APOD: Rings and Seasons of Saturn (2021 Sep 19)

Comments and questions about the APOD on the main view screen.
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APOD Robot
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APOD: Rings and Seasons of Saturn (2021 Sep 19)

Post by APOD Robot » Sun Sep 19, 2021 4:05 am

Image Rings and Seasons of Saturn

Explanation: On Saturn, the rings tell you the season. On Earth, Wednesday marks an equinox, the time when the Earth's equator tilts directly toward the Sun. Since Saturn's grand rings orbit along the planet's equator, these rings appear most prominent -- from the direction of the Sun -- when the spin axis of Saturn points toward the Sun. Conversely, when Saturn's spin axis points to the side, an equinox occurs and the edge-on rings are hard to see from not only the Sun -- but Earth. In the featured montage, images of Saturn between the years of 2004 and 2015 have been superposed to show the giant planet passing from southern summer toward northern summer. Saturn was as close as it can get to planet Earth last month, and this month the ringed giant is still bright and visible throughout much of the night

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Sa Ji Tario

Re: APOD: Rings and Seasons of Saturn (2021 Sep 19)

Post by Sa Ji Tario » Sun Sep 19, 2021 2:27 pm

There are six illuminated images on one hemisphere and another six on the other, the seventh is missing "without rings" because it is perpendicular to the Sun.

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johnnydeep
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Re: APOD: Rings and Seasons of Saturn (2021 Sep 19)

Post by johnnydeep » Sun Sep 19, 2021 4:54 pm

Sa Ji Tario wrote:
Sun Sep 19, 2021 2:27 pm
There are six illuminated images on one hemisphere and another six on the other, the seventh is missing "without rings" because it is perpendicular to the Sun.
I don't get it. Even though none is shown, there could well have been an image showing the rings (or showing their absence) edge on here, right?
--
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Chris Peterson
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Re: APOD: Rings and Seasons of Saturn (2021 Sep 19)

Post by Chris Peterson » Sun Sep 19, 2021 7:12 pm

johnnydeep wrote:
Sun Sep 19, 2021 4:54 pm
Sa Ji Tario wrote:
Sun Sep 19, 2021 2:27 pm
There are six illuminated images on one hemisphere and another six on the other, the seventh is missing "without rings" because it is perpendicular to the Sun.
I don't get it. Even though none is shown, there could well have been an image showing the rings (or showing their absence) edge on here, right?
Nothing is missing. We have 12 images approximately equally spaced in time, each taken a year apart. One of the images is very close to Saturn's equinox, but not exactly.
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orin stepanek
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Re: APOD: Rings and Seasons of Saturn (2021 Sep 19)

Post by orin stepanek » Sun Sep 19, 2021 7:22 pm

saturn2004to2015_peach_960.jpg
Ah! Beautiful Saturn! What a beautiful sight! Saturn has it's own disc
drive!
cat-close-up-picture-id1063469124.jpg
Getting kinda nosy Kitty!
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Smile today; tomorrow's another day!

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johnnydeep
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Re: APOD: Rings and Seasons of Saturn (2021 Sep 19)

Post by johnnydeep » Sun Sep 19, 2021 8:12 pm

Chris Peterson wrote:
Sun Sep 19, 2021 7:12 pm
johnnydeep wrote:
Sun Sep 19, 2021 4:54 pm
Sa Ji Tario wrote:
Sun Sep 19, 2021 2:27 pm
There are six illuminated images on one hemisphere and another six on the other, the seventh is missing "without rings" because it is perpendicular to the Sun.
I don't get it. Even though none is shown, there could well have been an image showing the rings (or showing their absence) edge on here, right?
Nothing is missing. We have 12 images approximately equally spaced in time, each taken a year apart. One of the images is very close to Saturn's equinox, but not exactly.
Allright. I suppose I was really asking whether it's possible to see Saturn's rings completely edge on from Earth. This says the answer is yes - https://science.nasa.gov/science-news/s ... ar_saturn/
The same thing that's happening now [in 2009]: we're experiencing a "ring plane crossing." As Saturn goes around the sun, it periodically turns its rings edge-on to Earth—once every 14-to-15 years. Because the rings are so thin, they can actually disappear when viewed through a small telescope.

In the months ahead, Saturn's rings will become thinner and thinner until, on Sept. 4, 2009, they vanish
I thought Sa Ji Tario might have been implying that's not possible.
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Re: APOD: Rings and Seasons of Saturn (2021 Sep 19)

Post by Joe Stieber » Sun Sep 19, 2021 9:24 pm

Saturn was in conjunction with the sun on Sept 17, 2009, so on Sept 4, the day of ring plane crossing, it was a daytime object that would have been essentially impossible to see or image from earth's surface. Before and after Saturn disappeared into the solar glare, the rings looked like a thin bright line running through the planetary ball as viewed in a modest amateur telescope (12.5 inches aperture in my case).

Sa Ji Tario

Re: APOD: Rings and Seasons of Saturn (2021 Sep 19)

Post by Sa Ji Tario » Mon Sep 20, 2021 3:37 am

Didn't it happen to Galileo when the "ears" disappeared?

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Re: APOD: Rings and Seasons of Saturn (2021 Sep 19)

Post by MarkBour » Tue Sep 21, 2021 8:10 pm

Great sequence of beautiful shots. 3 questions come to mind:
  1. Does this show the greatest angle at which we ever see the rings (from Earth)? I've never seen them tilted further than the end images shown, but I've not been observing Saturn for very long.
  2. It looks like the images toward the middle sometimes show a noticeable black edge to the rings as they cross the planet's disk. I'm guessing this is not really the rings themselves, but their shadow on the cloud tops of Saturn, making what looks like a black stroke there. Is that right?
  3. If Saturn were to turn so that the rings were completely sideways (so that Saturn's equatorial plane was perpendicular to our line of sight) would they perhaps appear fainter, rather than as bright and glorious as in these images? I'm thinking that they are so thin that they might have more transparency and not reflect as much light per pixel as they do when seen at an angle. They would still certainly reflect at least as much light in total, but it would be spread over a larger area, so it seems they could actually dim somewhat. (I assume analysis of some Cassini images could answer this question, but I'm just asking it, I'm too lazy to go work on that ...)
Mark Goldfain

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neufer
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Re: APOD: Rings and Seasons of Saturn (2021 Sep 19)

Post by neufer » Wed Sep 22, 2021 9:26 pm

MarkBour wrote:
Tue Sep 21, 2021 8:10 pm
  1. Does this show the greatest angle at which we ever see the rings (from Earth)? I've never seen them tilted further than the end images shown, but I've not been observing Saturn for very long.
  2. It looks like the images toward the middle sometimes show a noticeable black edge to the rings as they cross the planet's disk. I'm guessing this is not really the rings themselves, but their shadow on the cloud tops of Saturn, making what looks like a black stroke there. Is that right?
  3. If Saturn were to turn so that the rings were completely sideways (so that Saturn's equatorial plane was perpendicular to our line of sight) would they perhaps appear fainter, rather than as bright and glorious as in these images? I'm thinking that they are so thin that they might have more transparency and not reflect as much light per pixel as they do when seen at an angle. They would still certainly reflect at least as much light in total, but it would be spread over a larger area, so it seems they could actually dim somewhat. (I assume analysis of some Cassini images could answer this question, but I'm just asking it, I'm too lazy to go work on that ...)
From Earth the (penumbra) ring shadow is always thicker than the (brighter) rings:
https://apod.nasa.gov/apod/ap110308.html wrote:
Explanation: How thin are the rings of Saturn? Brightness measurements from different angles have shown Saturn's rings to be about one kilometer thick, making them many times thinner, in relative proportion, than a razor blade. This thinness sometimes appears in dramatic fashion during an image taken in infrared light.
Note, however, that the ring can be quite dark from the side in violet-light images:
https://apod.nasa.gov/apod/ap180402.html wrote:
Explanation: The violet-light image.
  • This probably implies that the "one kilometer thick" ice ring
    is shrouded in in a much thicker (100 km?) dust ring:
https://apod.nasa.gov/apod/ap040712.html wrote:
Explanation: What are Saturn's rings made of? This and other images show that inner rings have more dirt than outer rings. Specifically, as shown above, the thin rings in the Cassini Division on the left have relatively high dirt content compared to the outer parts of Saturn's A ring.
The "outer parts of Saturn's A ring" is, indeed, quite bright beyond the Keeler Gap:
https://apod.nasa.gov/apod/ap200527.html wrote:
Explanation: The featured image is Saturn's A ring, with the broad Encke Gap on the far right and the narrower Keeler Gap toward the center.

https://apod.nasa.gov/apod/ap191012.html
https://apod.nasa.gov/apod/ap151203.html
Perhaps, thanks to Daphnis and its waves,:
  • "some of Saturn's ring particles temporarily collide, effectively
    recycling ring particles by bringing fresh bright ices to the surface
    ":
https://apod.nasa.gov/apod/ap120916.html wrote:
Explanation: Some of Saturn's ring particles temporarily bunch and collide, effectively recycling ring particles by bringing fresh bright ices to the surface. Seen here, Saturn's rings were imaged in their true colors by the robotic Cassini in late October.
https://en.wikipedia.org/wiki/Rings_of_Saturn#Keeler_Gap wrote:

:arrow: Near Saturn's equinox, Daphnis and its waves cast shadows on the A Ring.

<<The Keeler Gap is a 42-km-wide gap in the A ring, approximately 250 km from the ring's outer edge. The small moon Daphnis, discovered 1 May 2005, orbits within it, keeping it clear. The moon's passage induces waves in the edges of the gap (this is also influenced by its slight orbital eccentricity). Because the orbit of Daphnis is slightly inclined to the ring plane, the waves have a component that is perpendicular to the ring plane, reaching a distance of 1500 m "above" the plane.>>
Art Neuendorffer