Sky Talk July 2009: Phenomena of Jupiter’s Satellites

Everyone has heard of a “three-ring circus” but over the next several months a “four-ring” one will be performing nightly overhead when skies are clear. It’s the fascinating phenomena resulting from the ever-changing positions of Jupiter’s retinue of four bright Galilean satellites (those discovered by Galileo with his primitive telescope 400 years ago in 1609).

Jupiter’s four bright moons in order of distance from the planet are Io, Europa, Ganymede and Callisto. The outer ones can be seen in steadily-held or image-stabilized 7X and 10X binoculars when distant from the planet and all four in the smallest of telescopes at 15X or more. As each of them does their fascinating orbital dance about Jupiter, the following events involving them can occur:

  • Eclipse Disappearance (ED): satellite is eclipsed by the planet’s shadow
  • Eclipse Reappearance (ER): satellite reappears from eclipse
  • Occultation Disappearance (OD): satellite passes behind Jupiter
  • Occultation Reappearance (OR): satellite reappears from behind Jupiter
  • Transit Ingress (TI): satellite passes in front of Jupiter’s disk
  • Transit Ingress (TE): satellite moves off Jupiter’s disk
  • Shadow Egress (SI): shadow of satellite falls on Jupiter’s cloud tops
  • Shadow Egress (SE): shadow moves off Jupiter’s cloud tops

Since there are four satellites involving eight events, it’s possible in theory for these lovely gems to display up to 32 different phenomena! In practice, it’s not unusual to see as many as half a dozen of them during the course of a several hour viewing session.

A small telescope like the Scientifics’ 60mm refractor (#30823-61) will easily show the eclipses and occultations even at its lowest magnification, while those involving transits of the satellites and their shadows can be viewed under steady seeing conditions at its highest magnifications. Times of the various events can be found by going to the following web site provided by Sky & Telescope magazine.
(Note that these times are given in Universal Time — that of Greenwich, England, which is five hours ahead of EST and fours hours ahead of EDT. This means those hours have to be subtracted from the times listed, taking note of the date as well which may be the evening before that shown after converting!)

There are also events involving the satellites themselves. Every six years, Jupiter’s equator is presented exactly edge-on to us and since the satellites lie in the same plane they can actually occult and eclipse each other! While this isn’t the case right now, they can still pass very close to one another. Here’s just one example: on the evening of July 23rd, as Jupiter rises above the southeastern horizon, Io and Europa will appear to be almost in contact. Indeed, since atmospheric seeing is generally poor close to the horizon, they may appear as a single image. But as the planet gets higher you will see them pulling apart. Then from about 9:30 to 11:30 p.m. EDT the two moons will form a triangle with Callisto, which then takes turns with Europa playing tag with Io! All the while, Ganymede watches from a distance on the same side of the planet. Jupiter and its satellites make the perfect target to impress visitors with — and to demonstrate that contrary to popular belief, the sky is not static and unchanging but alive with fascinating activity!

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of five books on stargazing.

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