Sunday, January 22, 2012

Understanding Jovian Eclipses

[This article concerns Jovian eclipses, not transits. Although a Jovian eclipse can mean the same thing as a transit in this sense, this article talks about the disappearance and reappearance of the Jovian moons, not the transit of Jupiter’s surface.]

Considered the most intriguing and fascinating astronomical phenomena yet, the eclipse is highly acclaimed not just here on earth, but also in the rest of our solar-system. When a celestial body passes into the shadow of another body, preferably larger; or an object casts a shadow upon another, an eclipse is formed. From there, the term ‘eclipse’ can be forwarded into two separate definitions: the solar eclipse, or the lunar eclipse. When the Moon passes in front of the Sun, respectively on the ecliptic, a shadow is cast upon the earth; a solar eclipse is formed. When the Moon passes into either the umbral or penumbral shadow of the earth, again respectively on the ecliptic; a lunar eclipse occurs. Although these definitions discreetly define actions in a terrestrial sense, these same exact phenomena occur on the other seven planets in our vast solar-system, but with another name.

When an object passes into the shadow cone of Jupiter, it disappears from view (note the striped moon to represent being eclipsed instead of being solid colored, not eclipsed)
The Jovian Eclipse is perhaps one of the most unique extraterrestrial happenings, for it involves one of the many Jovian moons, mainly the Galilean moons, Jupiter, and the Sun; rather just the Moon and earth. Because a solar eclipse and a lunar eclipse have terrestrial definitions and terms, therefore, a Jovian Eclipse is defined as “the passing of a Jovian moon into the shadow-cone of Jupiter where it is eclipsed out of sight, until reappearing.” These eclipses can be total, in which the eclipsed moon is eclipsed indefinitely; or partial, where the eclipsed moon is partially eclipsed. But, you must make note that the Galilean moons (the four most famous moons, which we do know to eclipse) have very little orbital inclinations. With Io, having an inclination of 0.050º, Europa with 0.471º, Ganymede with 0.204º, and Callisto with 0.205º, it is doubted that partial eclipses can occur, because each of the moons’ orbits are so perfectly aligned with Jupiter’s equator; therefore, no partial eclipses. Aside from the types of Jovian eclipses that occur, series of eclipses take place, nevertheless imitating ‘Saros’ here on the earth. Because the moons that do eclipse are rather diminutive compared to Jupiter itself, an observer on Jupiter would not notice much, so a Jovian observer would consider a Jovian eclipse as a phenomena of theoretical interest, or something unimportant to note. Nevertheless, the Jovian moons’ ‘Saros’ is pure academic interest as well.

Seeing a Galilean moon eclipse is an enchanting experience you won’t forget. But, certain requirements are needed for you to observe one; it’s hard for Jupiter to produce such key-viewing times, like the one 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 for eclipses. That’s what makes eclipses so amazing,” Matthew Winter writes from Astronomical Events: Eclipses, Transits, Occultations, and Conjunctions. In order for Jupiter to produce a good eclipse viewing season, it first must be at either eastern or western quadrature. Quadrature is simply a fancy term for the time when a planet forms a right triangle with the earth and Sun. Western Quadrature occurs when the planet is towards the west of earth, and Eastern Quadrature is vice versa. Jupiter forms a twelve degree angle on its end, while the ninety degree angle is on earth’s end. Then, we are able to see the moons eclipse in and out of Jupiter’s shadow cone, rather than at opposition. At opposition, when the earth, Sun, and the planet of note (Jupiter) are all aligned in a straight line, Jupiter’s shadow cone is behind the planet, making eclipses invisible from earth. Precise timings are location is everything in viewing a Jovian 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!,” from Astronomical Events: Eclipses, Transits, Occultations, and Conjunctions.

Jupiter’s shadow cone is brought into our view at quadrature, it’s not at opposition, which is when Jupiter would make the earth, Sun, and Jupiter in a straight line.
August 1, 2011 brings us to Jupiter’s Western Qudarature, which is a key time to view these events. Around west quadrature, Jupiter rises beautifully in the east around midnight and shines high up at dawn. But mind you, Jupiter takes over eight years to orbit, so returning to Eastern Quadrature will take a few years, so patience is a good aspect in waiting for the return of the Jovian eclipses.  Returning to 1999, Capitol Skies (the newsletter of the Madison Astronomical Society) tells us this about the Jovian eclipses during that epic season. “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).” You can visit a myriad of online resources to locate the times of Jovian eclipses, for what more is there to miss? A moon may be eclipsing now!

Here is a basic explanatory site of Jupiter’s phenomena:

Galilean moons simulator from the Transits Page (click ‘now’ for most current events):

[To download the Word Document of this story, come to this page where it can be downloaded.] 

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Credit: NASA