Throughout 2020, the planets Jupiter and Saturn have been close together in the sky. Over the summer, they were within a few degrees of one another, bright in the southern sky, and they will continue to approach one another throughout the fall. On the evening of Dec 21, 2020, Jupiter and Saturn will be in conjunction (closest approach). This page describes specifics for the event. There is much more to learn! Refer to the general Jupiter-Saturn conjunction page to learn all about these conjunctions over a 3000 year interval starting in 0 CE.
MAIN TAKEAWAY: THE ACTUAL CONJUNCTION IS A RARE EVENT DEFINITELY WORTH SEEING, BUT WILL BE A CHALLENGE TO OBSERVE FROM SOME LOCATIONS. IT'S EASY TO FOLLOW THE PLANETS IN THE WEEKS BEFORE THE CONJUNCTION THOUGH. NEAR DEC 21, JUPITER AND SATURN WILL APPEAR LIKE A BINARY PLANET IN THE DUSK SKY. HOWEVER, THEY WILL *NOT* SUDDENLY LOOK LIKE A BRILLIANT 'CHRISTMAS STAR'.
Diagram of the Conjunction of Jupiter and Saturn on December 21, 2020, as
viewed from three different latitudes when the Sun is 6 degrees below the horizon.
The diagram below, adapted
from the program Stellarium, shows what the conjunction will look
like in a telescope at the end of civil twilight in Houston.
Views from other locations a few hours one way or
another will look similar, though Jupiter's moons will move a bit, and
Jupiter is moving up relative to Saturn in the image. At closest approach
the separation between the planets is about 6.3 arcmimutes in Houston,
(essentially the same in Europe: 6.1 arcminutes). For reference,
the full Moon is about 30 arcminutes. Theoretically the unaided eye can
resolve down to 1 arcminute, but that is for equally bright objects and
for perfect optics within the eye itself and a flawless retina.
With a more realistic limit of 2 - 5 arcminutes it should be just possible
for most people with decent vision to resolve both planets.
View of the 2020 Jupiter-Saturn conjunction to scale. This is how it will
appear in a telescope. The western horizon is towards the bottom.
Adapted from graphics from Stellarium.
Perhaps because this event occurs around Christmas, a lot of people seem
to be talking about this conjunction as being the famous Star of Bethlehem.
I'm not a biblical scholar, so can only address the astronomical aspects.
There is an obvious danger in that if you look for unusual celestial
events in a given time interval you are going to find them. For example, in
the past few decades we've had two naked eye comets (Hale Bopp and Hyakutake)
with tails in back to back years (1997/1998); our Jupiter/Saturn conjunction
(2020); very close approach of Mars (2003); two solar eclipses back to back
that cross the US (2017 and 2024); spectacular Leonid meteor shower (1966);
great alignment of outer planets (1980s), transit of Venus across Sun (2012),
plus other things I've undoubtedly omitted. All time periods have something
unusual.
Most biblical scholars place the birth of Jesus somewhere between 4 BCE and
6 BCE. As far as conjunctions go, Jupiter and Saturn align once every 20
years or so. They aligned in 14 CE and in -6 (7 BCE), neither alignment being close.
There have been roughly 100 such alignments in the past 2000 years.
So from an astronomical standpoint, there is nothing at all special about
these two conjunctions. The one in 7 BCE was a triplet, meaning three of
them occur within a few months of one another (May 29 southeast at dawn;
Sep 29 up all night, mainly south; and Dec 5 in the south and southwest
in the early evening for this case; exact dates depend slightly on ephemeris).
Triplets are rather rare, but occur in a little over 10% of the conjunctions.
The most recent triplet was in
1981, and actually that one had similarities to the one in 7 BCE, with
closest separations of about a degree. There was a somewhat closer triplet
in 1940 (about 40 arcminutes), and quite a good one in 1683 (12 - 16 arcminutes).
The next one is in 2238/2239. In Roman
times, we had triplets in 146/145 BCE, 7 BCE, 332/333 CE, 411/412 CE,
and 452 CE. In this latter group, only the 146/145 BCE triplet featured
close conjunctions (10 - 15 arcminutes), and the rest had separations of
about a degree.
If, as in 7 BCE and 1981 CE,
the conjunctions aren't particularly close, the visual effect is less
impressive. You've probably never heard of the 1981 triplet conjunctions.
I've seen them, and they were kind of interesting in that the two planets
were within a degree or two of one-another for quite a while. But it wasn't anything
spectacular, and nowhere near as close as to what we're seeing in 2020.
The overall direction in the sky varies within a triplet,
the first one being in the southeast in the morning skies
and the last one more to the southwest in the evening skies.
Of course, there may be astrological (rather than
astronomical) or political reasons for attributing significance to a particular
constellation or time of year, and I'll leave it to others to speculate
about those.
Another possibility I've seen put forth is a lovely alignment of Jupiter and Venus on June 17, 2 BCE.
This was a particularly close conjunction and would have been easily visible
in the western skies after dusk. The planets would have merged into a single
object with the unaided eye. Venus, as it usually is, would have been
brilliant. Jupiter was much fainter though, and even when they overlapped
the combined brightness would have increased by too small an amount to
notice over what you would get with just Venus. An observer in Europe/Palestine
would have seen Venus, with a fainter Jupiter a little over the diameter of
the full Moon away on one night, just Venus the next night, and Jupiter
again close to Venus the following night but on the other side. It would have
seemed as if Jupiter had disappeared for one night.
Unlike Saturn and Jupiter, which align once every 20 years, Jupiter and Venus do
it every 13 months or so. Because there are so many more conjunctions, some are
very close, and it is even possible for Venus to transit across Jupiter, something
that happens irregularly every 300 years or so. The conjunction on 6/17/-1 was
a near-transit, with Venus just clipping Jupiter for observers in the southern
hemisphere. For an ancient astronomer there would be no
difference between a very close conjunction (< 2' separation or so) and a transit
(< 0.4' separation) because they didn't have telescopes. Transits occurred in
the years 363, 428, 829, 1210, and 1818, and will again in 2123. Separations < 2' are
more common, but are still rather rare, and happen maybe every 50 years or so.
There was one in 42 CE for example, though that one was only 12 degrees from
the Sun and would have been very difficult to see.
The most obvious candidate for a Star of Bethlehem would be the sudden appearance
of a brilliant star. This does occur from time to time when a star nearing the
end of its life explodes in a supernova. There are also less catastrophic events
known as novae, that occur in close binary systems when a white dwarf star accretes
matter from a close companion. Novae are more common, but are much fainter intrinsically
so need to be quite close to appear brilliant in the sky. Examples of historical
supernovae brilliant enough to be seen during the day (brighter than Venus) occurred
in 185 CE, 1006 CE, and 1054 CE, visible for 8 months or more, left observable
remnants with expanding shells you can measure to find out their ages.
There are no records of such a bright supernova or nova around 4 BCE. Something
was recorded in Chinese text of an object that lasted for 2 months around that
time. This could have been a comet or a nova. If it was a nova it would have
had to have been very close by (inside around 100 pc or so)
to be as bright as Venus or even Jupiter,
and there are no plausible candidates I am aware of for that. If it were as
bright as Venus it should have been visible for longer than 2 months. The object was
in the summer sky near the constellation Aquila, and some have speculated that
the Taylor-Hulse binary pulsar is the leftover from a supernova there. However,
the T-H pulsar is quite far away and there is a lot of dust along the line
of sight. Optimistically it would not have been as bright as Jupiter, and
may have been too faint to see at all. There is no accurate age for the T-H
pulsar so there is no reason to believe it occurred at this time. Novae do
occur now and then; we had a nice one in 1975 about 2nd magnitude,
visible for about 2 weeks. And of course, comets come and go, some brighter
than others. I think we can safely say there was no really bright supernova
during this time, though there could well have been a 1975-like nova, or
a decent comet. It is hard to believe that a nova or supernova as bright
as Venus could have slipped by without many more records.
Bottom line: Jupiter and Saturn would not have made a very impressive Star of Bethlehem
in 7 BCE, at least to our modern eyes. They were a wide
pair about a degree apart. They did do a triple conjunction, something uncommon
but not extremely so, but the conjunctions were
not nearly as impressive (close) as the previous triplet was in 145 BCE.
The 7 BCE triplets would not have indicated a specific direction, but would
have been anywhere from southeast to southwest depending on which triplet you
observed and at what time of the night.
You might argue that Jupiter/Venus would have made for a better
`Star', as something quite special, though not
unprecedented, happened in the summer of 2 BCE, and that conjunction was in a clear
westerly direction. The dates don't quite match up
with the birth of Jesus for either one though. The Jupiter/Saturn 7 BCE
conjunction is better, but may be a bit early, and the Venus/Jupiter one in 2 BCE seems late,
as King Herod was supposed to have died in 4 BCE.
It's important to keep in mind that conjunctions
in general don't suddenly appear like a brilliant 'star'. They look
like two planets close together. If the planets are unresolved, it'll look
like the brightest of the two usually, because there is almost always a large
difference in brightness between the brighter one and the fainter one.
But ancient astrologers may have been looking for something other than a
pretty sight in the sky, for example, which constellation the conjunction
might appear in. So it is hard to know what effect a wide conjunction
of Jupiter and Saturn, or a close one with Venus and Jupiter, or something else
entirely, might have had on their interpretations of events.
What's the best plan to see it?
To be successful in observing this conjunction
you will need to have a clear southwestern horizon and no low clouds in the distance.
Observers without a telescope it may find it a challenge
to resolve both planets, but it can be done. This is an event that could
be impressive to see, but you will have to be prepared and
binoculars will likely be very helpful for seeing it well in most skies.
If you can set up a small telescope on them before it gets fully dark
that will be optimal, as then you will be able to see both planets together in
the field of view along with Saturn's rings, and the brightest moons of
both planets.
It was cloudy!
If it is cloudy or you miss it for some reason don't worry, the conjunction
is an ongoing event! Any time from December 17 through Christmas will find the
two planets closer together than the diameter of the full Moon, and that is
closer together than they usually ever get during most of the other years
that have these conjunctions. Interestingly, one of the five closer approaches
mentioned above occurs only three conjunctions from now, on March 15, 2080.
That one has almost exactly the same separation as this one does, but will be much easier
to see as it will appear 44 degrees from the Sun, well up in the morning sky.
The major challenge there is you'll have to stay alive for another 60 years
to see it!
Here's what it will look like in a small telescope
Jupiter/Saturn Conjunctions and the STAR OF BETHLEHEM