Once Every 15 Years: Don’t Miss the Rare Events on Saturn!

There is something special brewing in Saturn’s large family. Soon we will be able to witness the shadow of its largest satellite, Titan, make its way across the planet’s globe. Why are these events so rare in our lifetimes? How do you make sure you won’t miss this unique event? Let’s delve deeper in Saturn’s secrets in all their detail.

Saturn Is a Unique Planet

Though now we know that all gas giants – Jupiter, Saturn, Uranus, and Neptune – have their own ring systems, it’s only the rings of Saturn that are accessible for small amateur telescopes. The honor of discovering Jupiter’s rings in 1979 goes to Voyager-1; the rings of Uranus revealed themselves in 1977 to American astronomer James Elliot when they briefly outshone the surrounding stars; in 1984, the rings of Neptune were discovered in the similar fashion by groups of astronomers led by André Brahic and William B. Hubbard.

Meanwhile, Saturn’s rings were first found all the way back in 1610 by Italian polymath Galileo and his small 20x telescope. Although at the time, he had no idea what he stumbled upon, let alone imagine that scientists of the 21st century would still be pondering the mysteries of his unexpected discovery.

Modern scientists use the preprint service arxiv.org for more expedient publications. But in the 17th century, long before the introduction of the Internet, researchers faced a trickier path: they encrypted their results through anagrams to announce their findings before the official publication. For instance, Galileo described the odd shape of Saturn with the anagram “smaismrmilmepoetaleumibunenugttauiras” which deciphers as “Altissimum planetam tergeminum observavi” meaning, “I have observed the highest of the planets three-formed”. But why was Saturn the highest? Well, in 1610, Uranus and Neptune were still undiscovered, and Saturn was indeed the highest, or furthest, known planet in the Solar System.

Due to the small size and relatively crude optics of his modest instrument, Galileo described Saturn as having “ears” and was unable to find at least a somewhat rational explanation for their nature. It took another 49 years for the correct description of Saturn’s rings to come along by the efforts of Christiaan Huygens. And only in 1959, James Clerk Maxwell finally proved that the rings are not monolithic but rather consist of small individual objects.

Rings and Satellites

Considering that Saturn’s rings are formed by variable objects whose size ranges from dust specks to boulders, it begs the question: which of these objects should then be classified as satellites? It would hardly make sense to assign this label to every tiny rock orbiting the planet.

The current official number for Saturn’s satellites is a whopping 146 objects ranging between 2 and 3 kilometers, with the latest of them being confirmed only last year. Some of them carved space for themselves inside the rings, while others orbit far beyond their margins. The International Astronomical Union allows to classify objects larger than 1 kilometer as satellites, meaning that Saturn may have hundreds more of them, inside the rings and beyond them. Nearly all large satellites orbit Saturn almost in the same plane as the rings, as well as the planet’s equator.

Consequent annual observation of this cosmic ‘Lord of the Rings’ shows that the planet undergoes some changes. The first thing that begs attention is the angle of inclination of its rings, or as the astronomers say, their unfolding. The secret behind this change is very simple. Saturn’s axis is inclined towards its orbital plane at the angle of 26.7°, retaining its relative position to distant stars, sort of like a gyroscope.

However, throughout its solar year, which spans about 29.5 Earth years, Saturn changes position relative to the central part of the Solar system. And similar to Earth, Saturn also experiences equinoxes and solstices. As Saturn’s year progresses, during its equinox, its rings incline to us edgewise twice; during Saturn’s solstice, the rings unfold twice completely as seen from either Southern or Northern Hemisphere on Earth. Consequently, about every 15 years, the gas giant presents us with all its naked glory as it sheds its famous tutu of rings. 

It shouldn’t come as a surprise then that its rings are incredibly thin, astronomically speaking. Their thickness ranges between 5 and 30 meters, so when they are inclined to use edgewise, we are unable to see them at all. This is what baffled Galileo back in 1612 who compared the planet to the mythical giant Saturn, infamous for devouring his children in an attempt to defy his prophesized downfall at their hands. Could you imagine Galileo’s surprise when just the following year in 1613 he witnessed the return of Saturn as it appeared three-formed! 

At the moment, Saturn’s rings are gradually folding and will completely vanish on March 23, 2025. Unfortunately, we won’t be able to witness this display as the planet will be nearing its solar conjunction set to occur on March 12, so it will completely hinder any observations for the upcoming future.

At the end of spring and throughout the summer, the rings will slowly unfold, but then, in November and December, they will stretch into a thin line once more! This visible fluctuation is caused by the 2.5° inclination of Saturn’s orbit relative to Earth’s elliptic plane. As a result, we observe small annual changes in Saturn’s skirts, as they sway slightly to the south and then to the north.

It’s only fitting that the most remarkable planet in the solar system gets a similarly remarkable satellite — Titan. Firstly, Titan is not only Saturn’s largest satellite but also the second largest and most massive satellite in the solar system. With a diameter of 5150 kilometers, it’s almost 1.5 that of the size of our Moon and even larger than Mercury. Secondly, its mass is estimated at about 96% of the total mass of all orbital objects of Saturn, including satellites and rings. And lastly, Titan is the only satellite in the solar system to possess a dense atmosphere and liquid on its surface, albeit these oceans are not made up of water but rather methane and ethane.

But Titan isn’t the only nightly gift for observers. In general, small amateur telescopes are able to pick up 5 of Saturn’s satellites: aside from Titan, there are Tethys, Dione, Rhea, and Iapetus. The last four are rather small, ranging between 1060 and 1530 kilometers, so they appear as tiny stars lacking any clear spherical shape. Saturn is currently 9 astronomical units away from us, and even Rhea, the largest of the four satellites, measures only 0.2 arc seconds. By contrast, Titan measures almost 1 arc second, so it’s possible to discern its tiny disc with medium amateur telescopes, provided the sky is clear.

Titan’s Attempts to Overshadow Saturn

Even though the most remarkable planet will lose some of its visual appeal this year due to the folding of its rings, we might get a consolation treat of sorts by way of a rather unique event – Titan’s shadow crawling over the planet’s surface. In astronomy, these events are called transits.

In reality, any other relatively sizable nearby satellite, such as Dione, Tethys, Mimas and Enceladus, regularly cast their shadows upon Saturn’s surface. But all these objects are too small to catch through amateur telescopes. Only the most skilful astrophotographers are able to immortalize these moments with the help of large instruments as well as special photography and processing techniques.

Titan orbits Saturn at a distance of a whopping 1.2 million kilometers from the planet’s center, moving almost in the same plane as Saturn’s rings. This is why the satellite’s shadow only falls upon the planet’s surface when the rings are inclined edgewise, just like now. As you already know, the rings shift into this position only every 15 years, meaning the window of opportunity to witness Titan’s shadow transits occurs with the same frequency. Though we can only glimpse the shadow in question when the satellite passes between the Sun and Saturn. We have already covered similar events in the Saturn system. But Titan’s shadow is indeed unique, and shows itself very, very rarely.

This unique giant of a satellite needs 15.945 Earth days to complete the full orbit around its parent. When the rings fold for long enough, for the duration of about a year, Titan completes quite a number of orbits around Saturn, hence why transits occur in a series. On the other hand, the fact that Titan’s orbital period spans a handful of Earth days means that these events occur almost at the same time. This, in turn, makes them inaccessible for some Earthlings: during Titan’s transits, Saturn will be either hiding below the horizon or appear in daylight sky.

Finally, for our main course. When can we see Titan’s transit over Saturn’s surface? Below are the dates for this season’s transits visible in Ukraine and rest of Europe and given in the yyyy/mm/dd format with UT times from ingress to egress. The most favorable conditions for Ukraine and rest of Europe are marked with 🔭. 

2024/11/04: 21:08 UT to 22:51 UT

2024/11/20: 19:44 to 22:54 🔭

2024/12/06: 18:32 to 22:30 🔭

2024/12/22: 17:30 to 22:04 🔭

2025/01/07: 16:30 to 21:33 🔭

2025/01/23: 15:31 to 20:58 🔭

2025/02/08: 14:37 to 20:22 🔭

2025/02/24: 13:44 to 19:44

Keep in mind that we won’t be able to see the transit’s egress for any of these events as Saturn will have set below our horizon by that point. Transit’s ingress is also a fairly difficult thing to catch, so it’s best to start preparing for the hunt 30 to 40 minutes after the ingress time, when Titan’s shadow will have already begun its journey across the planet’s surface.

The first transit on November 4 is tangential, meaning that the shadow won’t fully fall on the planet but only partially traverse it, so this event is not very suitable for observation. The last event in the series is similarly unfavorable, with Saturn moving too low across the western sky, dipping below the horizon soon after sunset.

It’s also important to note that Saturn is gradually moving away from us during this series of transits, so its visibility is shrinking in size, which in turn reduces the size of both Titan and its shadow. So you won’t get a better opportunity to observe this event after 2024.

Observation, Observation, Observation…

All of this leads to one crucial question: what is the best observational instrument for this rare event? Unfortunately, small amateur telescopes and binoculars are not up to par for this task. The shadow’s angular size doesn’t exceed 1″, which budget telescopes can’t catch. Another issue is our planet’s atmospheric turbulence that blurs the image, reducing the quality of detail. So if you want to observe Titan’s shadow transit over Saturn, make sure to stock up on plenty of patience and look for the calmest possible atmospheric conditions. 

One of the key features to consider when choosing a telescope is its aperture, or the diameter of its main mirror or lens, since it defines the instrument’s ability to pick up small details on another planet. Large aperture means greater resolution, which is a diffraction limit determined by the wave nature of light. In this case, an instrument’s aperture should be at least 120-150 mm, though a smaller telescope could also prove to be useful, provided the atmosphere is clear and stable.

So is it all as dire as that without a telescope? Of course not! First of all, you could always turn to your local observatory, if there is one near you. Another option is to form a group with people you might know that have a telescope with the right aperture. In Ukraine, there are many astronomy enthusiasts in urban and rural areas that would always welcome other space lovers among their midst. And then there is also the more desperate option — to buy your own telescope. After all, why not use this opportunity to get this dream item off your wishlist? 😉