See Saturn’s largest moon Titan and its shadow pass in front of the ringed planet.
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?
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.
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.
Titan transit, 1 August 2024
To give you a tricky taster of what’s to come, there’s a transit of Titan visible on 1 August 2024.
It’s far from ideal, occurring as the Sun is rising, but it does give you the opportunity to locate and keep Saturn in the eyepiece of a telescope on a tracking mount.
This way, you can experience what the planet looks like under bright sky conditions and whether or not you can see Titan under such circumstances.
The 1 August 2024 Titan transit begins at 05:44 BST (04:44 UT), the rising Sun being 1 degree above the horizon at this time.
Saturn will be 26 degrees up above the south-southwest horizon at the start of the transit, subsequently leaving the disc at 09:09 BST (08:09 UT) when Saturn will only be at 2 degrees above the western horizon.
As a result, it’s the initial part of the transit that gives the best opportunity.
Get Saturn in the eyepiece around 05:00 BST (04:00 UT), when it should be easily visible in the brightening dawn twilight, around 30-degrees up almost due south.
Titan will be easy to see at this time, the bright moon sitting just off the planet’s southeast limb. Select a low- to medium-power eyepiece.
Too high a magnification will lower contrast and make life very difficult as the sky begins to get brighter.
Then, just stay with the planet, watching Titan get ever closer to its edge until at 05:44 BST (04:44 UT) the transit begins.
As ever, when observing a daylight event through a telescope, you must be careful not to catch a glimpse of the Sun through the eyepiece, as doing so could damage your eyesight.
If you’re not sure what you’re doing, do not attempt it.
If you’re an imager, one way to assist the view is to use a red or infra-red pass filter together with an infra-red sensitive camera.
This can be used to darken the blue sky and restore a degree of contrast for a better and clearer result.
Future Titan transits
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.