Once-in-6-year-alignment Makes Jupiter’s Moons Dance In Shadows

Jupiter and his Great Red Spot photographed on January 3rd through a 14-inch telescope. Credit: Paul Maxson

Now that Jupiter’s up in the east by 9 o’clock local time, we have lots of opportunities to observe it before bedtime. That’s good because the Jupiter system is currently edge-on to the Earth and Sun, allowing us to see the planet’s brightest moons eclipse and occult one another now through August.

Io, Europa, Ganymede and Callisto all orbit very close to the plane of Jupiter’s equator. From our perspective on Earth, the moons usually pass a little above or below one another and escape each other’s shadows. But every six years or so, Earth and Sun cross the plane of the satellites’ orbits putting us “level” with Jupiter’s equator.

Instead of missing one another, the moons appear to merge into one during occultations and cast their shadows on one another during eclipses. This cyclic but relatively rare planetary alignment last happened in 2009 and won’t again until 2020.

The six varieties of eclipses and occultations possible among Jupiter’s four brightest moons now through August. Credit: Dave Dickinson

While you may not be able to resolve the four brightest moons in your telescope, you’ll have no difficulty watching them approach one another and meld into either an extremely close “double moon” or a single object during an occultation. Minutes later, the pair breaks apart as each moon follows its own track around the mothership.

Io eclipses Ganymede back on August 16, 2009. Credit: Christopher Go

During an eclipse, one moons will cast a shadow on another, causing it to fade the same way our moon dims when entering Earth’s shadow during a lunar eclipse. Assuming a fairly deep eclipse, you’ll be able to watch a Jovian moon fade and then re-brighten in a matter of minutes.

Again, you won’t see the shadow itself because the moons are so tiny, but the drop in brightness is clearly visible especially during deep eclipses.

With Jupiter coming to opposition on February 6th you’ll have lots of opportunities to catch at least one of each type of phenomenon. Not to mention, that all the moons cross over the Jupiter’s bright equatorial zone as they orbit the planet, making shadows they cast on the cloudtops easier than ever to see. Those are called shadow transits, and we’ll feature those soon, too.

Io eclipses Ganymede on Christmas night 2014. Credit: Paul Maxson

Below is a list of the best upcoming mutual events of the four brightest satellites for locations across North America. To view them, you’ll need at 3-inch or larger telescope.

A drop of 0.5 magnitude or larger during an eclipse or occultation should be apparent to the eye by carefully comparing before and after views. For occultations, you can also have the pleasure of seeing the moons in close embrace

Two for the price of one. Io occults and then eclipses Europa in this animation of still photographs taken on September 28, 2009. Credit: Brian Combs

For a customized table of events when Jupiter’s easily visible in a dark sky from your location, click over to this list of observatories, do a Control-F (Command-F on Mac) and type in the name of a larger city within a few hundred miles of your location. Next, copy the 3-digit code number and then paste it into the window on the IMCCE table creation page. Click enter and you’ll get a handy list of every event visible from your location through August.

To help you pick which eclipse events are worth your time, make sure the “Δm” (change in magnitude) is 0.5 or greater. Times listed in the table are Universal Time. Subtract 5 hours for EST, 6 for CDT, 7 for MST and 8 for PST. Also, each moon is listed by number rather than name. Io=1, Europa=2, Ganymede=3 and Callisto=4, so “2e4″ means Europa eclipses Callisto.

More event information and some great animations are available at SAF Planetary Observation Commission’s page.

Events – Times are CST:

Jan. 15 – Io eclipses Callisto starting 6:13 a.m., ending 6:39 a.m. Magnitude drop: 0.5
Jan. 18 – Ganymede occults Europa starting 8:31 p.m., ending 8:37 p.m. Mag. drop: 0.5
Jan. 23 – Callisto eclipses Ganymede starting 3:06 a.m., ending 3:20 a.m. Mag. drop 1.4!
Jan. 25 – Ganymede occults Europa starting 11:13 p.m., ending 11:19 p.m. Mag. drop 0.5
Jan. 28 – Europa eclipses Io starting starting 12:18 a.m., ending 12:27 a.m. Mag. drop 0.5
Jan. 29 – Io occults Europa starting  8:31 a.m., ending 8:35 a.m. Mag. drop 0.6

Nice sequence showing Io occulting Ganymede on December 21, 2014. The moons meet and part over 22 minutes. Credit: Paulo Casquinha

Bear in mind, these are the most easily observed events. There are many more! I’ll post a new list at the beginning of every month through the summer. Let us know if you get to see one of these. Good luck!

9 Responses

  1. Edward M. Boll

    Jupiter always delights. It’s moons make it more amazing. Yesterday, Mercury and Venus were the same altitude. Too cloudy tonight for Finlay. I may try for it again before fading to mag. 11.

  2. Jeremy Hamer

    Your piece on the occultations and eclipses of the Jovian moons is fascinating. I will try to see some. On the website you gave, I found the list of events for my location. The type of occultation is given as nEm and nOm with E for eclipse and O for occultation with n and m being the satellite number. I haven’t been able to find out which satellite has which number (0 to 4). Can you help?

    1. astrobob

      Jeremy,
      Thanks for writing. Here are the moons and their numbers: Io=1, Europa=2, Ganymede=3 and Callisto=4

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