See a Rare Transit of Iapetus on July 17–18

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.

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.

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.

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.

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 ingress	July 17, 21:12 UT (5:12 p.m. EDT)
Centered over Cassini's Division	22:38
Transit begins	23:48
Central meridian passage	July 18, 4:42 UT (12:42 a.m. EDT)
Transit ends	9:58
Start of C-ring ingress	11:05
Mid-point B-ring	13:25
Centered over Cassini's Division	14:36
End of A-ring egress	16:02

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.

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.

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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