by Rex Parker, Phd director@princetonastronomy.org
Join us for the great conjunction tonight, Friday, December 18 on solstice day, December 21 right after sunset (4:37pm). You canjoin the live astro video session via Zoom produced by the AAAP astro-video group. If conditions are favorable we will stream from the club’s Washington Crossing Observatory with the Celestron-14 as well as with additional member telescopes. Weather permitting, we will begin shortly after sunset at 4:37pm as soon as the skies are dark enough to use the imaging cameras. It will be tricky because of the low position in the southwestern sky, only 17 degrees above the horizon at 5:00pm. A conjunction occurs when two planets reach their minimal angular separation in our skies. This one will be remarkable because the Jovian planets will be only ~0.1 degrees apart, close enough to fit within the field of view of the telescope/camera setup. Conjunctions of Jupiter and Saturn happen every ~20 years but this one will be the tightest since 1623!
The link below will take you to Zoom meeting information posted on the website. Friends and family are welcome to join the session (if we hit our limit you may need to wait). If weather dictates, the date may shift a couple days either way – stay tuned to the website announcement.
The conjunction of Jupiter and Saturn will take place in the late afternoon and early evening of Monday, December 21. On that day, the Sun will set at 4:35pm.At that time, Jupiter and Saturn will be low in the southwestern sky, just 20degrees above the horizon. Also, at sunset, the sky is still bright like daytime. For those observers with binoculars, it is possible to find Jupiter at sunset. But, it’s a challenge. Fortunately, darkness comes quickly and by 4:45pm, or maybe 4:50pm, Jupiter should be easy to find in binoculars and also findable without binoculars. At 4:50pm, Jupiter and Saturn will still be 18 degrees above the horizon. It’s low but not terribly low. The separation between the two planets will be just roughly 6 arcminutes. So, for those who have a telescope the conjunction will be an awesome sight. And, for those members who are into astrophotography, it will be a once in a lifetime opportunity to take pics of the event.
But, there will be some serious challenges. Jupiter will be 2.5 magnitudes brighter than Saturn. That’s a factor of 10 times brighter. So, a photograph that nicely shows Jupiter will have Saturn looking very dark. To make a good picture, one will need to take images with different exposures and then do some sort of “high dynamic range (aka HDR)” combo of the images. In addition to Jupiter and Saturn themselves, we’ll also get to see some of their moons. But, the moons will be even fainter and that makes the HDR imaging an even bigger challenge. Also, the fact that this event will only be about 18 degrees above the horizon will mean that the atmospheric “seeing” is likely to be bad. Shown here are two screenshots from the planetarium program Cartes du Ciel (aka Sky Charts) showing how things will look at 4:50pm. One picture just shows Jupiter, Saturn, and their moons. The other picture shows the various stars that are also in this field of view. Jupiter has four bright moons. From left to right they are Callisto, Io, Ganymede, and Europa. Saturn has lots of moons. From left to right, they are Lapetus, Hyperion, Rhea, Mimas, Enceladus, Dione, Tethys, and Titan. Of Saturn’s moons, Hyperion is the dimmest. It’s magnitude 14.9. That magnitude can be seen in, say, 10 second long astrophotographs, but is not visible visually through most amateur telescopes. And, the not completely dark sky will also be a problem. Saturn’s brightest moon is Titan at magnitude 9.0. If the skies are dark enough, that moon could be seem visually through a telescope. The four moons of Jupiter are all about magnitude6 and things of that magnitude do appear in astrophotographs taken at dusk.
As we are all aware, there will be a close conjunction of Jupiter and Saturn in the evening sky of Monday Dec. 21, which coincidentally is the same day as the winter solstice. Jupiter passes Saturn once in about every 20 years as seen from Earth, but in most cases they are not nearly so close together in the sky as they will be this time.
About 30 years ago I became interested in such conjunctions as close as or closer than 6 arc minutes, and I researched them with the aid of advanced ephemeris technology. (The upcoming one will be 6.1 arc minutes at closest approach, but I included it as being in the foreseeable future.) I wrote up my results and sent an article to Sky & Telescope for consideration in the Astronomical Computing feature moderated by Roger Sinnott. A corollary was my finding that Jupiter hadn’t actually occulted Saturn as far back as 4000 BC, and won’t do so until at least the year 2800 AD.
Because the ecliptic passes near Regulus (Alpha Leonis, often called the “Royal Star”), I was especially intrigued by close conjunctions that have occurred in its immediate neighborhood, i.e. within 10 degrees. Such events might have had significance for contemporary astrologers. The events of 1794-3 BC struck me as having been possibly associated with the rise of Hammurabi, the “lawgiver king” of Babylonia (reigned ca. 1792-1750 BC). The fine 940 BC triple conjunction near Regulus might have had some connection with King Solomon (reigned ca. 970-931 BC); the lion has long been associated with Israel, and both the five-and six-pointed stars have been called the Seal of Solomon. The coming event is not near Regulus, but is still quite interesting as it will be the closest such conjunction readily visible since the year 1226.
The following table is abridged from my original data. My article appeared in the March 1991 issue of Sky & Telescope, pages 305 to 307. Those who may be interested in the entire article may be able to make copies for their own private use from back issues kept in one of the branch libraries of Princeton University. In the past, these branches have been open to the public free of charge; this may again be the case after the Coronavirus issue has passed. Unfortunately I have no photocopies available.
Date
UT(hr)
Separation (arc min.)
Celestial Long. (Deg.)
Elongation From Sun (Deg.)
Remarks
4/6/3780 BC
0
5.3
266
80 W
Fine double (T)
6/28/3501 BC
20
4.5
91
23 E
Fine double
3/9/3441 BC
20
5.9
100
137 E
Fine double (T)
3/22/2926 BC
19
1.5
273
65 W
Naked-eye merge
6/5/2647 BC
21
3.9
96
45 W
Spectacular (R)
3/16/2072 BC
20
-2.4
281
57 W
Possible Merge
10/7/1794 BC
23
2.7
106
74 W
Possible Merge (TR)
1/19/1793 BC
18
-5.4
103
178 E
Fine double (TR)
5/1/1793 BC
2
1.3
101
76 E
Naked-eye merge (TR)
12/26/1278 BC
3
3.3
280
16 E
Spectacular
9/4/940 BC
1
3.5
110
42W
Spectacular (TR)
12/28/424 BC
10
-1.5
298
17 E
Naked-eye merge
8/11/86 BC
18
3.7
115
20 W
Spectacular (TR)
6/29/26 BC
1
16.1
123
30 E
(TR, M)
3/6/372 AD
13
1.9
294
53 W
Possible Merge
3/5/1226 AD
4
-2.2
303
49 W
Possible Merge
12/21/2020 AD
18
6.1
300
30 E
Fine double
3/15/2080 AD
1
-6.0
312
44 W
Fine double
8/24/2417
16
-5.4
125
27 W
Fine double
Notes to the table: All dates before 2020 AD are in the Julian rather than the Gregorian calendar. A negative sign under “Separation” indicates that Saturn was or will be south of Jupiter at the time of closest approach. Celestial longitudes are measured along the ecliptic eastward from the vernal equinox of date. “T” means that the event is part of a “triple conjunction” of the two planets; “R” means the event was near Regulus, and “M” means that there was also a “massing” of other planets in the vicinity. This was the case with the 26 BC event (not a particularly close one), but included because of this as well as nearness to Regulus. Events closer to the sun than 15 degrees were omitted. Please see the original of the article for full details and accompanying illustrations.
On Monday, Dec. 21, Jupiter will “lap” Saturn in their mutual race around the Sun. At the closest approach, their angular separation will be just 6 arcminutes. That’s the closest Jupiter/Saturn conjunction in about 400 years. And, it will happen on the day of the winter solstice. This will be an interesting event to see especially if viewed through a telescope. In preparation for the event, I have in recent weeks taken some pics of Jupiter and Saturn using my 10-inch Ritchey-Chretien telescope. Shown here is a mosaic image I made from two distinct pictures, one of Jupiter and one of Saturn taken at about 4:45pm on December 10. I made the mosaic showing what a 6-arcminute separation would look like. By the way, one of the really cool things was that my picture of Jupiter also has Ganymede, Europa and Callisto in it. That’s pretty cool given that it was only just a few minutes after sunset when I took that picture. It was not yet dark outside.
Amateur Astronomers Association of Princeton 