Watch Io Snuff Out Ganymede At Jupiter Tonight

The moon kisses up to Aldebaran last night (Feb. 25) during evening twilight seen from Duluth, Minn. Credit: Clint Austin
The moon kisses up to Aldebaran last night (Feb. 25) during evening twilight seen from Duluth, Minn. Credit: Clint Austin

First, my apologies. I so wanted to alert you to the half moon’s pass of the bright star Aldebaran last night. But there were network problems with the blog, and I wasn’t able to post.

No doubt many of you noticed it all the same. A quick look up at the moon and you couldn’t help but see the star a little more than one lunar diameter to the southwest. The farther north you lived, the closer they drew together. In far northeastern Canada the moon occulted Aldebaran. Checking the moon several times overnight, it was amazing to see how quickly it departed Aldebaran, forced by its perpetual orbital motion to “go east, young moon, go east”.

Tonight our satellite moves a fist further east in Taurus the Bull and beams atop Orion the mighty hunter at nightfall. It’s 8 days past new phase and absolutely resplendent with craters. Sic your telescope on it and marvel at the ruggedness of all that ancient terrain bludgeoned by forgotten meteorites and asteroids.

The view through the telescope this evening just before Ganymede is eclipsed by Io's shadow. Created with Stellarium
The view through the telescope this evening just before Ganymede is eclipsed by Io’s shadow. The deepest part of the eclipse will occur around 9:35 p.m. Created with Stellarium

East of Orion you’ll find the blazingly bright planet Jupiter right along the border of Leo and Cancer. I’ve written before about this being a special season for Jupiter’s moons. Because Earth’s equator is aligned with Jupiter’s, and the brightest moons orbit above the planet’s equator, we can see them eclipse and occult one another in what astronomers call “mutual events”.

Tonight, little Io will cast its shadow on the largest Jovian moon, Ganymede. While not a total eclipse, it’s close, with a good deal of Ganymede in shadow at maximum (although not 97% as I wrote earlier). This should be easily visible in a small telescope at low to medium magnification. The eclipse begins at 9:31 p.m. CST (3:31 UT) and ends at 9:40 p.m. (3:40 UT). Jupiter will be very well placed for viewing across all of the Americas at the time.

Now here's something cool - a double mutual event. Europa eclipses then occults Io on January 28 captured by Theo Ramakers of Oxford, Georgia.
In this double mutual event, Europa eclipses then occults Io on January 28 captured by Theo Ramakers of Oxford, Georgia. The eclipse is quick in the time lapse, occurring about 1/2 second in. Look for the shadow passing across the top of Io.

Get that scope out at least a half hour beforehand and let it cool down if you’re in a cold climate otherwise Jupiter will look all mushy. Then start watching about five minutes before the eclipse begins, so you can get familiar with Ganymede’s normal brightness.

During the eclipse you won’t be able to see Io’s shadow with your eye, but Ganymede will fade by one magnitude and then re-brighten as the shadow first covers and then departs its 3,275-mile-wide globe.

Wishing you clear or at least partly cloudy skies tonight!

7 Responses

  1. Edward M. Boll

    No, I missed out on the moon and Aldebaran. It has been 30 degrees below normal. Tonight will be the coldest night of the Winter. And March is only 2 days away. 20 below tonight is possible. On March 8, 1995 we had 22 below 0. It looks like I will miss out on the new comet too unless it shows itself in a dark sky at 8th or 9th magnitude. And even then the Moon will cause problems for evening viewing.

  2. Edward M. Boll

    I am resigned to the fact now that I am sure that Lovejoy is the brightest comet. With still no report on 2015 D1, I am quite sure that it cannot be brighter than magnitude 7, probably dimmer, if it is to be found at all. I am still optimistic on it.

  3. Hi, Bob – I missed the Ganymede dimming, but before the clouds came in, Aldebaran next to the moon was very striking from here in the NYC area.
    My friends and I have been talking about the role of the Jovian spring equinox in the shadowing of the moons. Isn’t Jupiter’s equator (and the orbital plane of its brightest moons) aligned with the ecliptic at its equinox so the shadows of the moons can fall on each other?
    Thanks for keeping an eye out for the SOHO comet!

    1. astrobob

      Hi Bob,
      Yes, Jupiter’s “edge-on” to Earth right now so the shadows of the Galilean moons, which orbit in the planet’s equatorial plane, fall upon one another. Likewise, they occult each other.

  4. CollinofAlabama

    I and friends, too, enjoyed an Io transit, then Io occultation (or Europa), then Io partial eclipse of Europa on Sunday, February 8th. The event is recorded here …

    http://www.cloudynights.com/topic/491681-the-kings-ball/

    One should take the time to see one of these Galilean Jovian Mutual Events. Check out the Sky & Telescope article on the subject here …

    http://www.skyandtelescope.com/sky-and-telescope-magazine/beyond-the-printed-page/mutual-events-jupiters-satellites-201415/

    A fantastic phenomenon not to be missed! There’s one coming up on the evening of Friday March 20th, 20:43 & 21:09, CDT. Make sure to catch it if you can.

    1. astrobob

      Hi Collin,
      I’ll check your event out on cloudynights in a bit. I’ve watched a few of the mutual events this winter and reported on a couple in my blog. Thanks for the heads up. The March 20th one looks nice!

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