A rare shadow-casting transit of Titan almost eluded this neophyte astronomer.
A transit happens when one object passes in front of another, and in this case it’s a dance between Titan and Saturn, with Titan’s orbit and inclination just right to cast a large shadow on Saturn’s cloud tops.
Titan is the largest moon in the solar system after Jupiter’s Ganymede. I tried to catch the transit early on the morning of August 19th only to be thwarted by high clouds. They moved in (of course) just as the transit was about the happen.
I thought I was done. But it turns out that there were actually a series of such transits recently at intervals of every sixteen days – each transit a little higher in latitude on Saturn’s disc. This time I managed to catch one on the morning of September 4th starting at about 1:30 AM Eastern Daylight Time.
Transits in the Jovian system (Jupiter) are not uncommon and I’ve seen quite a few. But it’s harder to catch one on Saturn and the next transit of Titan after October 2025 will not occur until the year 2040.
This bleary-eyed astronomer finally lucked out on September 4th, catching a bunch of images from 1:30 AM to about 4:00 AM local time before heading to bed. The seeing was pretty decent that night from my back deck in Petersham, Massachusetts. So, I just kept taking as many shots as I could at various exposure and camera gain settings.
Most of the exposures were quite short, in the range of 10 to 30 milliseconds. These are actually taken from individual frames of 3-minute movies containing thousands of images each. The technique is called “lucky imaging”, where once in a while Earth’s atmosphere steadies up long enough to catch some frames that aren’t dancing around too much.
I processed eight such movies to produce this short animation showing the progression of Titan over a period of about 2-1/2 hours.
I’m also working on some high-dynamic range images, where I’ve got over-exposures of Saturn in order to bring out Titan and several other moons. I’m hoping that I can combine these in a pleasing way while masking out the over-exposed Saturn. Stay tuned!
The Neophyte Astronomer 