Titan Shadow Transit Season Underway

I'm embarrassed to admit that I've yet to see a Titan shadow transit despite having observed the planet at four ring-plane crossings starting in 1966 at age 12. That's why I'm so looking forward to the current season, when there will be 10 opportunities to make it happen. In the past, either the timing was wrong, the seeing lousy, clouds nixed the view, or I wasn't aware of when to look. Maybe you've experienced similar hurdles. Failure isn't necessarily a bad thing in astronomy. It often sharpens the blade of desire.

Titan transits occur only during the relatively brief time Earth and Saturn both lie nearly on the same plane, which coincides with when the rings appear approximately edge-on. Currently, we see the rings' south side, with the ring plane tilted between 2.7° (May 14th) and 3.2° (May 31st). The rings have been difficult to tease out since the planet's return to the morning sky. Not only have they been backlit by the Sun until recently, but Saturn also continues to lay low in the eastern sky at dawn, especially from mid-northern latitudes. Poor seeing coupled with bright twilight have made it difficult to discern much more than a pale-yellow ball. I finally succeeded on May 14th, when the rings flickered in and out of sight in my 10-inch Dob at 168×. They were nothing short of wispy — on the verge of invisibility.

Shadow transits are common at Jupiter, and they occur when one (or more) of the four Galilean satellites casts its shadow on the gas giant's cloud tops. The size of the dark spot is closely related to the size of the moon. Ganymede is the largest, with an apparent diameter of about 1.7″, and Europa the smallest at about 1.0″. Through the telescope, Ganymede's shadow is a definitive black dot, while Europa looks more like a pinpoint.

Saturn's moons likewise cast shadows, with Titan's the easiest to see because it's the largest, with an apparent diameter of 0.8″at opposition on September 21st. This means you'll want to use at least a 3-inch telescope and magnification of 200× or more to discern the inky speck. At times, both moon and shadow will occupy the disk simultaneously. Titan itself appears somewhat smaller than its shadow and paler in tone. Even after the shadow portion of the transit series ends on October 6th, you can continue to observe the moon pass in front of Saturn every 16 days, through January 25, 2026.

Titan's one of the most fascinating places in the solar system. With a diameter of 5,150 km (3,200 miles), it's larger than Mercury. Nitrogen and methane dominate its atmosphere, which is 1.4 times denser than Earth's. It's also teeming with lakes and rivers filled with ethane and methane, which are liquid at Titan's frigid surface temperature of –179°C (–290°F). If you were sufficiently protected against the cold and jumped into a Titanian lake, you'd be in for a big surprise. Liquid methane is only about half as dense as water, so you'd sink . . . and fast!

Upcoming titan shadow transits (UT)

When the current transit series began last November, timing favored Eastern Hemisphere observers. Now through early October, skywatchers in the Americas will get a crack at the shadow. Most of the transits occur after midnight, especially for the eastern half of the U.S. The table above lists the Universal Times (UT) of the start, midpoint, and end of each event. To convert to local times, use this UTC time zone converter. Notice that the transits occur every 16 days, which is Titan's orbital period.

Titan and its shadow gradually migrate northward across Saturn's oblate disk this spring and summer. The May 15th transit occurs across the planet's Equatorial Zone (EZ) and lasts nearly 6 hours. But on October 6th, when Titan's shadow nibbles the northern limb, the event lasts only about 2 hours.

Rhea's also fair game

Rhea, the planet's second largest satellite, casts a shadow just 0.3″ in diameter. Telescopes in the 8- to 10-inch range should be able to tackle it in excellent seeing. Also, because Rhea orbits much closer to Saturn, transits are more frequent. To find out when they'll happen, use a stargazing app like Stellarium or SkySafari and run through a simulation. I pick the date I plan to observe and then open the clock/calendar and step through time, noting when Rhea's shadow enters and exits Saturn's disk from my location.

There's really just one rule for seeing a Titan shadow transit. Jump at the first opportunity. That's my plan. If I miss it this time around, the next series won't happen for another 15 years!