On August 21, millions of Earthlings will gather to watch as a total solar eclipse sweeps across the centerline of the United States over the course of 90 minutes. For many, it will be once-in-a-life-time spectacle. Unfortunately, it won't be visible in Australia. But if you had a spacecraft on hand, you wouldn't need to wait decades for the next total solar eclipse to arrive at a town near you — you could simply jet off to Mars, Jupiter, Saturn or even Pluto. That's because there's a veritable zoo of solar eclipses occurring all across our solar system, all the time.
Illustration: Sam Wooley/Gizmodo
To be fair, none of these extraterrestrial eclipses is quite like the total solar eclipse here on Earth, where a quirk of celestial geometry causes the Moon to stack perfectly over the Sun, leaving a fiery ring of coronal jets to illuminate the sky. Some satellites, such as Mars' moon Phobos, are too small to engulf the Sun from the perspective of an observer on the planet, resulting in what astronomers call a transit. In other cases, such as that of Saturn's moon Titan, the angular size of the satellite in the sky is far greater than that of the Sun, resulting in a solar occultation. But that's just the basics — eclipses, it turns out, come in all shapes and sizes, and studying them can tell us a lot about our cosmic neighbourhood.
Tiny transits at Mars
For most other planets, we have to imagine what a solar eclipse would look like from the surface (or, in the case of a gas giant, the cloud-tops). But when it comes to Mars, we've actually seen quite a few.
Image: NASA/JPL-Caltech/Malin Space Science Systems/Texas A&M Univ.
In 2004, NASA's Spirit and Opportunity rovers captured six solar transits events from the surface of Mars — four involving the potato-shaped moon Phobos, another two starring the even runtier satellite Deimos. They were, according to a paper published in Nature the following year, the first direct images of satellites transiting the Sun from the surface of another planet. (Phobos eclipses had been observed indirectly even earlier, in data collected by NASA's Viking lander and Soviet-led missions to Mars.)
Transit of Deimos, captured by Curiosity on 18 September 2012. Because of its much higher orbit, Phobos appears much smaller relative to the disk of the Sun as compared with Curiosity. Image: NASA / JPL / MSSS / Emily Lakdawalla
According to Mark Lemmon, an astronomer at Texas A&M University and co-investigator on the Spirit and Opportunity rovers, early solar eclipse observations helped NASA refine the positions of Mars' moons in the sky. "Despite observing Phobos and Deimos for 120 years from Earth, they're small and far away," Lemmon told Gizmodo. Before NASA's rovers landed, "the uncertainty about where Phobos would be at any given time was about as big as Phobos." Once eclipse observations had improved the orbits of the two moons, the European Space Agency's Mars Express Orbiter was able to point its camera accurately enough to capture high-res images of both, Lemmon said.
Eclipse-watching on Mars has only gotten better, especially since NASA's Curiosity rover landed in Gale Crater in August 2012. Just look at this footage Curiosity captured in 2013, of 23km-wide Phobos racing across the Sun, transforming our beloved star into an eerie cyclops monster over the course of 32 seconds:
"I think that's the best shot of an eclipse [on Mars] we have," Lemmon said of the Curiosity video, adding that the rover has continued to make solar eclipse observations since, including the eclipse shown below, which occurred this past May. Even though Spirit and Opportunity refined the positions of Mars' moons more than a decade ago, there's still plenty we can learn by watching their transits.
"Even now, while there's no missing where Phobos and Deimos are, their orbits are changing all the time due to the push and pull of gravity," from Mars, Lemmon explained. "In particular, Phobos raises a tiny tide on Mars — a slight displacement of the rock surface, which in turn leads to a gravitational pull on Phobos, changing its orbit. That is why Phobos is spiralling in toward Mars and will eventually be destroyed."
Indeed, eclipses may be critical to figuring out how soon Phobos faces annihilation — and when Deimos will be cast away into deep space.
Hazy eclipses at Saturn
With 62 confirmed moons, Saturn's skies offer myriad eclipse-viewing opportunities, from tiny solar transits to massive occultations to moons stacked atop other moons. But of all the gas giant's many satellites, few produce an eclipse as otherworldly as Titan, a massive methane cauldron that challenges our understanding of the kinds of places life might emerge.
It was the Voyager 1 spacecraft that spotted the first solar occultation at Titan in 1980, according to a paper published in The Astrophysical Journal. As Titan swept across the Sun, Voyager captured some of the light that filtered through its hazy atmosphere, which scientists used to confirm that the moon's skies are composed mostly of nitrogen. Since NASA's Cassini probe arrived in orbit around Saturn in 2004, we've witnessed many other Titan eclipses, which we've used to probe the chemistry of the moon's thick haze.
Some of what we've learned even has implications for understanding planets beyond our solar system.
Titan's shadow on Saturn, as seen by Cassini in 2009. Image: NASA/JPL/Space Science Institute
In 2014, analysing visible and infrared spectra collected by Cassini during solar occultations, researchers demonstrated that Titan absorbs, refracts and scatters sunlight in ways that may obscure information about deeper parts of the atmosphere. This, the researchers wrote in a paper published in PNAS, could have ramifications for elucidating the atmospheres of exoplanets, particularly "super Earths". "Haze has a dramatic effect on the transit spectra," the researchers wrote, noting that it "substantially impacts the amount of information that can be gleaned". This information could prove incredibly useful when the James Webb Space Telescope starts peering into the atmospheres of distant planets over the next few years.
But as valuable at the science is, astronomers are mostly drawn to Saturnian eclipses because of their sheer beauty.
"In most cases, we imaged eclipses because they are just wondrous events, at Saturn as they are on Earth," Cassini imaging lead Carolyn Porco told Gizmodo. "It was part of my desire, from the very beginning of the [Cassini] mission, to turn our image-taking responsibilities at Saturn into a video documentary of everything there was to see there, including celestial motion."
The rare, 42-year eclipse at Uranus
Solar eclipses are a fairly common for Jupiter and Saturn, but not so for Uranus, a planet which, in flagrant defiance of celestial convention, circles our Sun tipped over on its side, its spin axis almost perfectly aligned with its orbital plane. Because of Uranus' funky tilt, its poles are alternately illuminated during its 84-year trip around the Sun. The moons, which circle Uranus in the band of rings stretched across the planet's equator, only align edge-on with the Sun ever 42 years, making solar eclipses a rare event for this lopsided Ice Giant.
Hubble Space Telescope view of Uranus, its rings, and several of its moons. Image: NASA/Hubble
Rare, but not impossible to catch. In 2006, just as Uranus was approaching its summer equinox, the Hubble Space Telescope caught a never-before-seen-glimpse of the moon Ariel traversing the face of the ice giant and casting a shadow, or umbra, on the planet's blue-green cloud tops. From the "surface" of Uranus, it would have looked like a solar eclipse.
Image: NASA, ESA, L. Sromovsky (University of Wisconsin, Madison), H. Hammel (Space Science Institute), and K. Rages (SETI)
"These observations were planned only to study the atmosphere of Uranus — the detection of Ariel and its shadow were purely serendipitous," Heidi Hammel, Executive Vice President of AURA, who helped analyse the image while working at the Space Science Institute, told Gizmodo. "This moon shadow image is more a beauty shot than a science result."
Lawrence Sromovsky, astronomer at the University of Wisconsin-Madison who also helped analyse the image, noted that Ariel's shadow creates a region of totality about the same size as the moon itself — a very different situation from what we see during an eclipse on Earth, where the area of total eclipse is fairly small, and surrounded by a much larger region of partial eclipse. This, he explained, is due to the fact that at Uranus, Ariel is roughly ten times bigger in the sky than the distant Sun.
As Uranus continued to approach the summer equinox, there were other eclipses of other large moons, including Umbriel, Titania and Oberon. But Ariel's eclipse was the money shot, and it's likely to stay that way for a while — the next equinox at Uranus won't be until 2049. Maybe, if we're lucky, the weird world will have had another visitor by then.
Years of solar eclipse at Pluto
Pluto may seem like a cosmic castaway, taking 248 years to complete a single orbit some 3.7 billion miles from the Sun. Thankfully, it's got Charon, a moon approximately half its size with which it's locked in an endless gravitational embrace. And because Charon is so close — just 19,570km from Pluto — when eclipse season hits, eclipses happen a lot.
"When Pluto's in eclipse season, twice during its 248 year orbit, because Pluto and Charon essentially orbit around each other every six days, you get an eclipse every six days," Planetary Society astronomer Bruce Betts told Gizmodo. "Of course, they're tidally locked," meaning the two bodies never rotate with respect to one another, "so you'd have to be on the Charon-facing side to see it." But if you were on the right side, you'd see solar eclipses quite regularly, and for years on end.
From 1985-1990, Pluto and Charon were constantly eclipsing each other. Astronomers used these events to determine the size and surface brightness of the two objects. Image: NASA
While regular eclipses would make Pluto terrible for solar farming, they have proven damn useful for astronomers. Before the New Horizons spacecraft zipped past Pluto in 2015, some of our best intel on the dwarf planet came from Charon's incessant photobombing.
"The last eclipse season was not that long ago, and astronomers used the event — Charon in front of Pluto, Pluto in front of Charon — to resolve the two bodies," Betts said. "They got a better idea of their size, brightness and colour."
Indeed, the mutual eclipses of Pluto and Charon astronomers observed between 1985 and 1990 led to some of the very first "maps" of Pluto, including the image below, which shows a bright south polar region we now know to be a mixture of nitrogen and methane ice.
Pluto, circa 1990. Image: NASA
For comparison, this is what Pluto looks like today, two years after the New Horizons flyby.
Image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Center/Chandra X-Ray Center
"The whole thing is pretty amazing," Betts said, noting that the sunlight is so weak at Pluto astronomers had struggled to get any resolution at all on the icy world. "But the fact that it was so hard to study Pluto is what drove them to do it."
There are many other incredible eclipses around the solar system we didn't get into here — take Jupiter, whose four largest moons line up between the Sun and the gas giant with every single rotation due to the planet's lack of axial tilt. "It never stops having eclipses," Betts said. Or Neptune's mysterious moon Despina, which a citizen scientist discovered transiting the sun when he re-analysed old data from the Voyager 2 flyby in 2009.
Then, there's the eclipse here on Earth. "In terms of eclipses across Solar System, ours is pretty special," Noah Petro, a planetary geologist at NASA's Goddard Spaceflight Center, told Gizmodo. "We have this configuration where the Moon perfectly [overlaps] the Sun, so that we can see the solar atmosphere. And it's not only the size of our Moon, but the Moon at this point in time — in early lunar history, the Moon was too close."
So, as you're making your travel plans to hit the centerline of the eclipse on August 21, enjoy the fact that what you're about to see is special. But it's also more than an Earthly phenomena. It's a connection to a far grander celestial choreography that promises to dazzle observers for aeons to come.