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Tethys' shadow on Saturn's rings - 19/6/25

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#1 happylimpet

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Posted 23 June 2025 - 07:47 PM

Lucky lucky lucky.

 

Firstly, kudos to kevinbreen who gave me a heads up for good seeing which tempted me to attempt Saturn in the very low dawn sky. Its still very badly placed in the UK, but the jetstream gods were truly onside and the seeing was remarkable for ~23 degrees altitude. I used Baader R and 610 filters and QHY5III200m, 12" newtonian.

 

Just happened to check the moon positions in CdC the night before (hoping against hope for a Titan transit) and noticed a grey streak across the A ring at 0300. Quickly realised it was the shadow of Tethys. Cant be common right? So fingers crossed went for it - and got it! I even collimated on a nearby star beforehand, which was needed as the collimation shifts between Mars in the west (usual target) and saturn here in the east.

 

The AS stacks are 70% culls, having used PIPP to reject worst 30% frames.  Animation uses 43% stacks for the bright run-through, and 17% stacks for the 'disk' images. Maybe a faint equatorial belt structure visible moving across. Animation covers 45 mins - the same data repeats twice at different stretches, one for the rings, one for the disk.

 

But the main thing is the dark diagonal streak of Tethys' shadow moving to the left across the lower left ansa. Subtle but once you see it, pretty clear.

 

optimised normal,bright3.gif  2025-06-19-0300.0-Saturn-NR.png sat0300_2.jpg

 

Images are noisier at the end as the sky was quite bright - it was blue throughout.

 

WinJupos gets the shadow position quite wrong - not massively so in the scheme of things, perhaps by a Tethys diameter (~500km) or so, but it puts the shadow on the far side of the rings. CdC appears spot on at a casual glance.

 

I think Tethys is one of the few moons that can do this (Iapetus certainly can) as the orbit is tilted by a full 1 degree - enough to allow a shadow on the rings even when the system is tilted, but small enough that its far from a round shadow (as per Iapetus). I think most of the other big moons are much closer to the equatorial plane, but now i type this, I realise i havent checked.


Edited by happylimpet, 24 June 2025 - 02:01 PM.

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#2 Doug_Hole

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Posted 23 June 2025 - 09:18 PM

Facsinating! 


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#3 leoyasu

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Posted 23 June 2025 - 09:31 PM

That’s quite the catch!!!
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#4 Winteria

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Posted 23 June 2025 - 11:55 PM

Very cool! Thanks for sharing.


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#5 CeeKay

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Posted 24 June 2025 - 04:26 AM

Awesome!


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#6 kevinbreen

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Posted 24 June 2025 - 04:34 AM

Well done Master Haigh!
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#7 Lacaille

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Posted 24 June 2025 - 05:13 AM

Gosh that’s interesting!
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#8 Foc

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Posted 24 June 2025 - 05:16 AM

A great achievement from carefully checking the captures.


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#9 Kokatha man

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Posted 24 June 2025 - 07:51 AM

waytogo.gif waytogo.gif waytogo.gif waytogo.gif


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#10 happylimpet

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Posted 24 June 2025 - 01:59 PM

reverse1.gif

 

Reversing GIF for clarity!


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#11 KMH

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Posted 24 June 2025 - 03:00 PM

Wow - very cool!!

 

Kevin


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#12 Mike Spooner

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Posted 24 June 2025 - 03:37 PM

Amazing catch!!waytogo.gifwaytogo.gif bow.gif


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#13 dcaponeii

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Posted 24 June 2025 - 04:11 PM

I liked this when I saw it this morning but it really deserves a comment.  WELL DONE!!  Really interesting and excellently processed capture.


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#14 John Boudreau

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Posted 24 June 2025 - 04:43 PM

Superb work in recording this Nick!

 

I had been wondering if anybody was going to catch one of these --- congratulations, and you were certainly well prepared to take full advantage of the opportunity!


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#15 Astrohunter8se

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Posted 24 June 2025 - 04:48 PM

Incredible shot! That really is top notch in every way


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#16 KiwiRay

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Posted 24 June 2025 - 04:50 PM

Excellent result, Nick!


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#17 Spacedude4040

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Posted 25 June 2025 - 05:46 AM

Ditto to all the above comments waytogo.gif


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#18 Mike Phillips

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Posted 25 June 2025 - 01:14 PM

I saw these in WinJUPOS when planning for this season and glad to see someone got it!!  Great work!

 

Mike in NC


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#19 Mirzam

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Posted 25 June 2025 - 01:25 PM

I’m curious about the contrast of the shadow against the rings.  Have you looked at histograms? Do you think this would be possible to detect visually?

 

JimC



#20 leoyasu

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Posted 25 June 2025 - 02:39 PM

I'm here again, this event got me all hyped up. I checked WJ, and during the last days of June and all of July, there are quite a few shadow ring transits. I'm going to try my best to catch one!


Edited by leoyasu, 25 June 2025 - 02:39 PM.

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#21 AstroDan2015

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Posted 25 June 2025 - 06:26 PM

Excellent animation. It's the first time I ever saw anything like this. Congratulations! waytogo.gif


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#22 happylimpet

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Posted 26 June 2025 - 03:45 AM

I’m curious about the contrast of the shadow against the rings.  Have you looked at histograms? Do you think this would be possible to detect visually?

 

JimC

This is far too noisy to do any kind of analysis like that. Visually? Who knows....I'm sure with a biggish scope and excellent, excellent seeing. I doubt i ever would be able to here.

 

Thanks for all the marvellous comments folks!


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