Galilean Moons

December’s Astronomical Occultations

So what in blazes is an astronomical occultation? Simply put, it is merely a reference to an event taking place when one celestial objects gets in the line of sight of another celestial object as seen by an observer. When it involves the Sun and Moon, we just call it an eclipse. However, astronomers, not those dealing with the occult, call such an event an occultation. This month of December offers astronomers an unusual series of occultations involving Jupiter’s moons, Io, Europa and Ganymede. Learn more about it in the Astrocast.TV vodcast.

Jupiter with the Rising Moon

Late tonight and tomorrow night, in the hours before dawn, watch for Jupiter and the Moon to rise together on your E to ESE horizon.  Tonight, you’ll find Jupiter less than two degrees beneath the gibbous Moon; tomorrow, the two are nearly side-by-side, with Jupiter just a couple of degrees W of the Moon.

As an added treat, grab your binoculars or telescope for a close-up look at Jupiter’s four Galilean moons.  Tonight, from left to right, Callisto, Europa, and Ganymede will be aligned immediately E of Jupiter, while Io will be that lone moon on the other side.  Tomorrow night, the alignment is similar, but Europa is now that single moon W of Jupiter, while Callisto, Ganymede, and Io form an east-side trio.

If you like challenges, try spotting Neptune in that one-degree region NNW of Jupiter and ENE of Mu Capricorni, a fifth-magnitude star slightly NW of Jupiter.  At ninth-magnitude, Neptune is faint, but distinguishable from the background stars as a bluish-tinted pinpoint of light.

Did You Know (3)

Did you know that Jupiter officially has the most number of moons, 63 to be precise. The four most famous (and largest) moons of Jupiter are called the Galilean moons as they were first discovered by Galileo. It should be noted that only 49 of these moons of Jupiter have an official proper name. Saturn officially has 60 moons, and 52 of those have official proper names. These numbers are likely to change, especially as more data from the Cassini spacecraft are analyzed by scientists. Also, if astronomers ever change the official definition of “moon,” there will be an upheaval of these numbers.

The Formation of Jupiter’s Moon Europa

In a paper released today, which sometime next year will be a chapter in a book on the Jupiter moon we call Europa, astronomers discuss the likely events which led to the formation of Europa and the other Galilean moons of Jupiter. Astrobiologists are still eager to get beneath Europa’s frozen water (mostly) ice crust and learn if there is a water ocean below. Many astrobiologists consider Europa one of the best candidates in this solar system (other than Earth) where microbial life may have formed, and perhaps still exist. Learn more about the formation of Jupiter’s moon Europa online at

April 2014
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