Perhaps you're well acquainted with observing Jupiter's Galilean moons transiting the gas giant, but have you ever witnessed a transit of Saturn's moon Titan?
Saturn’s largest moon is Titan, an object that shines at a small telescope- (and potentially binocular-)
friendly eighth magnitude.
Its orbital period around Saturn is 15 days and 22 hours, which means from Earth we get to see it in roughly the same position relative to Saturn every 16 days.
We're entering into a period when it's going to be possible to see Titan transit Saturn on several occasions, including 'shadow transits', when the shadow of the moon will be cast onto the ringed planet from our perspective on Earth.
Titan is one of those bodies that has planetary scientists continuously amazed.
We think Titan may have fizzy lakes, that Titan has a sea level, and there are even questions as to whether life could exist on Titan.
And, NASA has announced it’s sending a drone-like spacecraft called Dragonfly to explore Titan.
This makes it all the more fascinating to watch Saturn’s largest moon pass across the planet, and you can see this for yourself over the coming months.
Saturn equinox and ring plane crossing
Saturn takes 29.4 years to orbit the Sun and has an axial tilt of 26.7 degrees.
The tilt direction remains the same relative to the Sun throughout the orbit.
When one pole points maximally towards the Sun, that’s when Saturn experiences a solstice: the time when the hemisphere containing that pole is experiences its summer, the other its winter.
In between these extremes, Saturn’s apparent tilt angle – the degree of tilt we actually see though the eyepiece - gradually decreases until the planet effectively appears sideways on to the Sun: a Saturnian equinox.
As Earth is fairly close to the Sun from Saturn’s perspective, a Saturnian equinox has the planet appearing edge-on to us as well, a time when it appears to undergo what’s known as a ring plane crossing.
A good time for Titan transits
The jostling positions of both Earth and Saturn means we get to see these events either singularly or as a triple, the latter being better placed for viewing from Earth.
On 23 March 2025, Saturn will undergo a single ring plane crossing event, and although this isn’t going to be well positioned for observing, the very fact it’s happening means we get to see some rather special and infrequent interactions between the planet and its larger moons, including atmosphere-enshrouded Titan.
Near a Saturnian equinox the rings appear very thin, disappearing from view for a short time as the ring plane crossing occurs.
Saturn’s major moons orbit more-or-less equatorially, which means that during an equinox, their orbits would appear as very narrow ellipses or, at the point of the ring plane crossing, a line.
This allows the moons to transit Saturn’s disc and opens up the possibility of spotting moon shadow transits.
We’ll be mentioning these infrequent events in forthcoming issues of BBC Sky at Night magazine and on this very website, concentrating on the rare and exciting possibility of spotting Titan and its shadow transit Saturn.
For more advice, read our guide on how to observe Saturn through a telescope.
Observing Titan transits
Observing a transit of Titan across Saturn can be frustrating.
Titan's orbital period of 15d22h54m means you get similar circumstances approximately every 16 days.
But if timings are off, where events occurs when Saturn is below the horizon or in daylight, you can go through a whole cycle of potential Titan transits without seeing any of them properly.
As Saturn’s inclination is now quite reduced, we’ll see Titan passing very close to the north and south limbs of the planet, a precursor to some excellent interactions between moon and planet later in 2024 and into next year, as Saturn approaches its next ring plane crossing in March 2025.
At the end of 2024, we start to see Titan’s shadow transiting the planet, two events visible in November (4 and 20 November) together with two in December (6 and 22 December).
But transits of Titan itself are poorly timed for the UK, many occurring in daylight in the first half of 2025.
There’s no need to worry though, because at the end of that year, despite being several months after the ring plane crossing in March, conditions repeat for Titan transits and there are several that are very well timed for the UK.
Upcoming Titan transits
Now is a great time to get acquainted with Saturn and its moons ahead of the exciting and fairly rare events to come.
These can be challenging and require a moderate-aperture scope combined with steady seeing.
From November 2024 onwards, Saturn’s tilt angle will be such that Titan’s shadow makes an appearance, crossing Saturn’s disc in dramatic fashion.
The first Titan shadow transit occurs on 4 November 2024, when just a small part of the shadow appears to scrape across the southern polar region of Saturn’s globe.
The next occurs 16 days later on 20 November, a more tangible event, with the moon’s shadow fully visible on Saturn.
These events can be seen through a small telescope, the 20 November one being a better option for such instruments.
We’d recommend at least 100mm of aperture and, of course, the larger the scope, the better the view.
Seeing will make a difference here too, and despite Saturn being able to achieve a peak altitude of 30° from the UK when it’s due south, the timing of the Titan shadow transits may not sync with this optimal altitude, leading to a lower-altitude, more unsteady view.
Imaging setups using high-frame-rate cameras and an atmospheric dispersion corrector should work well for this.
An atmospheric dispersion corrector is highly recommended for colour camera imaging, but can also provide benefits for mono cameras using imaging filters.
Fingers will be well and truly crossed for clear skies as these rare transits take place, but if the weather doesn’t cooperate, two more transits occur on 6 and 22 December.
Successive events will have it moving further north and more prominently on Saturn’s disc.
We’ll need clear skies for this to be observable but if that doesn’t happen, don’t worry, 2025 will hold a good number of additional chances.
Titan’s orbit becomes too tilted for these events to occur beyond Jan 2026 until near the next triple ring plane crossing events on 15 October 2038, 1 April 2039 and 9 July 2039.
With Saturn now getting higher in UK skies, this is a great time to observe this beautiful planet.
If you manage to observe or photograph Titan transiting Saturn, get in touch and let us know, or send us your images. Email contactus@skyatnightmagazine.com.
