A full season of mutual events for Jupiter's four largest satellites

Mutual satellite events can occur in any of six ways, depending on slight differences in the moon's angular sizes and relative positions.   Sky & Telescope
Mutual satellite events can occur in any of six ways, depending on slight differences in the moon's angular sizes and relative positions.
Sky & Telescope

As Jupiter shines brightly down from the sky during its 2014–2015 apparition, quite often Jupiter's four big Galilean moons will occult and cast their shadows on each other. A "mutual events season" like this happens about every 6 years, when Earth and Sun cross the plane of the satellites' orbits.

Some of these events will dim the shadow-eclipsed moon, or the combined light of two moons during a partial or total occultation, enough for the change to be visible in a small telescope for some minutes. Photometric recordings of the events, even shallow ones, provide a very accurate way to refine the satellites' gradually changing orbits. Their orbits are morphing in interesting long-term ways due to interactions among the satellites and between them and Jupiter, including tidal energy dissipation within the satellites themselves. For instance, this is why Io is so volcanic.

What to see and when to see it

North American viewers can use the tables provided in Sky & Telescope to see when mutual events will occur. The greater the drop in magnitude, the more potential for viewing drama. A drop of 0.5 magnitude will be observable if you compare carefully with the other satellites, while drops of 1 magnitude or more will be much plainer.

To get the complete table of event predictions for this season, go to the IMCCE (Institute of Celestial Mechanics and Calculation of Ephemerides) table creation page for this project and click the "Show" button.

But what you really want is the list of only your events — those visible when Jupiter is up during darkness at a location near you. Open this list of observatories, do a Control-F search on the list for an observatory or city within a few hundred miles of you, and note its 3-digit observatory code. Then back on the on the table creation page, enter the observatory code in the box in place of the "500".

In the table you get, look for events with a large change in magnitude: the "Δm" column. A Delta-m of 0.7 magnitude or more (at least a 50% or so drop in light) ought to be fairly readily visible by eye.

The table also gives the start and end times of the event and the altitude of Jupiter above the horizon and the depression of the Sun below the horizon. See the explanation of the table's column headings.

There's lots more on the main site to browse, including additional useful tools.

Get involved

The international campaign to record light curves during this mutual-event season is being coordinated by the IMCCE in Paris. Read all about it at the main site and especially the Handbook for observers.

And here's a PowerPoint telling why and how to join the timing campaign, including the equipment you'll need, courtesy of the International Occultation Timing Association (IOTA).

Track Jupiter's four largest moons with Sky & Telescope's app, JupiterMoons.


You must be logged in to post a comment.