A good night to see Iapetus

[Nik271]

I am observing Saturn tonight, the seeing is excellent and my 120mm SW Mak is pushing x200. I can clearly see Iapetus, it is diagonally opposite to Titan with Saturn in the middle. Dione and Rhea are also visible with averted vision, despite boing much closer to Saturn.

Iapetus is famous for being 2 magnitudes brighter when west of Saturn (like tonight, it's about mag 10). East of Saturn it is only mag 12 so not easily seen in small scopes.

 

Edited October 10, 2022 by Nik271

[IB20]

Yep with the 200P I can clearly see Titan, Rhea, Dione and Iapetus. Tethys popping in and out too. 
Looked great with the 17.5mm Morpheus, superbly framed.

With the small Tak I managed Titan and Rhea with Dione occasionally visible. The northern hemisphere banding always pops out more with the refractor than the dob, being a dimmer image helps.

[RobertI]

1 hour ago, Nik271 said:

Iapetus is famous for being 2 magnitudes brighter when west of Saturn (like tonight, it's about mag 10). East of Saturn it is only mag 12 so not easily seen in small scopes.

I didn’t know that! Educational and informative - nice repot. 🙂

[markse68]

I didn't either- thanks for that Nik. I have seen Iapetus with my 150p but only with averted vision- likewise a couple of the closer moons- but not sure which

Mark

[Zermelo]

10 hours ago, Nik271 said:

I am observing Saturn tonight, the seeing is excellent and my 120mm SW Mak is pushing x200. I can clearly see Iapetus, it is diagonally opposite to Titan with Saturn in the middle. Dione and Rhea are also visible with averted vision, despite boing much closer to Saturn.

Iapetus is famous for being 2 magnitudes brighter when west of Saturn (like tonight, it's about mag 10). East of Saturn it is only mag 12 so not easily seen in small scopes.

 

Thanks Nik, I did see Iapetus last night and was a little surprised, but I'd forgotten about that albedo variation.

[Stu]

11 hours ago, Nik271 said:

Iapetus is famous for being 2 magnitudes brighter when west of Saturn (like tonight, it's about mag 10). East of Saturn it is only mag 12 so not easily seen in small scopes.

Thanks for that Nik, it wasn’t something I was aware of. Worth looking out for to note in future.

[Stu]

Just checking back on a past post I made, I seem to have got it in the 4” when it was at mag 11.4 (according the SkySafari) at x148. After a quick check on SkySafari, it doesn’t seem to accurately reflect your info Nik, showing Iapetus at mag 11.7 still so it is possible it was brighter when I saw it. The date was 7th June 2015, position in the attached screenshot.

 

[Nik271]

Looks like you saw Iapetus when it was exactly north of Saturn so in superiour conjunction. I would expect it to be a bit brighter than mag 11.7 at that time, the darkest it gets is 11.9 which is at eastern elongation.

Once it moves to the west of Saturn it should be mag 11 or less.

Actually there can be a small discrepancy from 2015 to now because of the inclination of the Saturnian moon system. Currently we are seeing it more edge on so the difference of brightness will be more pronounced.

[Stu]

1 hour ago, Nik271 said:

Looks like you saw Iapetus when it was exactly north of Saturn so in superiour conjunction. I would expect it to be a bit brighter than mag 11.7 at that time, the darkest it gets is 11.9 which is at eastern elongation.

Once it moves to the west of Saturn it should be mag 11 or less.

Actually there can be a small discrepancy from 2015 to now because of the inclination of the Saturnian moon system. Currently we are seeing it more edge on so the difference of brightness will be more pronounced.

Thanks Nik. What causes the difference in brightness, albedo variations on the surface presenting to us??

[Nik271]

Yes, one side of Iapetus is much darker than the other, for reasons which are still not well understood. Iapetus is tidally locked to Saturn and and thus its brightness varies with the same period as its rotation around Saturn (about 2.5 months I think). 

The interesting thing is that the dark Iapetian half is not the same as the hemisphere facing Saturn or opposting it, otherwise the maximum and minimum of brightness will occus at conjunctions.

Since they happen at elongations people guessed and later it was confirmed by probes that the dark hemisphere is the 'forward' facing as Iapetus goes around Saturn.

So an observer on Saturn will see something like a Yin-Yang Iapetus which never changes 🙂

[markse68]

it looks like it wasn't masked off properly and got a bit of overspray when they were painting the background sky matt black

https://solarsystem.nasa.gov/resources/2381/iapetus-3d-model/

Mark

Edited October 11, 2022 by markse68

[Zermelo]

Some members will be aware of the story about Iapetus in 2001:A Space Odyssey.

In the book version (but not the film), alien signals are traced from Luna to Iapetus (though it's rendered "Japetus" in the text), to which a probe is dispatched for investigation. The variation in albedo was long established at the time of writing (1960s), and Arthur C. Clarke invented an extraterrestrial origin for it in the novel. His 'explanation' was a topography with a very stark contrast between black and white regions, at the centre of which was a second alien monolith.

In 1980, when Voyager 1 arrived in the Saturnian system and imaged Iapetus, the first pictures were eerily similar to Clarke's description (abetted by some processing artefacts). Carl Sagan, who was working at NASA, sent a copy to Clarke, with the message "Thinking of you ..."

[IB20]

Just reviving this topic as it was quite noticeable last evening how easy Iapetus was to see. I was using a 76mm scope at 136-106x. It’s at far Western elongation and showed a similar brightness to Rhea which is recorded as mag +9.8 on SkySafari, so over a mag brighter than it’s recorded +11.1. 

A nice discussion post on CN here about the “phenomenon” https://www.cloudynights.com/topic/212334-observing-iapetus-enigmatic-magnitude-estimates/#entry2717345

Does anyone know of any resources for a more accurate Iapetian mag estimate? Presuming that SkySafari and Stellarium do not provide dynamic mag estimates.

Edited August 27, 2023 by IB20

[Zermelo]

Sky and Telescope have a web page (and iPhone app) that gives positions but not magnitudes. I doubt that any of the planetarium tools would have dynamic magnitudes.

There is this NASA tool that isn't as pretty, but you can search for Iapetus and it does list mags in the ephemerides:

Edited August 27, 2023 by Zermelo

[Nik271]

I was not aware that past observations estimated Japetus at magnitude 9 and even brighter. To me at brightest it seemed to be mag 10 when at western elongation.

Magnitude 11.15 seems too low for the moment, given @IB20 's observation. Just shows making an accurate prediction is tricky indeed.

[IB20]

Perhaps it’s a citizen science project for us all?! 
It does seem tricky to estimate however as although I felt Rhea was ever so slightly brighter, Iapetus caught my eye more as it was further away from Saturn’s radiating brightness; which conversely may have made Rhea appear at lesser magnitude. Dione was closer to Saturn than Rhea on the opposite side and I had real difficulty consistently seeing that.

[The60mmKid]

I felt excited by the name of this thread because I used to live on the previous site of the Iapetus Ocean, which covered parts of the current landmass of the northeastern United States and southeastern Canada. Frankly, I think I would have preferred it greatly when there were a lot of sharks and so forth there.

Despite my confusion, I've enjoyed reading and learning about the actual topic of this thread.

[Nik271]

Last night I observed Iapetus with my 70mm and 100mm refractors. With the 70mm at x105 I could just about make Iapetus with averted vision. Saturn was only 12 degrees above the horizon and there was a lot of moisture in the air. With the 100mm at x175 Iapetus was a lot easier to spot even with direct vision. There was a dim star of slightly brighter magnitide south of it, Stellarium lists it at mag 10.5, so I estimate Iapetus at magnitude 10.6  last night. 

[John]

I observed Iapetus tonight with my ED120 refractor. Lots of "moonwash" from the nearby 99% illuminated moon but Iapetus looked approximately as bright as Dione this evening which is around magnitude 10.6 I think. Certainly straightforward to see with direct vision.

 

[johnturley]

Looking at Saturn last night, I also could see Iapetus quite easily along with Titan, Rhea, and Dione. I could also make out what looked like a brightish 'moon' to the right of Titan (as seen in a telescope giving an inverted image), which according to Stellarium, roughly corresponded to the marked position of Hyperion,. However as it appeared only about a magnitude fainter than Titan, and brighter than Iapetus and Dione, I assume that it must have been a star (not shown as such on Stellarium) and not Hyperion. 

Did anyone else see this. 

John 

Edited August 30, 2023 by johnturley

[IB20]

38 minutes ago, johnturley said:

Looking at Saturn last night, I also could see Iapetus quite easily along with Titan, Rhea, and Dione. I could also make out a brightish moon to the right of Titan, which according to Stellarium, roughly corresponded to the marked position of Hyperion, however as it appeared only about a magnitude fainter than Titan, and brighter than Iapetus and Dione, I assume that it must have been a star (not shown as such on Stellarium) and not Hyperion. 

Did anyone else see this. 

John 

There’s a mag +10.5 star listed in SkySafari.

[johnturley]

17 minutes ago, IB20 said:

There’s a mag +10.5 star listed in SkySafari.

Yes, almost certainly that star, thought it appeared far too bright to be Hyperion, which incidentally I've never been able to see visually.

John 

[John]

I saw that star as well. I assumed that Hyperion would be below the limit that my ED120 could catch whereas that star was about the same brightness as Dione (and Iapetus).

I've never seen Hyperion either but I have seen Enceladus quite a few times when it is towards it's maximum elongations from the planet and it's rings. Not last night though.

 

Edited August 30, 2023 by John

[johnturley]

35 minutes ago, John said:

 

I've never seen Hyperion either but I have seen Enceladus quite a few times when it is towards it's maximum elongations from the planet and it's rings. Not last night though.

 

Me too, I don't know whether its a question of increased light pollution, or my deteriorating eyesight, or a combination of both, but in the 1970's with the 10in Newtonian I had then, I used to find Rhea, Tethys, Dione and sometimes Enceladus easy to spot. In recent years however, even with my 14in I've found Dione and Tethys quite hard to spot, and not seen Enceladus recently. 

For most of the 1970's though, Saturn was quite high up from the UK, so hopefully visibility of the fainter satellites will improve over the coming years, as Saturn moves higher up, and the near edgewise rings in 2025 will also help.

John 

Edited August 30, 2023 by johnturley

[johnturley]

11 hours ago, johnturley said:

Looking at Saturn last night, I also could see Iapetus quite easily along with Titan, Rhea, and Dione. I could also make out what looked like a brightish 'moon' to the right of Titan (as seen in a telescope giving an inverted image), which according to Stellarium, roughly corresponded to the marked position of Hyperion,. However as it appeared only about a magnitude fainter than Titan, and brighter than Iapetus and Dione, I assume that it must have been a star (not shown as such on Stellarium) and not Hyperion. 

Did anyone else see this. 

John 

Are their any experts on Stellarium out there, that might know whether it is possible to reduce the limiting magnitude of satellites, such as those of Saturn, displayed in 'Ocular View', so that in the case of Saturn for example it only displays the original pre-spacecraft 9 satellites, and increase the limiting magnitude of background stars stars displayed.

I find it confusing that 'Ocular View' in Stellarium in the case of Saturn, displays lots of approx 15th magnitude minor satellites, which are not visible in amateur telescopes, but did not display a 12th magnitude background star that can easily be confused with a brighter satellite. I know that Stellarium does display background stars in Ocular View down to a certain magnitude, as the other week it did display the 5th Galilean moon that Jupiter appeared to have, (which was actually an approx 6th mag star in Aries), but did not show the 12th magnitude star in Aquarius, which was close to the predicted position of Hyperion.

John 

Edited August 30, 2023 by johnturley