Saturn Satellite Spotlight

Have you caught a glimpse of Saturn at the start of this apparition? The 6th planet from the Sun is gradually climbing out of the dawn twilight to greet early morning risers and enthusiastic planetary imagers. Even if you have not set the alarm for a personal early morning rendezvous, you may have seen the work of other imagers online. And if you have, your take on it is likely to be along the lines of "gees, the rings have really closed up!", followed by a little sadness that for the next few years Saturn won't be the jaw-dropping star party favorite. 

Saturn 05/01/2024 by Clyde Foster

But, take heart - the arrival of fall for Saturn's northern hemisphere brings with it opportunities to see interesting and infrequent alignments of its moons as they circle the planet. Like Jupiter's moons, Saturn's contingent can undergo the phenomena of eclipses, transits (both the moon and its attending shadow), and occultations\re-emergences. However, unlike Jupiter with its minimal 3° axial tilt where such Galilean moon activity is commonplace, these events can only happen for Saturn when it is heading for a ring-plane crossing (i.e., when the Sun appears to traverse the Saturnian equator - an equinox for the planet). Given the ringed planet's nearly 30-year orbit and 26¾° axial tilt, we only get this opportunity roughly 15 year intervals. 

Unfortunately, to date I have not been able to find a convenient ephemeris online that enumerates these upcoming occurrences. In the absence of that I've leveraged WinJUPOS to manually inspect the locations of Titan, Rhea, Dione, and Tethys as they orbit the planet for the next few months. So full disclaimer, I certainly could have overlooked some events in my investigation!

Europa appears as a white dot
against Jupiter's clouds in this
 IR image

With the exception of Titan, these satellite events are going to be a bit harder to detect in amateur telescopes due to their size (roughly a quarter that of the Galilean moons) and their brightness (roughly 100 times dimmer than their Jovian counterparts). Capturing them by imaging will likely be a bit easier since the SPLICE technique yields details that the eye cannot pick up. The use of an IR filter (imaging only, not visual) will also likely help, especially for transits since it tends to make the moon stand out with higher contrast against the planet. 

Here then is a my list of some of the more favorable and interesting events for those of us along the east coast from now until September 1st. I'll try to update the list with additional events from September through January when I have some time to research them. 

Keep in mind that the larger and brighter the moon, the better odds are for catching the occurrence.

Eclipse Events

The eclipse events, where the moon slides into Saturn's shadow, are likely going to be the easiest of the phenomena to observe visually. Make sure you use high magnification to increase the distance of the moon from the glare of the planet and/or its rings to improve its visibility. Start your observing a few minutes before the event to locate the moon, and then keep watching as it slowly fades from view. 

Also note that the eclipse events that occur near quadrature are ideal, because we get to "see" the largest extent of the shadow cast by the planet. That in turn means the moon enters the shadow farther away from the glare of the globe, making your observation a bit easier. Since quadrature for Saturn happens June 9th, the further into the summer you go the less favorable the circumstance for viewing the eclipse. 

Saturn's 2nd largest moon, Rhea, poised to enter eclipse

Transit Events

Transit events are tougher because once the moon is fully backlit by the Saturn cloud deck the contrast drops significantly. We see the same situation with the larger and brighter Galilean moons that often are very difficult to pick out once they are on Jupiter's face. For this reason I have limited the events in my list to times when the smaller moons (i.e., not Titan) are just creeping onto or sliding off the planet because the limb darkening effect will provide the best contrast and increase your opportunity to see it.

Another interesting Earth-Sun-Saturn geometry effect that you'll be able to follow over the summer involves the appearance of the shadow of the rings cast on the planet. At the outset of summer you should be able to see the shadow quite clearly under (south) of the rings as they cut across the planet. However, as summer wears on Earth will climb northward in respect to Saturn, causing the shadow to retreat under the rings (think of it as early in summer we get to peek under the rings, but then as we ascend in our orbit we view it more straight on). 

We can use that favorable early summer geometry to enhance our chances of seeing Tethys transit Saturn because it will appear to trek along the path of the rings' shadow from our vantage point. This high contrast between tiny moon and dark shadow should be optimal for witnessing it moving across Saturn, appearing tucked just under the rings. By August the moon moves across illuminated cloud tops, which will make it impossible to see in all but the largest telescopes.

Above, Tethys glides along the rings' shadow
Below, the shadow has retreated and Tethys will
be harder to see

 

Observing Titan this summer will be another way for us to appreciate the shifting geometry between the 3 objects. We start off the summer with Saturn's largest moon undergoing transits and occultations, but by August as we have ascended in our orbit they stop and become near-misses. 

Titan starting its transit at end of June

By August the giant moon misses the planet

Of course, we can have not only transits of a moon across Saturn but the moon's shadow as well. That sets up another high contrast situation with dark shadow backlit by bright cloud tops of the host planet. However, the small size of Tethys and Dione mean that their shadows will be so tiny as to be very difficult to see/image in anything under 12" in aperture. Titan's shadow should be quite easy, but sadly it looks like we are sitting on the wrong side of Earth to witness any of those this year. That leaves it all up to Rhea which is about 50% larger than siblings Tethys and Dione. I'll be accepting the challenge and getting out there during some of the Rhea transit events to determine if I can visually see it in my 10" Newtonian, or maybe image it.

Rhea undergoing a transit end of August

If the idea of observing these Saturnian moon events piques your interest I'd urge you to get a copy of WinJUPOS for yourself so that you have a tool to visualize them. So remember, no need to feel glum over losing the splendor of the rings, take advantage of these uncommon views we only get during Saturnian equinox season!