Extra-Planetary Eclipses and Transits

Our universe is a beautiful collection of celestial bodies revolving and rotating around the sun. Earth’s eclipses are best seen from earth, but who would think that other planets have eclipses as well? Every other planet in the solar system has natural satellites (except for Mercury and Venus—they grant us the favor of transits, though!) Why not their moons eclipse as well? Well, if our moon eclipses, then it’s true that other planet’s moons eclipse as well. Called extra-planetary eclipses, other planet’s moons eclipse wherein some planets with many moons do quite often. Take Mars for an example, March 5th 2004 was a historic day for NASA when they released the first pictures ever taken of an eclipse as seen from the surface of another planet. The pictures were taken by the Opportunity land-rover a day earlier. The pictures showed the moon Deimos passing like a tiny speck over the sun's disk. Actually, this is considered a transit, but it can also be referred to as an eclipse. Mars’ moons are way too small to create totality of darkness on the planet, unlike our moon (which was perfectly designed to eclipse perfectly!) The same goes for Jupiter and Saturn. They are so big that their tiny moons can only cast small shadows that look like specks of dust on the planet’s surface.

Phobos Transit as seen by Opportunity

But concentrating on Mars, Cornell University astronomer Jim Bell said “While Deimos would produce just a small speck traveling across the surface of the sun, we expect Phobos, because it's much closer to the surface (of Mars), to blot out half or more of the sun, it's a very quick event—it happens in 30 or 40 seconds or so.” It’s true that eclipses occur much more often on Mars than earth, but nothing compares to our moon eclipsing. Because Phobos, one of Mars’ moons, is only approximately twelve by fourteen miles in apparent size and orbits very swiftly, eclipses of Phobos would only last about thirty seconds because it takes it seven hours and thirty-nine minutes to orbit the planet. If the Martian day is twenty-four hours and thirty-seven minutes long, Phobos can eclipse twice daily! Annular eclipses are its only option because like annular eclipses of our moon, Phobos can never cover the entire Sun, but can fit inside it. Deimos, though, is considered not to eclipse, but to transit. Because it is five by six miles in apparent size, it is only viewed as a small object moving across the sun’s surface; this could be noted as an eclipse, but under more technical terms, it’s a transit.

 According to Jan Curtis, in his Predictable Astronomical Events, “Io (I), Europa (II), Ganymede (III), and Callisto (IV) act as a mini solar system. The moons’ commensurabilities with one another cause them to orbit with near perfect resonance. This means that these four giants cannot eclipse, occult, or be at greatest elongation with Jupiter all at the same time, although combinations of these phenomena occur.” Jupiter perhaps gives us a whole other solar system! With sixty-three moons and counting, Jupiter and its moons eclipse singularly (one moon at a time) non-stop—note that dual eclipses occur. Io, named from Ovid’s Metamorphoses—a fair virgin whom Jupiter (king of the Roman gods) fell in love with, even though he was already ‘married,’ has an orbital period of 1.769 days. This means every two days it will transit across the surface of Jupiter. Often eclipsing, Io is very ‘busy’ because it spends 10.8% of its orbital period transiting, eclipsing, and occulting daily! Europa, also named (as well as the rest of the Galilean moons) from Ovid’s Metamorphoses, who Jupiter falls in love with, has an orbital period of 3.551 days and spends 6.75% of its orbital period transiting, etc. Then Ganymede, with an orbital period of 7.155 days and spending 4.2% of its orbit ‘busy,’ and Callisto, having an orbital period of 16.689 days and spending only 2.5% ‘busy’ both transit, etc. but, not as often because their orbits are more elongated, then those of Io and Europa.

Eclipses of the Galilean moons are perhaps the most spectacular. Although you can view Jupiter clearly with the naked eye, but viewing the Galilean moons requires more equipment, such as a small telescope or even a good pair of binoculars. “...Strung along Jupiter’s waistline,” The Rough Guide to the Universe describes these moons so vividly. But, how do these moons play their role in the universe? Discovered between 1610 and 1612, Galileo first observed four moons of Jupiter. After observing them for a while, he was soon able to create tables and predictions of the movements of these satellites that were actually correct. But later in the 1600s, Giovanni Domenico Cassini (the satellite ‘Cassini’ that went to explore Saturn in 1997 was named after him), also contributed works on Jupiter’s moons. Using his charts, he determined Io’s correct longitude; and as measured today, he was just off one degree. So, these four moons (Io, Europa, Ganymede, and Callisto) have an incredible history, let alone they have been orbiting throughout the ages.

More on eclipses, what actually is a Jovian eclipse? Although much different than an eclipse of our moon, Calsky (a website that calculates astronomical events) defines a Jovian eclipse as this: ‘if a moon enters the shadow cone of Jupiter, it gets eclipsed and disappears pretty fast.’ So, whenever a Jovian moon (or a Galilean subset—Galilean moons are a subset of Jovian moons; Jovian moons include all moons of Jupiter—all sixty-three) enters Jupiter’s shadow cone, it will be eclipsed. Eclipses can be partial, in where only a portion of the moon is eclipses, or total, where the whole moon is eclipsed. One such example of series (yes, Jovian eclipses follow their own ‘Saros’ too!) of Jovian eclipses occurred in 1999. Jupiter and Saturn returned to the sky in autumn of that year; Jupiter was at its best to view in a long time. Jupiter and its moons were easily observed with a small telescope, and two of three phenomena could easily be seen. Transits and occultations were easy to see, but not so much eclipses. That’s what makes eclipses so amazing. Capitol Skies (the newsletter of the Madison Astronomical Society) tells us this about Jovian eclipses: “Jupiter provides a virtual smorgasbord of events to watch. Of the three events, eclipses are by far the most scientifically interesting, because the timing of the precise disappearances and reappearances of the moons can be used to calculate orbital elements of the moons. For this reason, precise timings of the Galilean moon eclipses are sought by the Association of Lunar and Planetary Observers (ALPO).”

Just like eclipses here on earth, precise timings are everything; not as much location. If you can see Jupiter, then you can see an eclipse. Because its shadow cast is very magnificent, when a Galilean moon passes behind it, it disappears completely from view, but has not been occulted. As well as that, because the shadow cast is so steep, the Galilean moons may be viewed entering and exiting the shadow on the same side of the planet! But the closer Jupiter is to opposition (when Jupiter is opposite from the Sun in the sky), the closer these eclipses happen to the disk of Jupiter, making them harder to view. Depending on Jupiter’s position relative to the earth and to the Sun, the eclipses may be viewed easily and is best viewed when Jupiter is near quadrature. Quadrature is simply being ninety degrees west or east from the Sun; away from ‘hard-to-see’ opposition.

Saturn, Uranus, and Neptune have moons that eclipse as well, but aren’t widely as known as Jovian eclipses. Pluto, on the other hand, has three moons that eclipse. It’s rare for a dwarf planet to have moons, but Pluto does; as also does Eris. Eris is a dwarf planet far beyond Pluto (ninth largest body to orbit the Sun), and has one moon: Dysnomia, which orbits every 15.8 days.  Nothing is clearly known whether its moon eclipses, because it’s just too far away to detect. More than likely, it does, along with other events! But, Pluto and its three moons (Charon, Nix, and Hydra) have another story to tell. February 1985 and October 1990 were the years in which Pluto experienced solar eclipses that lasted for hours; but on average ninety minutes. In order to view this event, you had to be on Pluto, and I doubt anyone was there.

Basically, solar eclipses on Pluto are caused by Charon (which is big enough to make Pluto experience totality—Charon is more than one-half the size of the dwarf planet!), passing in front of the Sun, blocking its light. Of course Hydra and Nix can eclipse as well, but they are much smaller, so Pluto would not experience totality on its surface; it’s more of a transit. But, Nix and Hydra did not eclipse then, because they weren’t even discovered until 2005! An eclipse of this sort can only occur when one of the satellites’ orbital nodes are lined up with the apparent position of the Sun, as seen from Pluto. Because of the fact that all three of its satellites orbit in the same plane, the times at which this is possible are the same for all three. Pluto has a steep axial tilt, so eclipses can only occur around the dwarf planet’s perihelion and aphelion (the closest and farthest point of a planet’s orbit).

It’s easy for Charon to eclipse, but not Hydra and Nix. We know that they are smaller than Charon, but we don’t know how big the really are. That’s when eclipses helped. During that period between 1985 to 1990, the solar eclipses that Charon and Pluto’s other moons made, helped determine their actual sizes. That was a great step in research about Pluto, because no man-made satellite has ever been there before. NASA sent out New Horizon’s mission which is expected to reach and explore Pluto in 2015. That will definitely be a great accomplishment! Eclipses were able to be viewed every day, and while Pluto experienced a solar eclipse, earth was able to view a transit of Charon across Pluto’s surface. During the start of this five year period, the eclipses began blocking the North Polar Region of the dwarf planet, but as the year progressed, the equator was blocked. By means of carefully measuring the brightness over time, it was possible to determine surface features. Pluto has a highly reflective south polar cap, a dimmer north cap, and dark features around its equator. All these were discovered by the help of a solar eclipse—a great step in research!

When the eclipses began, in order to measure the diameters of the moons, apparent diameters cast on the dwarf planet were taken, and the Hubble Space Telescope helped by taking pictures and sending them back to earth for studying purposes. Hubble’s images showed that Pluto’s diameter is 1413 miles wide; Charon’s is 728 miles wide. Although Nix and Hydra weren’t measured, we now have the correct diameters of these bodies in which we knew nothing about previously! The next eclipses will occur in October 2103, so none of this generation will be able to view them. They peak in 2110, and finish in January of 2117. Much more will be learned about this distant planet during those eclipses as well. Maybe Nix and Hydra will choose to eclipse next time.

As magnificent as transits of Mercury and Venus are, there are those that are more magnificent. How would you like to observe earth transit? Think about this for a minute, other planets can transit the Sun; but, we can’t see them. We can’t see Uranus transit, but it does as viewed from Neptune. Therefore, Neptune can transit as not viewed from Uranus, but from Pluto. Ranging from mutual planetary transits, solar-system planetary transits, simultaneous transits, extra-lunar transits, and so forth, much more occurs than what the average person knows. Mars might be transiting the face of Neptune as viewed from Mercury right now for all I know! A planet, the Sun, and another planet are all that it takes to create a special transit.

But before we discover true rarities, exo-lunar transits must be made known first. An extra-lunar transit simply put us a transit of another planet’s (other than earths) moon across the surface of it. As mentioned countless times already, the Jovian moons put on quite a show daily. One moon will either eclipse, occult, transit, or do two or all three in a single day. Io does frequently transit, as well as Europa, Ganymede, and Callisto. So does Saturn’s moons. On February 24th 2009, something happened so rare; it will probably never repeat itself again. “On Feb. 24th, there's going to be a quadruple transit of Saturn's moons,” says Keith Noll of the Hubble Space Telescope Science Institute. “Titan, Mimas, Dione and Enceladus will pass directly in front of Saturn and we'll see their silhouettes crossing Saturn's cloud tops—all four at the same time.” Sure enough, all four moons cast shadows as they transited as well. At the end of that magnificent event, each moon and shadow had been on the surface of Saturn, a true gem! Another event happened on Jupiter, but earlier. March 28th 2004 hosted a triple-transit of Io, Ganymede and Callisto all traveling across Jupiter’s disk. They also cast beautiful shadows that made this event rare as well. Something even more peculiar occurred to Saturn. Dione transited Titan, as seen by the Cassini probe; and in the background, Prometheus became occulted by the rings of Saturn. It’s not that common moon will transit another moon (exception—Saturn’s Enceladus transited Titan on February 5th 2006) and rings occult another. Not much like that happens every day!

Jovian dual transits are another wondrous thing. Although occurring in periods just about twice a year, two Jovian moons will line up with each other and transit the surface. Although three have occurred in 2011 already, the next ‘season’ starts May 7th 2011. Io will either transit with Europa or Ganymede; but more frequently Europa. Dual shadow transits, dual-transits just with both the moon’s shadows on Jupiter’s disk, instead of one, start on May 14th 2011 and run until June 26th 2011, while dual-transits end on June 15th. This will be a great time to photograph and record information.

Rare triple-transit on Jupiter

[This is a special interest story, written by Matthew Winter from his research paper entitled Astronomical Events: Eclipses, Transits, Occultations, and Conjunctions.]