The Dec 21, 2020 Conjunction of Jupiter and Saturn

Hacker News The Dec 21, 2020 Conjunction of Jupiter and Saturn

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.


How special is this?

Whenever there is an astronomical event, everyone seems to want to know how special it is. As noted on the Jupiter-Saturn conjunction page, conjunctions between these two planets occur regularly, and are spaced out by about 20 years and so always seem rather rare. Apparently Kepler originally referred to `Great Conjunctions’ as being between Jupiter and Saturn (thanks to Manfred Gaida of the German Aerospace Center for that). Not all of the Jupiter-Saturn conjunctions are particularly ‘great’, in the sense that in many such alignments the planets don’t get that close together. But it is fair to say that this conjunction is truly exceptional in that the planets get very close to one another. In the three thousand year interval from 0 CE to 3000 CE, only seven Jupiter/Saturn conjunctions were/will be closer than this one. Two of those (in 769 CE and 1623 CE) were too close to the Sun for Saturn to be seen without a telescope, and even Jupiter would have been difficult or impossible to detect from most locations with the naked eye for these events. If you are looking for a positive spin, the last time the two planets appeared this close to one another in the sky and were observable (i.e. not in the Sun’s glare) was on the morning of March 4, 1226! That was back in the Middle Ages, when the Notre Dame Cathedral was first being built. For this conjunction, both planets will be visible in the same field of view in most small telescopes, along with some of Jupiter’s and Saturn’s moons (see image below). In fact, they will be so close it may be a challenge to separate them with the unaided eye for some people.

How easy is it to see?

A difficulty with the December 21, 2020 conjunction for many observers is that its separation from the Sun is only 30 degrees, and for northerly latitudes such as those in the United States, Canada, and Europe, those 30 degrees are at an angle to the horizon that is not vertical. Now consider that you want the Sun to be at least 6 degrees below the horizon for skies to be dark enough, plus for most locales anything below an altitude of 10 degrees is very difficult to observe, and you’ll appreciate there is not a lot of room left over to see the event. It can be done with good weather and horizons, but seeing this well will take a bit of planning and perhaps some luck with weather. Of the two planets, Saturn is significantly fainter than Jupiter, and will be the most difficult to see. But once you find Jupiter, Saturn will be right next to it. My sense is that as long as it is clear and you go someplace where you can watch the sunset, you will be successful in observing this event.

How much does location matter?

Viewing conditions are best close to the equator, though no matter where you are there is maybe an hour or so to observe this conjunction before the planets sink into the haze. For example, in Houston TX, latitude ~ 30 degrees north, civil twilight ends (i.e. it starts to get dark enough to see the brightest stars and planets) around 5:53 pm. Jupiter should be visible by then, and Saturn just emerging from the twilight. At that point Jupiter and Saturn will be 19.7 degrees above the horizon. It will be fully dark an hour later, but by then the planets are only 9 degrees above the horizon. These elevations are low, but manageable if you find somewhere with an unobstructed view to the southwest. It is a bit more difficult in, say, New York City, latitude ~ 40 degrees north. In this case civil twilight ends at 5:03 pm and the planets will be 16 degrees above the horizon. An hour later they are only up 7.5 degrees. Up at latitude 51 degrees in London UK, the planets will be up about 11.5 degrees at civil twilight, and an hour later they are at 5.3 degrees, a difficult observation for sure. But go down to Caracas Venezuela, latitude ~ 10 degrees north, and the pair is up 23.5 degrees at civil twilight, which isn’t too bad, and is about as good as it gets this time.

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.

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.

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.

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

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.

Jupiter/Saturn Conjunctions and the STAR OF BETHLEHEM

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; Oct 1 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.

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.

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