Great Conjunction - Jupiter & Saturn - 2020 DEC 21

[Hornblower]

With only a rudimentary understanding of solar system dynamics, one might expect Jupiter to occult Saturn every ~20 years (every time it laps Saturn). But of course that's not what actually happens. This was the closest separation in 400 years, and yet the two planets still didn't exactly align. So why is this? With respect to the solar system's invariable plane, I see that Earth's orbit is inclined 1.57%, Jupiter's is .32%, and Saturn's is .93%. Is that the main factor that explains why the planets rarely occult each other?

Yes

Are there additional reasons?

No, to the best of my knowledge.

[profloater]

I think you have it, remember these are earth centred observations, so the line up of three planets gets disturbed by all those inclinations.

[profloater]

And both Saturn and Jupiter have elliptical orbits : their aphelion being about 10% further from the sun than their perihelions. That changes timings of the earth centred conjunctions.

[Trebuchet]

Although I missed the closest approach, I enjoyed seeing them last night. Might be good again tonight, I'm thinking of connecting the camera to the telescope.

[schlaugh]

Wouldn't the precession of the respective orbits also have an influence on the appearance of a conjunction?

[Centaur]

With only a rudimentary understanding of solar system dynamics, one might expect Jupiter to occult Saturn every ~20 years (every time it laps Saturn). But of course that's not what actually happens. This was the closest separation in 400 years, and yet the two planets still didn't exactly align. So why is this? With respect to the solar system's invariable plane, I see that Earth's orbit is inclined 1.57%, Jupiter's is .32%, and Saturn's is .93%. Is that the main factor that explains why the planets rarely occult each other? Are there additional reasons?

As Hornblower and profloater noted, the answer is, "Yes, it's a huge factor." An additional reason would be the small angular sizes of the planets when viewed by an observer on Earth.

While occultations of Saturn by Jupiter are exceptionally rare, we also don't see mutual planetary occultations very often in the cases of any pairs of planets. And we are currently in an unusually long dry spell. The most recent case was an 1818 occultation of Jupiter by Venus, with no observation reports. The only report ever of a mutual planetary occultation was by an amateur astronomer who observed Venus occulting Mercury in 1737. The next mutual planetary occultation will be when Venus occults Jupiter in 2065, but that will be almost impossible to observe with an elongation from the Sun of only 8˚.

[Spacedude]

Here's another perspective...almost at maximum distance from Earth in respect to orbits.

download (19).jpg

[Robert Tulip]

Occultations will only occur when the planetary nodes are conjunct. Jupiter's south node is at 280° and Saturn's is at 293° RA, a separation of just 13°. With the conjunction at 300° this is is why the declination of the conjunction was so close. They have the same declination some time next week, a week too late for an occultation. The nodal periods of Jupiter and Saturn are about thirty thousand years (~1°/century).

[chornedsnorkack]

Heliocentrically, Jupiter and Saturn could only occult at their mutual node.
But we are on Earth, and Earth orbit is inclined to orbits of Saturn and Jupiter. Where in sky can Jupiter and Saturn transit?

[Centaur]

Occultations will only occur when the planetary nodes are conjunct. Jupiter's south node is at 280° and Saturn's is at 293° RA, a separation of just 13°. With the conjunction at 300° this is is why the declination of the conjunction was so close. They have the same declination some time next week, a week too late for an occultation. The nodal periods of Jupiter and Saturn are about thirty thousand years (~1°/century).

Two planets' ecliptical nodes do not have to be conjunct for a mutual planetary occultation to be observed from Earth. That would only be true for the very special case of an occultation occurring almost precisely on the ecliptic. When Mars occults Jupiter in 2223, their nodes will be separated by 51°.

MarsJupiter.JPG

[Robert Tulip]

Two planets' ecliptical nodes do not have to be conjunct for a mutual planetary occultation to be observed from Earth. That would only be true for the very special case of an occultation occurring almost precisely on the ecliptic. When Mars occults Jupiter in 2223, their nodes will be separated by 51°.

MarsJupiter.JPG

Thanks Centaur, apologies for my mistake.

[Spacedude]

^^^ Now that will be cool! Jupiter with have a temporary new pink spot to go with it's red spot. Too bad it's not in 2023.

[grant hutchison]

^^^ Now that will be cool! Jupiter with have a temporary new pink spot to go with it's red spot. Too bad it's not in 2023.

Courtesy of Celestia:
2223-12-02 12.27.30.png

Grant Hutchison

[skysurfer5cva]

On Saturday the 19th I set up one of my telescopes in the backyard to do some observing and photography. Transparency was excellent, but seeing (atmospheric stability) was poor. How poor, you ask? So poor that one image of Saturn shows rectangular rings, Jupiter's Galilean Moons sometimes faded briefly out of view, and images of the Moon taken seconds apart show different craters sharp or "muddy."

On Monday the 21st, it was foggy down here in California's San Joaquin Valley, so I drove towards (but not to) Kings Canyon National Park and set up my gear near a rarely used helipad at 4300 ft, well above the fog. Due to the inversion layer, it was 16°F warmer at the helipad (59°F) than at my home in Fresno (43°F). Transparency was even better than at home on the 19th and seeing was good near the horizon and excellent overhead. So, Jupiter and Saturn suffered just a bit from atmospheric wiggles, but the Moon was close to being rock solid (pun intended).

Here is a link to a few pics from my little adventure. For some reason, CQ's Insert Image button would not upload an image after I had selected it, so here is link to a public Facebook album I made a few days ago for this event: https://www.facebook.com/media/set/?...9266731&type=3 As I understand it, you do not need to have a Facebook account to view this album.

[schlaugh]

Lovely images. The moon is crisp and the conjunction is very well defined. But those landscape shots are just as good if not better!

[skysurfer5cva]

schlaugh....

Thanks for the kind words. I had a good time observing the conjunction and taking pics. I also I showed a couple of the non-annoying visitors the conjunction through the telescope. :-)