators became available, might instead solve that puzzle by expressing the equation in terms of the denominators being equal: (1/X) = (5/60) – (2/60) = (3/60) thus X = 20.

That once every 20 years, however, is for any Jupiter-Saturn conjunction. Most often there are some degrees between them, perpendicular to the ecliptic. Really close conjunctions like this one occur about every 400 years. The last such event was in 1623, but there are no records of it being witnessed, and that is not surprising because the planets were very close to the direction of the Sun in that episode.

Looking through a telescope
If you have access to a telescope, it would be well worthwhile watching this dance of the planets. I hasten to add though that you should not try pointing your telescope in their direction until after the Sun has completely disappeared below the horizon.

Many astronomical groups will be setting up telescopes for the public to take a peek, weather allowing. In Nelson, I know that there will be a group gathering at the very highest point on Princes Drive, from where a view across Tasman Bay and the northern limit of the Southern Alps is available, and arrangements have been made for the few street lamps there to be switched off so as to facilitate viewing.

Here is a NASA diagram showing what the planets and their natural satellites will look like at the time of nearest approach:

Graphic indicating how Jupiter and Saturn will appear around the time of the Great Conjunction on Monday/Tuesday December 21/22.

Again I hasten to add that one should not be expecting seeing anything as vivid as that composite view. It is wonderful to see the planets ‘live’ through a telescope with your very own eye, but the best images by far are obtained by the Hubble Space Telescope or the spacecraft we have sent to explore those planets up-close.

Historical significance of Great/Triple Conjunctions
Such comings-together of the two bright giant planets have been noted by astronomers and astrologers across antiquity. The reasons for these events were of course not fully understood until modern times, and so a variety of meanings were read into the occurrences. Of especial note to the ancients were what is known as Triple Conjunctions, when Jupiter and Saturn were witnessed to come close to each other not just once, but three times spaced over a matter of weeks. This really was a dance of the planets, made possible by the fact that sometimes the Earth is ‘undertaking’ the outer planets on the inside (closer than them to the Sun) and so there is an apparent reflex movement. Such Triple Conjunctions can also occur with other pairs of planets involved, for example Mars and Jupiter.

Although there was a Jupiter-Saturn Triple Conjunction in 1980-81, there will not be another until 2239.

The great astronomer Johannes Kepler witnessed a Jupiter-Saturn conjunction in 1603 (i.e., that 20 year cycle-time before the last Great Conjunction of 1623), and calculated that a Triple Conjunction between those two planets would have occurred in 7 BCE. From that derives the suggestion that the biblical Star of Bethlehem might be linked to that conjunction having been seen at that time.

There were other arguments that supported Kepler’s interpretation of the conjunction(s) of 7BCE, one of them being that the planets were at that time in the constellation Pisces, and that constellation was astrologically associated with Israel and the belief that a messiah-king would be born in Israel.

The Massing of the planets
The following year (which we now term 6 BC or BCE) Jupiter and Saturn were still in Pisces, and were joined there by Mars. This is sometimes referred to as a Massing of the planets: for some months they appeared close to each other in the sky, at one stage being spread by only about 10 degrees.

Not only that, but shortly thereafter Jupiter was twice occulted by the Moon, four weeks apart. That is, the Moon in its orbit around Earth cut off all visibility of Jupiter for a short time as the planet passed behind it. For people who thought that the sky was ‘perfect’ with the stars and planets being arranged on a heavenly sphere, this was disturbing and unusual.

Myself, I have no religious beliefs, nor superstitions (really), but I am interested in the historicity of what might have occurred at this time in the near/middle East, and what astronomy can tell us about the lifetime of Jesus. In considering the above astronomical facts, one might consider that these events might have ‘primed’ the Magi (Wise Men) that celestial events were indicating something unusual.

The Comet of Bethlehem?
The biblical story (in the Gospel of St Matthew) has a star appearing in the east and leading the Magi westwards to Bethlehem.

It happens that ancient Chinese records have a comet appearing early in March in 5 BCE in Capricornus, which was then in the east. The comet was visible for at least 70 days (consistent with the timescale for the travel of the Magi) and it moved from the east to the south, again consistent with the biblical story.

While this is only mentioned briefly here, the identification of the Star of Bethlehem with this comet has been widely written about, analysed, and criticised. This formed the substance of a chapter of a book I wrote two decades ago about the history and astronomy of the calendar, but there have been many other discussions of the facts and their interpretations published elsewhere.

Does 5 BCE fit?
Just to finish off this short discussion of the birth of Jesus, I might anticipate that some readers might imagine that a comet appearing in 5 BCE cannot fit. That is a false impression.

It is well-known to chronologists that the dating of our era (AD/BC, or CE/BCE) is ‘wrong’ in that Jesus was born some years earlier than the zero mark in that dating scale (about which, more below). This error is apparently due to the Scythian monk Dionysius Exiguus (Dennis the Little!), who was charged by the Pope in the early sixth century with deriving a set of tables for future dates of Easter, having made a simple error: he interpreted a record of Jesus having been born in a certain year of the reign of Augustus Caesar as counting from when that first emperor assumed that name (in 27 BCE) rather than from when that person actually took control of the Roman Empire (in 31 BCE), at which time he was still known as Octavian. Thus a four-year slip-up occurred.

A date for the birth of Jesus shortly before the middle of April in 5 BCE is consistent with the biblical and other records, such as the reign or Herod, and the date of Passover.

Why Christmas Day is December 25th and does not coincide with the solstice
To conclude, again a brief summary of a matter which could occupy thousands of words.

It happens that there are several astronomical connections that lead to December 25th being Christmas Day (despite, as written above, it being apparent that Jesus was born around mid-April, and some years earlier than our dating system would suggest). Here we go.

The Archangel Gabriel is said to have descended to tell Mary that she would bear the Son of God on a day that is now termed the Feast of the Annunciation, or Lady Day, or the Feast of St Mary. This is March 25th, which is the traditional date of the equinox (and indeed the date of New Year in England and Wales until 1752). If one counts forward the nine-month human gestation period then one comes to December 25th, which is the traditional date of the solstice, which was about the end of the week or so when Romans celebrated Saturnalia. Note the astronomical connections!

A Jewish boy would be circumcised and named usually on the eighth-day post partum counted inclusively, which means January 1st for a birth on December 25th. In the ecclesiastical calendar, January 1st is the Feast of the Circumcision. Thus our common dating system is based on that: birth on December 25th in 1 BCE, and the circumcision on January 1st CE (or AD).

The only thing left to explain is why nowadays the solstice occurs not on December 25th, but rather at a time that ranges across December 21-22 and more (the full range on the Gregorian calendar is about 53 hours across the 400-year cycle of the calendar). Thus, for example, this year the solstice is at 10:02 UTC on December 21st (23:02 NZDT).

In short, the solstice occurs several days earlier than December 25th, that being its traditional date. Similarly the equinox in March occurs at a time ranging between the 19th and the 21st, rather than on the traditional date of the 25th. How come?

The answer is that when the Catholic Church was reforming the calendar in the 1570s-80s, supplanting the Julian calendar with the Gregorian (named for Pope Gregory XIII) from 1582, the decision was made to hinge its zero point on the dates of the equinoxes and solstices in 325 CE, when the Council of Nicaea occurred and the Easter computus was adopted. This was more than three centuries after the birth of Jesus, and in the interim the Julian calendar (one leap year every four years) had forced these astronomical events to arrive earlier on the calendar. The Gregorian system (97 leap years in its 400-year cycle) is a better approximation for the actual year length, but in the 16th century it was decided to fix the phasing of our dating against the deliberations of the Nicene Fathers rather than the actual era of Jesus.

The reasoning behind this has been the subject of considerable debate… but that’s why Christmas Day does not coincide with the solstice.