Starship Asterisk*

Jupiter Meets Saturn: A Red Spotted Great Conjunction

Explanation: It was time for their close-up. Last week Jupiter and Saturn passed a tenth of a degree from each other in what is known a Great Conjunction. Although the two planets pass each other on the sky every 20 years, this was the closest pass in nearly four centuries. Taken early in day of the Great Conjunction, the featured multiple-exposure combination captures not only both giant planets in a single frame, but also Jupiter's four largest moons (left to right) Callisto, Ganymede, Io, and Europa -- and Saturn's largest moon Titan. If you look very closely, the clear Chilescope image even captures Jupiter's Great Red Spot. The now-separating planets can still be seen remarkably close -- within about a degree -- as they set just after the Sun, toward the west, each night for the remainder of the year.

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The words "Last week" were changed to "Two days ago" on the main NASA APOD. We apologize for the oversight.

Great photo! Thanks for the annotation! A few more moons of Saturn are visible. Which ones? Tethys, Rhea, Dione, Iapetus?

Ann

Y'all said that Titan is visible with Saturn, but there are two more of its moon there. There's one just to the left of the rings, and another one just above and to the left of the rings. Which two moons are those?

-- Ron

This image is flipped left to right, or perhaps it was never un-flipped. (In addition to being upside down for northern hemisphere earthlings)

Actually, I just took a closer look, and I see not four but five of Jupiter's moons there, three on the left and two on the right. And as I said earlier, three of Saturn's moons.

-- Ron

I'm so pleased by all the worldwide attention the conjunction has been getting! So many of my family and friends who aren't ordinarily astronomy-focused have been talking about the conjunction and sharing pictures.

This afternoon it was clear where I live (finally!) and I was able to get a good look at the two brothers side-by-side in my 8" Dobsonian. They've clearly faded quite a lot since August, but it was still a pleasure to see them side-by-side. I'm no astrophotographer, but I wanted to share the picture I took with my phone camera through the eyepiece of the telescope anyway.

At the time I took this picture, the moons visible would have been (from left to right): Ganymede, Io, Callisto, and Europa.

jensenr@missouri.edu wrote: ↑Wed Dec 23, 2020 5:58 am Actually, I just took a closer look, and I see not four but five of Jupiter's moons there, three on the left and two on the right. And as I said earlier, three of Saturn's moons.

-- Ron

Jupiter has only got four big and bright moons, so the fifth light right next to Ganymede must be a star. Wonder what star it is?

Ann

it was HD191250. exciting name!

Seems extraordinary that this much vaunted sight, a 'near-single star' to naked eye, is so widely separated when magnified in a telescopic view!
I am not familiar with such views, but I expected to see them a few diameters apart, not a dozen of more.

Further to lighten my darkness, the visible moons are not on the same plane, when I thought that the planets' satellites would be more or less on the plane of their solar orbits. Is this another example of my ignorance?

Lovely; I was wondering about the pin pricks around Saturn! Since they weren't identified; I presumed they might be stars!

Ann wrote: ↑Wed Dec 23, 2020 5:49 am A few more moons of Saturn are visible. Which ones? Tethys, Rhea, Dione, Iapetus?

Good question! Sky & Telescope gives one way to identify Saturn's moons:
https://skyandtelescope.org/wp-content ... turn.html

I was fortunate enough to look out when there was a break in the clouds for five minutes. I saw the conjunction and I grabbed my binoculars and verified that I was correct in what I thought I was seeing. Then, the clouds came back in and that was it. This is a lovely telescopic view.

Using the Sky & Telescope tool, I couldn't get the orientation to match the APOD image, so I just had to assume that Titan is the bright one, and go in order from there. S&T shows Tethys and Dione close together at that time, and I only see one dot at that place in the APOD image, so I don't know which one it is. For those who want to check S&T themselves, I won't spoil the answer; just remember to set the date.

RJN wrote: ↑Wed Dec 23, 2020 2:13 pm

Ann wrote: ↑Wed Dec 23, 2020 5:49 am A few more moons of Saturn are visible. Which ones? Tethys, Rhea, Dione, Iapetus?

Good question! Sky & Telescope gives one way to identify Saturn's moons:
https://skyandtelescope.org/wp-content ... turn.html

Nice! The credit link to Damian Peach's twitter feed has some more discussion of the extra moons visible. The fifth Jupiter moon is apparently a star.

And from that page you linked to, and the image time of 12/21/2020 00:18UT, we get this (I couldn't get the orientation to match exactly):


So, in theory we should be able to see a fifth Saturnian moon - Enceladus - just below and very close to Saturn, but it must be too dim to reveal itself in this APOD.

EDIT: even after color inverting the APOD image, Enceladus is nowhere to be found:

Neslon wrote: ↑Wed Dec 23, 2020 8:00 am it was HD191250. exciting name!

Actually, for amateur astronomers it IS a truly exciting name: read about him here: https://en.wikipedia.org/wiki/Henry_Draper_Catalogue

Of course where I live at this time in December, there were snow flurries on Sunday night, and even today it is overcast and cloudy. I noticed something though in the picture. Wonder if it is a telescopic illusion? When I went to school nearly 45 years ago we were taught that the Great Red Spot is so large that 3 Earths could fit in it. Nowadays it must have shrunk since astronomy magazines say that 2 Earths can fit in it side by side. Yet when I look at the picture, Ganymede and Callisto can appear to be large enough to fit into the GRS. Yet Ganymede and Callisto side by side is the same diameter as one Earth. Even Europa with Ganymede look like they could fill the GRS. Is this an illusion or has the GRS shrunk even more?

This image is beautiful