In the early morning hours of Tuesday, August 19, Saturn’s largest moon, Titan, passed between the Sun and Saturn, casting an inky black shadow on Saturn’s cloud tops. Dyer Observatory’s Seyfert Telescope caught the rare solar eclipse (featured above) from 800 million miles away.
How Rare Are Titan Solar Eclipses?
Titan’s shadow on Saturn is considered a rare occurrence for a couple of reasons. First, a Titan eclipse “season” on Saturn (the time interval during which Titan, the Sun, and Saturn are aligned well enough so that eclipses are possible) occurs every about every 14 years, roughly half the 29.4-year orbital period of Saturn around the Sun. On Earth, we experience eclipse seasons about every 5.5 months, half of the orbital period of Earth. In order to have a solar eclipse on a planet, the Sun, the planet, and its moon have to be in a roughly straight line. Because Earth’s Moon has an orbit around Earth that is tilted a little more than five degrees to Earth’s orbit around the Sun, we can only have a solar eclipse when the intersection of these two orbits (the “node”) lies between Earth and the Sun, allowing the Moon can come in between us. For most of the year, the orbit of the Moon takes it above or below the Sun in our sky so that its shadow passes above or below the Earth. If the Moon’s orbit was positioned in the same plane as Earth’s orbit, we would have a solar eclipse every month at the new moon phase. The same thing happens with Titan and Saturn, just over longer periods of time. Titan’s orbit is almost in line with the Saturn’s equator and ring system, but Saturn itself is tilted about 26.7 degrees to its orbital plane (very similar to Earth’s tile of 23.5 degrees).
Another factor contributing to the rarity is that even though Titan eclipse seasons last around 11 months, the eclipses themselves only happen about every 16 days (the orbital period of Titan), so there are only about 20 occurrences every 14-15 years. At some point during the eclipse season, Saturn also typically reaches solar conjunction, the point where it is on the opposite of the Sun from us and appears near the Sun in our sky. As such, there is a few-month period where our Sun makes Saturn unobservable. The first shadow transit of the 2024-2025 eclipse seasonjust skimmed the bottom of Saturn on November 4 last year and favored locations east of the U.S. By February, Saturn had drawn close enough to the Sun to make it difficult for Earthlings to observe it. It would be about April before Saturn reappeared in the early morning sky ahead of the Sun that we’d have a chance at catching Titan shadow transits. In addition, as the shadow only takes a few hours at most to traverse the planet, an eclipse will only be visible to a small part of Earth that happens to be facing it (and if the sky is dark enough to see Saturn). In the end, you might have only a handful of opportunities to see an eclipse based on your position and where the Earth and Saturn are with respect to the Sun. Oh, and there is also a good chance that some of these opportunities will be ruined by clouds.
A Moon Like No Other
Discovered in 1655 by Dutch astronomer Christiaan Huygens, Titan is an outlier in the solar system. It is the only moon of any of the planets to have a substantial atmosphere, which is mostly composed of nitrogen but also contains contributions from methane and hydrogen. With an atmosphere denser than Earth’s (exerting about 1.5 times the atmospheric pressure on the surface) and a surface temperature around -180°C (-292°F), methane and ethane condense into pools that form large lakes on the rocky surface. In fact, the methane can condense, rain out on the surface, and evaporate to create a methane cycle very similar to Earth’s hydrologic cycle. The atmosphere also contains clouds and a thick haze layer that prevent direct visible light imaging of its surface from space; however, the surface has been mapped via radar and infrared observations. On January 14, 2005, the Huygens lander reached the moon’s surface after being released by the Cassini probe that orbited the Saturnian system from 2004 until 2017. During its descent, the probe made direct measurements of the atmospheric conditions (composition, temperature, pressure) and snapped images as the surface came into view. And due to the presence of the atmosphere, it also provided us our first and only sounds of the Titanic wind during its three hours of data recording and transmission.
As Saturn is nearly 10 times farther from the Sun as compared to Earth, the Sun appears 10 times smaller in the Saturnian sky and a hundred times dimmer. Still, even at this distance, the Sun would be a blindingly bright spot in the sky. Titan is the second largest moon in the solar system after Jupiter’s Ganymede, making it about 50% larger in diameter than our own Moon and slightly larger than planet Mercury; however, from Saturn it would appear only half the diameter of our Moon in Earth’s sky because it orbits about three times farther out. Larger orbits result in moons taking longer to orbit their planet, but Titan only takes about 16 days to make a trip around Saturn due to the immense gravitational pull of the gas giant’s 95 Earth-masses of gas.
If you could stand on Titan, not freeze to death, and somehow part the thick atmospheric clouds and haze to see the starry night sky, Saturn would be quite a spectacle. The globe of the planet would appear nearly six degrees wide, about 12 times as large as a full Moon. Since Titan, most of Saturn’s large moons (excluding Iapetus), and the rings have orbits that keep them very near the plane of Saturn’s equator, the rings would not be very impressive though they can cast a noticeable shadow on Saturn depending where Saturn is in its orbit. But, as Titan’s shadow traversed Saturn over the course of just a few hours, it would be easily visible on the planet as a slightly fuzzy, dark circle about half the size of the our full Moon. If you had binoculars, you might even pick up the shadows of some of the other, much smaller moons if they happen to be passing by. One other detail – you would need to make sure you were on the correct side of Titan. Like many of the large moons of the solar system’s planets, including our own moon, Titan is tidally locked to its planet so that it always keeps one side facing Saturn.
Upcoming Eclipses
Sadly, the 2024-2025 Titan eclipse season is drawing to a close. Two well-placed shadow transits are still to come in the early morning hours of September 4th and September 20th. Keep in mind that you do not need a tremendously large telescope to observe this – a small backyard telescope with decent magnification should show Titan’s shadow especially during moments where the atmosphere calms to provide brief moments of clarity. A final transit on October 6th is not nearly as favorable as only about half of Titan’s shadow skims the very top of Saturn’s globe. After that, the next shadow transit partially skims Saturn on July 29, 2038, occurring during daylight hours in the Americas but in the evening hours for areas such as Europe and Africa. The timing of the transits in 2038-2039 will not favor the Americas but will be good for locations on the other continents. We’ve had a good run this eclipse season here in the U.S, but we have to share Saturn. Happy observing!