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Thread: Great Conjunction - Jupiter & Saturn - 2020 DEC 21

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    Cool Great Conjunction - Jupiter & Saturn - 2020 DEC 21

    On 2020 DEC 21 at 18:21 UT Jupiter and Saturn will appear to have a geocentric angular separation of only 6.1 arcminutes, the narrowest since 1623 and until 2080. However, those separations are not small enough for Jupiter to occult Saturn. The last such occultation occurred in 6857 BC, while the next two will both occur during 7541.

    Great Conjunctions between Jupiter and Saturn occur about every 20 years. That’s their mutual synodic (lapping) period with a mean value of 19.86 years. Sometimes the conjunctions are triple due to the effect of apparent retrograde motion, but not this time.

    My Great Conjunction chart depicting the relative positions of Jupiter and Saturn during December 16-26 can be viewed at https://www.CurtRenz.com/jupiter.html

    Photos and descriptions of the upcoming conjunction would be welcome additions to this thread.
    Last edited by Centaur; 2020-Nov-13 at 11:33 PM.
    For astronomical graphics and data visit
    https://www.CurtRenz.com/astronomy.html

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    6.1 arcminutes... does that mean the two planets will be in the same field of view through binoculars? Small telescopes?

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    Quote Originally Posted by mapguy View Post
    6.1 arcminutes... does that mean the two planets will be in the same field of view through binoculars? Small telescopes?
    Depends on the optics used but think of it this way; the full moon is about 30 arc minutes. If your scope or binocs can see the full moon then yes, Jupiter and Saturn will be in the field of view.

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    The thirty degree distance from the sun at their conjunction on 21 December means Jupiter and Saturn will set one hour after the sun, so will only be visible for a short time. They are already within 4 degrees of each other (and have both just passed Pluto by RA), and now set two hours after the sun. The conjunction will get steadily tighter over the next month. They reach one moon width of separation, coincidentally in conjunction with the new moon, on 17 December.

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    Quote Originally Posted by Robert Tulip View Post
    They reach one moon width of separation, coincidentally in conjunction with the new moon, on 17 December.
    What will be their separation from new moon at conjunction?

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    It depends where you are.

    At 0640 UTC Thu 17 December, the declination difference from the Jupiter Saturn pair to the new moon is three degrees in the USA but only 2.5 degrees in Australia, according to SkyGazer 4.5.

    Jupiter and Saturn are close to the ecliptic. There is a total solar eclipse on 14 December visible in Argentina, when the moon crosses the ecliptic there. The moon then moves about 2 degrees south over the next three days.

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    I noted in my opening post that this will be the narrowest angular separation between the two since 1623. Actually, the pair in 1623 was separated by only 5.2 arcminutes, but at an elongation from the Sun of less than 13˚. That would have been quite difficult to observe by naked eyes. The next earlier yet better separation than in 2020 was in 1226 at 2.1 arcminutes separation and 49˚ from the Sun. The 2080 separation will be 6.0 arcminutes, barely less than this year's 6.1 arcminutes. But the 44˚ elongation from the Sun will be better than this year's 30˚.
    For astronomical graphics and data visit
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    So there will be two transits in 7541. What will be the respective separations? And how will the separations vary across Earth?

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    Quote Originally Posted by Robert Tulip View Post
    The thirty degree distance from the sun at their conjunction on 21 December means Jupiter and Saturn will set one hour after the sun, so will only be visible for a short time. They are already within 4 degrees of each other (and have both just passed Pluto by RA), and now set two hours after the sun. The conjunction will get steadily tighter over the next month. They reach one moon width of separation, coincidentally in conjunction with the new moon, on 17 December.
    Some current students have been imaging the giant planets frequently to determine orbits for their moons and use Kepler's 3rd law for masses, and decided they should show Pluto some love as well. On the evening of November 12, they got Pluto, all right - along with an impressive display of stray light from Jupiter, at that time 0.7 degree north. (I keep measuring fields for our various imaging setups seeking the best one for December 21 to get Jupiter and Saturn in the same field - we have a planetary modified webcam but its diagonal field is thaaaat much too small).
    Attached Images Attached Images

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    Jupiter Saturn and Moon, 17 December 2020 (from Australia)
    Attached Images Attached Images

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    Quote Originally Posted by Robert Tulip View Post
    Jupiter Saturn and Moon, 17 December 2020 (from Australia)
    Nice, thank you, hard to see here, weather and horizon.
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    Quote Originally Posted by profloater View Post
    Nice, thank you, hard to see here, weather and horizon.
    A succession of frontal systems arriving in the afternoons have made viewing impossible here for more than a week. We finally got a break this evening. As soon as Jupiter was visible to the naked eye in the dusk sky I put the binoculars on it and pulled up Saturn. Then a wodge of cloud came through, but half an hour later I had them as a naked-eye pair. Impressively close.

    Grant Hutchison

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    Not a chance here. Got a glimpse about three weeks ago, but no hope since. The weather has been, well, absolutely typical for this time of year in this location.
    Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.

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    A member of the Boon Companion's photographic society just posted a very nice telescopic view on the society Facebook page. She had the two planets in the same telescopic view, with Saturn showing its rings and Jupiter showing a disc and some moons.
    Essentially this view, which I just botched up in Celestia:
    2020-12-21 02_22_33-Celestia.png

    Grant Hutchison

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    Quote Originally Posted by Trebuchet View Post
    Not a chance here. Got a glimpse about three weeks ago, but no hope since. The weather has been, well, absolutely typical for this time of year in this location.
    Yeah, regrettably I won't be staying up for this one either. I take back all the nice things I said about rain.


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    Interesting to see how tilted the Saturn system is, compared to Jupiter, which barely has a tilt at all.

    I'm a bit disappointed that I can barely see Saturn with my naked eyes nowadays, probably for medical reasons. I must get to Specsavers sometime, if they ever lift the lockdown. But it looks good in binoculars.

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    Quote Originally Posted by eburacum45 View Post
    Interesting to see how tilted the Saturn system is, compared to Jupiter, which barely has a tilt at all...
    Indeed, the current axial tilt for Saturn relative to Earth is about 21˚, with an identical effect for its equatorial circling rings. The current case for Jupiter is less than 1˚, with the planes of the orbits for its four Galilean satellites similarly tilted.

    During periods of about six months separated by about six years, mutual events occur involving the Galilean satellites occulting and transiting each other, with similar effects from their shadows. The next such period will commence on 2021 JAN 03 and end on 2021 JUN 26. They will be included with my Nightly Events Timetable for Galilean Satellites: https://www.CurtRenz.com/jupiter.html
    Last edited by Centaur; 2020-Dec-21 at 06:21 PM.
    For astronomical graphics and data visit
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    days and days and days of CLOUDS!!!!!!!!!!
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    Quote Originally Posted by tusenfem View Post
    days and days and days of CLOUDS!!!!!!!!!!
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    Just came back inside after viewing it in a very clear sky following a cloudy & misty day just 2 hours ago. It's interesting to see but at first glance it appeared to be Jupiter and one of its moons. Since both planets are halfway to the other side of the solar system their apparent sizes aren't very impressive. But hey, I saw them :-)

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    Yep, saw them today with some binocs and it was enjoyable. Unfortunately I'm travelling this week, otherwise I'd have set up my scope and maybe even try some afocal imagery.

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    My Dad and I got in a peek with binocs just after sunset before the clouds rolled in.
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    The forecast clearing arrived this afternoon. After switching from CCD to a DLSR because of a shutter problem, I got a bunch of shots of the conjunction with our campus 0.4m reflector. These three span about a 200-fold range in exposure time; the dynamic range from Jupiter's clouds to Saturn's moons is a real challenge in capturing this event in one shot.
    Attached Images Attached Images

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    I got to see it last night with my eyeballs, binoculars, and my seemingly dying 4" Celestron... well, not sure if it was dying or the batteries I put in it were weak, I don't use it much. Had to keep it on track manually.

    Anyway, the noteworthy part was, one time I was looking through binoculars when a shooting star flew by just beneath the two planets. It was a "wow moment" and I did make a wish.

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    And tonight the planets are visibly separate objects to the naked eye.

    Kepler and Newton for the win.

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    An easy naked-eye pair against the dusk sky tonight. I'm impressed by the relative movement over the last three days--I'd expected it to be slower, for some reason.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    An easy naked-eye pair against the dusk sky tonight. I'm impressed by the relative movement over the last three days--I'd expected it to be slower, for some reason.

    Grant Hutchison
    Jupiter takes 12 years to orbit the sun, so it moves 30 degrees per year or 0.1 degrees per day. The moon is 0.5 degrees wide, so Jupiter moves on average one fifth of a moon per day. Saturn speed is 40% of Jupiter.

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    Earth also moves at about one degree per day so parallax shifts the apparent conjunction too.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Quote Originally Posted by Robert Tulip View Post
    Jupiter takes 12 years to orbit the sun, so it moves 30 degrees per year or 0.1 degrees per day. The moon is 0.5 degrees wide, so Jupiter moves on average one fifth of a moon per day. Saturn speed is 40% of Jupiter.
    Yes, I did the maths. I memorized basic planetary data when I was a kid and of course still have it in my head, despite not being able to retain more important information. So when I looked out last night and saw how rapidly the relative positions had changed, I did a quick sum in my head--Jupiter moving about a twelth of a degree per day, Saturn about a thirtieth. So assuming a spherical cow we get a relative motion in the ballpark of three minutes per day, a significant proportion of their separation at present.
    That should have been intuitively obvious to me, going in, but for some reason it wasn't.

    Grant Hutchison
    Last edited by grant hutchison; 2020-Dec-24 at 12:39 PM.

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    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?

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