It’s tiny. It’s challenging. But chances to see Iapetus transit the globe of Saturn are few and far between. The night of July 17-18 is one of them.

Iapetus multi-image
The arrows point to Saturn's moon Iapetus as it tracks west over a pale cloud zone in the planet's northern hemisphere on April 29, 2022. Observers will see a similar transit on the night between Sunday and Monday, July 17–18, lasting many hours.
Darryl Milika and Pat Nicholas

Saturn's the place to be this month. A little more than a week after its largest moon Titan occulted a star on July 9th, we're up for a rare transit of the planet's third-largest moon Iapetus. The event lasts about 19 hours — from approximately 21:00 – 16:00 UT July 17–18 (5 p.m. Sunday evening July 17th to 11 a.m. Monday morning July 18th EDT) — which means many geographic locations will see at least part of the event, including the entire U.S. from Maine to Hawaii.

No mincing of words here — it will be visually challenging because the moon spans a mere 0.23″ and will require at least a 10-inch telescope and excellent atmospheric seeing to observe. I suspect it will also test the mettle of planetary imagers, though prove easier to capture than by eye alone.

The two faces of Iapetus
Saturn's moon Iapetus, 1,460 kilometers in diameter, has a dual personality. One hemisphere is covered with bright ice, the other with darker material possibly ejected by impacts on the more distant moon Phoebe. These images are from NASA's Cassini orbiter.
NASA / JPL / Space Science Institute

This is actually Iapetus's second transit this year. The first occurred on April 29–30 but was not visible in the Americas. The next will happen on December 23 but only for observers in the eastern hemisphere. After that we have to wait until the 2037 transit, which is also problematic because it takes place close to solar conjunction. The next "easy" shot at seeing this uncommon event won't be until 2052! That makes this opportunity one you don't want to miss.

Saunter at Saturn

On July 17th at about 21:12 UT, Iapetus makes first "contact" with the northeastern edge of the outer A ring just 4.6″ from Saturn's globe. Keep in mind that despite appearances, especially when crossing the rings, Iapetus will pass in front of the planet the entire time. The yin-yang moon may prove difficult to spot near the bright rings because we happen to be facing the side coated in coal-black dust, dimming the target from magnitude 10 to 11.2. In a moment, we'll see how this also has advantages.

Iapetus transit single
Iapetus appears as a dark pinprick in Saturn's northern hemisphere during its last transit on April 29, 2022. North is up and east is left.
Darryl Milika and Pat Nicholas

At around 22:38 UT, Iapetus may pop more clearly into view for about 20 minutes as it crosses in front of the dark Cassini Division. A little more than hour later at 23:48 UT, it nestles into Saturn's limb, and the transit officially begins. Central meridian passage occurs at 4:42 UT July 18th (12:42 a.m. EDT), with Saturn well-placed for East Coast observers at ~25° altitude and a reasonable 15° for the Midwest. Atmospheric seeing is critical to a positive outcome, so the higher the planet the better.

Iapetus transit panel
Key scenes in the Iapetus transit are shown here from transit start to end. North is up. Click for a large version. North is up and east is left.
Stellarium

For more than 10 hours the moon will track west across the planet's lighter North Tropical Zone (NTrZ) just north of the prominent North Equatorial Belt (NEB). This is where Iapetus's dark hemisphere becomes a big plus — hovering above pale clouds, it will stand out in splendid contrast. But there's a caveat: Its orbit is inclined 15.5° to Saturn's equator. As the transit proceeds, the moon will gradually cross the dark NEB, temporarily becoming more difficult to see until it's "in the clear" again in the Equatorial Zone south of the belt. It's difficult to tell exactly when and how long it will spend over the NEB by simulations alone, but I estimate it will clear the belt by around 9:00 UT or roughly an hour before the transit ends at 9:58 UT.

Iapetus animation
Iapetus transits Saturn's globe from east to west in this 10-hour animation from 0:00-10:00 UT, July 18, 2022. North is up and east is left.
WinJUPOS

For observers in the eastern half of the U.S., twilight and sunrise will interfere before the transit ends, but farther west the moon marches on, passing over the inner C ring (starting ~11:05 UT), the bright B ring (mid-point at around 13:25 UT), and revisiting the Cassini Division at 14:36 UT before exiting the ring plane at around 16:02 UT.

Start of A-ring ingressJuly 17, 21:12 UT (5:12 p.m. EDT)
Centered over Cassini's Division22:38
Transit begins23:48
Central meridian passageJuly 18, 4:42 UT (12:42 a.m. EDT)
Transit ends9:58
Start of C-ring ingress11:05
Mid-point B-ring13:25
Centered over Cassini's Division14:36
End of A-ring egress16:02
This table lists the key phases and approximate UT times of the Iapetus transit.
Iapetus transit sketch
Dodging clouds and bad weather during the 2007 transit, UK amateur David Gray successfully spotted Iapetus through his 16.3-inch (415-mm) telescope. He writes: "My initial, and best, view was at 01:05 UT, when it looked a distinct almost black spot but little more than fleetingly. Then for the next 40 minutes or so was more of an intense condensation at the SEB(S) s. edge – just impinging." Based on his experience Gray surmises Iapetus might be visible under the right conditions in as small a telescope as ~6 inches.
David Gray

Several British Astronomical Association members visually observed the Iapetus transit on January 6–7, 2007. Two observers used 16-inch telescopes and clearly recorded the moon, while another suspected Iapetus at the correct position in his 10-inch. This gives me hope that with good seeing, high magnification, and careful, relaxed observation I might even see it myself. Of course, we'd love to hear if you do.

Iapetus shadow transit
Iapetus casts its shadow on Saturn's eastern ring ansa on July 17 UT, the morning before the moon transits the globe.
WinJUPOS

Bonus shadow transit

The morning before the transit there will also be a shadow transit of Iapetus. I ran a simulation using the free and excellent WinJUPOS program and found that the moon will cast its shadow on the A ring from about 5:00–9:00 UT on July 17th (1–5 a.m. EDT). The shadow then shifts to the front of the planet, but keeps close to the ring plane for several hours until emerging into view again south of the plane from 14:00–17:50 UT. I have no idea if this will even be visible or photographable. After a search I only turned up this Cassini image. Maybe you'll be one of the first to see or picture it.

Saturn locator
Saturn is well-placed in the southeastern sky for the Eastern Time Zone when Iapetus transits Saturn's central meridian.
Stellarium

If you're hanging your head because you don't have access to a larger telescope, your time will come. Saturn's largest moon Titan, which orbits in the equatorial plane like the rings, will cast a much more prominent shadow on the globe during the next ring plane crossing (edgewise presentation) starting in March 2025. Even a 4- to 6-inch scope in good conditions will reveal the amazing sight. Good luck this weekend!



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Comments


Image of Anthony Barreiro

Anthony Barreiro

July 18, 2022 at 4:26 pm

Paul Abel sketched the transit as seen through a 208 mm Dall Kirkham telescope in Leicester UK.

https://www.cloudynights.com/topic/832517-the-transit-of-iapetus/

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Image of Bob King

Bob King

July 18, 2022 at 7:53 pm

Thanks for posting, Anthony. Much appreciated. You wrote "208 mm" but suspect that was a typo — I see that he recorded it with a 508 mm (20-inch) scope.
I had my 15-inch on Saturn for more than an hour, but the seeing simply didn't improve to the point where I could definitively say I saw Iapetus. I don't think I've ever stared at anything that long while also continuously fighting hordes of mosquitos! Still, I don't regret the time. There's something about concentration and pushing limits I continue to find rewarding.

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Image of Anthony Barreiro

Anthony Barreiro

July 18, 2022 at 8:20 pm

Thank you Bob. Sorry about the typo. Yes, 508 mm.

I hope the mosquitos didn't feast too heartily while you were staring at Saturn.

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Image of Bob King

Bob King

July 18, 2022 at 10:46 pm

Anthony,

After a while I grew so tired of reapplying bug dope I just started slapping my head when they got close. All went fine. What a joy at the end of the night to drive 60 mph all the way home with the windows rolled down. I just received a nice report and photo from an observer who succeeded in getting a photo of Iapetus in his 10-inch Dob.

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bob kelly

July 25, 2022 at 12:46 pm

"It's tiny. It's challenging."
It was cloudy.
bob k

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Bob King

July 25, 2022 at 10:55 pm

Hi Bob,

Clouds — the greatest challenge of them all 😉

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