Achieving a Bucket List Item

There are certain astronomical events that intersect our lives only episodically, and Titan shadow transits has been one of those elusive moments for me. Years ago, as a young amateur seeing Jupiter’s moons cast their shadows on the giant planet, I wondered why I did not see that with Saturn. Of course, I soon realized that Saturnian moons do not do this routinely due to the planet’s axial tilt of 27°, lifting (or dropping) their orbits so that they don’t cut in front of the planet when it is facing the Sun. Only when Saturn is around the time of its equinox (when its rings appear very thin) do the moons begin to intersect the Sun’s plane as they move in front of their host planet. We are roughly halfway through such a season with the ringed planet having reached autumnal equinox for its Northern hemisphere last May. I know that only a few months remain of this opportunity, my last realistic chance for years to come. The next similar alignment won’t take place until 2038, at which point I’ll be a much older observer, perhaps with a little more wisdom but a little less stamina for predawn adventures.

Given that Titan orbits the planet every 16 days these transits will take place only once or twice a month. And given that the orbit is almost exactly 16 days you have to be on the right side of Earth with Saturn in a reasonably dark sky. Thankfully the Western hemisphere is well position for the current series. The window opened as Saturn emerged from Solar conjunction this spring. My concerns of missing it have been slowly nagging me as I watched the May and June windows be clouded out and the July 2^nd opportunity unavailable as I was away.

July 18th was on my calendar as the next transit, but as it approached it came with iffy forecasts: some sources predicted hopelessly thick clouds, while others suggested there might just be enough breaks to make an attempt worthwhile. I set my alarm anyway and, with a quiet stubbornness familiar to many amateur astronomers, rose at 3:15 a.m. to scan the skies. It was quite overcast but there still were occasional breaks through which Saturn would pop out for a few minutes. I rolled out the Cyrus 10” f/6 Newtonian and performed a quick mirror collimation.

I decided to prioritize visual observation rather than seeing it on a laptop screen, wanting to witness this event with my own eyes. When a sizable break appeared a little after 4 a.m., I popped in my 8mm TMB eyepiece and Meade 2x Barlow and brought the planet into focus. Perched on the first rung of my step ladder, I saw the planet as a light tan orb with the rings as a spike through it. I continued studying the planet, waiting for those fleeting moments of steady seeing. My first micro-glimpse was uncertain – did I see something or was it a trick from my aging eyes? But after a couple of minutes the dark dot continued to pop in & out of view, clinching it as the shadow of Saturn’s largest moon.

All too soon the clouds reclaimed the view. I swapped out the eyepiece for camera in hopes they would relent for a few minutes so that I could grab an image of the event. But it was an hour of frustration as the planet never emerged for an encore. But while the imaging effort failed, it was a pure delight and sense of accomplishment to witness that shadow cast across another world.

Last week the forecast for early Thursday morning caught my attention with partly cloudy skies and predicted above average seeing – rare conditions for this spring and early summer of 2025. Unable to pass on the prospect, I set out the Cyrus scope around 10 p.m. and settled in for a short rest with the alarm set for 3 a.m. As usual I ended up naturally waking up ahead of the alarm – odd how your body will do that! 

As I stepped outside I was greeted by warm, muggy air; just the sort of atmospheric hush that can often presage very good seeing. The malingering clouds were punctuated by generous breaks, one of which framed Saturn. I started the usual pre-observing sequence: mirror collimation, finder alignment before commanding the scope slew to Saturn. I centered it on the laptop screen and honed the focus, using the high-contrast edge of the rings' black shadow on Saturn’s bright equator as my guide.

Imaging began in the deep red/infrared wavelength using a Baader 685 filter, firing off five 2-minute video runs. Next I changed camera filters to begin a run of color captures. I did a quick stack and sharpen of one of the early video captures and was happy that it revealed delicate banding and even a hint of the Cassini Division. Encouraged by seeing conditions nudging up to 8/10, I pressed on to take a total of nine videos. Then with the laptop battery flagging I invested in capturing a nearby star’s Airy disk as a sharpening reference, managing two short videos before the battery gave up a little after 5 a.m. Time to pack it in and get ready for work!

The next day’s processing brought a pleasant surprise with the best Saturn images I’ve had in months. The R/IR set showcased layered bands, with the NEB and SEB standing out sharply. I was especially intrigued by the dark SPR, perhaps a lingering effect from Saturn’s long winter. The rings, though dim, opened enough to reveal the Cassini Division at their tips, while the planet’s shadow starkly cut off the rings where they ducked behind the globe.

The true showpiece, though, was the color session. Saturn’s pastel hues—blue to tan—came through beautifully. Again, the NEB and SEB dominated, while the SPR, though less sharply defined, was the darkest and bluest region on the disk. What caught my eye most was a subtle bluish belt in the NTR region, around latitude +50°, visible in each of the nine runs, so surely not an artifact. I’ll be curious to see if it turns up again in my next imaging session.

Observing opportunities given the lousy weather seem to come at a premium these days, especially when a quick shower and heading out to work follows your pre-dawn session. But these two sessions this month made every lost minute of sleep worth it. And among the many milestones, I can now add the thrill of witnessing a Titan Transit shadow—a sight I’ll not soon forget.