The meeting of our two giants is called the “Great Conjunction.”
Jupiter overtakes Saturn at intervals of roughly 20 years. The conjunctions vary in quality: how conveniently they are placed for viewing, and how close the planets come to each other.
The 2020 Great Conjunction
We can let this description stand until it needs to be replaced by the next, for 2040!
On 2020 Dec. 21 they passed so closely – only a tenth of a degree apart – that they could be seen simultaneously, with their families of satellites, in one telescope field. It happened in the evening sky, though rather low, the planets being 30° from the Sun.
This was the closest Great Conjunction since 1623! And the 1623 event, shortly after the 1609 invention of the telescope, was less than half as far out (13°) in the sunset sky.
The moment of the December 21 conjunction was about 13h Universal Time if measured in right ascension (parallel to the celestial equator), and 18h if measured in ecliptic longitude; and 18h UT is also the time of the closest approach, or appulse – the moment of minimum angular distance between the two bodies.
Observing it
On 2020 Dec. 21 they passed so closely – only a tenth of a degree apart – that they could be seen simultaneously, with their families of satellites, in one telescope field.
It happened in the evening sky,
(Enlarge this image by any means you can.)
18 Universal Time is 6 PM in Britain, but by American clocks it is 5 or more hours earlier and therefore in daylight. So the evenings of Dec. 20 and Dec. 21, respectively before and after the event, were the viewing opportunities that were closest to it and after dark.
The planets, appearing like a single brilliant star, were, at the place and time of the picture, 15° above the southwestern horizon. The Moon, nearly at First Quarter, was a guidepost to locating the planets. The stars Altair and Fomalhaut were about 3 magnitudes dimmer than Jupiter. Dimmer stars, the Milky Way, and certainly the below-naked-eye planets Pluto and Neptune, were not seen in the twilight, but they and the constellation forms, and the abstract lines of the ecliptic and the celestial equator, help to give a sense of where we are in the geography of space.
Arrows through the moving bodies, including the Sun, show their movement, relative to the starry background, over 5 days. A broad arrow shows how the sky will rotate downward, parallel to the celestial equator, in one hour.
The view at the same time on the next evening, Dec. 21, was almost the same; the Sun having advanced about a degree along the ecliptic, everything else will seem to have rotated slightly rightward and downward.
At a more northerly latitude such as Britain, the planets appeared a few degrees lower.
In the southern hemisphere, the view was similar except that the celestial equator slopes the other way and everything else is accordimgly “upside down.” This picture serves approximately for northern New Zealand and southern Australia.
Close up
Here is a highly magnified view
as through a powerful telescope, with a field only a fifth of a degree wide. (Your little finger held out at arm’s length appears one degree wide.) So the planets can be shown at true scale. Most conjunctions of bright planets and stars are much wider, so that pictures of this type for them would have to have fields several degrees wide, in which the planets to be visible would have to be exaggerated in size.) Each picture has celestial north at the top; is centered midway between the planets; and has a radius of 0.1° (6′). The planets’ apparent sizes are labeled in seconds.
At this time the planets appear not far out from the Sun on the eastern or evening side (their elongation, or angular distance from the Sun, is 31°). So they have to be observed not long after the Sun sets and not long before they set. The pictures are for a mid U.S.A. location (40° north, 90° west), at 45 minutes after sunset. The closest approach, to only about a tenth of a degree, happens during daylight for the western hemisphere. So on the evening of Dec. 20 Jupiter has yet to catch up with Saturn, and on the evening of Dec. 21 it has passed.
In this magnified view, even the major satellites of the planets can be shown, though they have to be exaggerated in size. You see them, even with most optical aid, as star-like points. To avoid crowding, Saturn’s inner four satellites are labeled with just their numbers (they are Mimas, Enceladus, Tethys, and Dione).
Physically, Jupiter and Saturn are 11 and 9 times as wide as Earth. But they are about 900 million and 1,600 million kilometers away.
Even a few days away from conjunction, the planets are close. For the naked eye, this will be a merging of two bright “stars” – magnitudes -2.0 and 0.7 – into an even more brilliant star that may seem to have a bulge on its northern side. In the telescope, it will be a rare chance to see two swirling satellite systems that seem almost in danger of intermingling!
The satellites
Here are the paths of the major moons, or natural satellites, of the two planets on the day of the conjunction.
Jupiter’s are the four “Galilean” satellites, which were among Galileo’s first discoveries with his telescope, in January 1610. The satellites’ paths are shown for the whole Universal Time day. The satellites themselves are shown at 0h UT (which in North America’s Eastern time zone is 7 PM on the preceding day) and at 18h UT, which is the time of the closest approach between the two planets. Jupiter’s satellites are exaggerated 5 times in size, Saturn’s 10 times. Equatorial north is up. A shorter line from the planet points to equatorial north, a longer line to ecliptic north.
2020, the year of overtaking
In each year of the 21st century, Jupiter has been following Saturn around the sky and gradually drawing closer. Jupiter spends about a year in each constellation of the zodiac, whereas Saturn dawdles in each for about 2 1/2 years.
In 2020, Jupiter joined Saturn in the southerly constellation Sagittarius. As Earth overtook them, they went into their apparent retrogrades loops, and had their moments of opposition (July 14 and 20). Before the end of the year, they reach Capricornus and their conjunction.
In this celestial chart, the paths of the planets slope increasingly close together into December. To help distinguish the paths, Saturn’s is drawn in red. The planets have arrived, together, at one of the two points where the orbital planes of Jupiter and Saturn slice through each other. The chance of that happening is very small.
Jupiter starts well behind (westward of) Saturn and ends slightly ahead. Both start in Sagittarius; Saturn enters Capricornus from late March to late June, then again in December; both end in Capricornus, where the conjunction happens.
This three-dimensional diagram
is from 35° north of the ecliptic plane. The boundaries of the zodiacal constellations (those through which the ecliptic passes) are drawn on an imaginary sphere of radius 10 astronomical units (Sun-Earth distances), not far beyond the 9 a.u. orbit of Saturn. The paths of Earth, Jupiter, and Saturn are shown over the course of 2020. Stalks are drawn perpendicularly from the planets northward to the ecliptic plane at the beginnings of the months.
The yellow line is the sightline from Earth past Jupiter to Saturn on Dec. 21. The gray line is from the Sun to Jupiter and Saturn at the date of their heliocentric conjunction, Nov. 2. The dashed line, with ram’s-horns symbol, is at longitude 0°, the vernal equinox direction, where the Sun appears for us on March 20.
Conjunctions through the centuries
Jupiter’s orbital period – the time it takes to circle the Sun – is 11.86 years, and Saturn’s is 29.46 years. So, after overtaking Saturn, Jupiter comes around to the same region of the sky about 12 years later; but Saturn has moved on, and Jupiter takes about another 8 years to overtake again, In other words, conjunctions follow each other at intervals of around 20 years. And each next conjunction happen about 2/3 of the way around the circle of the sky.
These are only approximate, because the geometry differs. The planets may both be advancing, as seen from Earth, or one or both may be in their apparent retrograde loops. Sometimes the conjunctions are triple, because Jupiter overtakes, then appears to fall back in its retrograde loop, and overtakes again. So we should really say that 20 years is the approximate interval between conjunctions or conunction groups. There are more conjunctions than 5 per century; including the members of triple conjunctions, there are about 6.2 per century.
In this version of the same spatial view
the lines are from Jupiter to Saturn at many dates of conjunction. They are sightlines from Earth, but to save clutter I omit the parts from the dates on Earth’s orbit out to Jupiter.
And to try to make the roughly-20-year rhythm clear, I show an early group of three (1940, 1961, 1981) in orange, a middle group (2000, 2020, 2040) in yellow, and a later group (2060, 2080, 2100) in green.
Two of the conjunctions of the twentieth-century group are of the triple kind: Jupiter appears to pass Saturn, retreat, and overtake again.
Thicker gray lines are at the true centers of the events: the heliocentric conjunctions. For 2020, this was November 2. The lines could be extended back to the Sun (missing the Earth). At those moments, Jupiter and Saturn are at the same direction (longitude) as seen from the Sun. The conjunctions as seen from Earth are displaced irregularly around these heliocentric conjunctions, because of where Earth happens to be in its faster little orbit.
If you draw lines connecting the positions of, say, Jupiter at successive Great Conjunctions, you get a series of triangles, which do not exactly coincide. Johannes Kepler recognized this and illustrated it in De Stella Nova, his 1606 book about the supernova of 1604.
Let us know
When did the two planets seem to become a single star, to your naked eye? How many of the satellites could you see at once in your telescope? How did the brightness of the low sky, or your local conditions, affect your experience? We’d like to know, and I think the best way for you to let us all know is to make a comment to my blog, https://www.universalworkshop.com/blog/
Table of selected dates
Listed here are the triple conjunction of 7 BC, often proposed as the “star” that, according to the gospel of Matthew, three wise men said they had seen in the east; the conjunction of 1345, which also involved Mars and which according to the king of France’s astrologers caused a plague; the very close conjunction of 1623; and conjunctions of the 20th and 21st centuries.
The column after the date means the type of event calculated: “long” (conjunction in longitude), “RA” (conjunction in right ascension), “appr” (appulse or closest approach). Modern sources usually use R.A. Earlier scholars used longitude, and I use it because it is usually closer to the closest approach. Closest approach is tricky to use because the angular distance between bodies can reach a minimum when there is no conjunction, such as when one is at its stationary moment. For comparison,l all three types are shown for 2020.
The other columns are the angular separation between the planets, in degrees (negative meaning that Jupiter was on the south); their magnitudes (brightnesses); the elongation (angular distance from the Sun, negative meaning westward or in the morning sky); and longitude around the sky, from the vernal equinox point.
The line | denotes triple events.
For ease of viewing, best is a large elongation, preferably positive. For the telescope, best is a small separation, as in 1623 and 2020.
UT separ magnitudes elo lon -6 May 29 12 long 0.985 -2.3 0.9 -74 351| -6 Oct 1 4 long 0.975 -2.9 0.5 162 347| -6 Dec 5 18 long 1.054 -2.4 0.9 94 346| 1345 Mar 24 12 long 0.348 -2.1 0.9 -53 319 1623 Jul 16 22 long -0.086 -1.8 0.3 13 127 1901 Nov 28 16 long -0.441 -2.0 0.6 38 284 1921 Sep 10 4 long -0.950 -1.7 1.1 10 177 1940 Aug 8 1 long 1.186 -2.4 0.3 -91 44| 1940 Oct 20 5 long 1.234 -2.9 -0.1 -164 42| 1941 Feb 15 7 long 1.288 -2.3 0.4 73 39| 1961 Feb 18 24 long -0.229 -1.9 0.7 -35 295 1980 Dec 31 21 long -1.047 -2.0 0.9 -91 190| 1981 Mar 4 19 long -1.056 -2.4 0.5 -156 188| 1981 Jul 24 4 long -1.104 -1.8 1.1 64 185| 2000 May 28 16 long 1.147 -2.0 0.2 -15 53 2020 Dec 21 13 RA -0.104 -2.0 0.7 30 300 2020 Dec 21 18 long -0.102 -2.0 0.7 30 300 2020 Dec 21 18 appr -0.102 -2.0 0.7 30 300 2040 Oct 31 12 long -1.131 -1.7 0.8 -21 198 2060 Apr 7 22 long 1.123 -2.1 0.2 42 61 2080 Mar 15 1 long 0.100 -2.0 0.8 -43 312
The 7 BC conjunction
The triple Jupiter-Saturn conjunction of 7 BC (May 29, Oct. 1, Dec. 5) is often suggested as the “star” that the wise men saw in the east, presaging the birth of a king of the Jews. Here is the view from Jerusalem on the evening of the third member of the conjunction.
