From the November 2014 issue

The feeling is mutual

Celestial sketcher Erika Rix encourages readers to record mutual events of Jupiter's moons by making a drawing every few minutes.
By | Published: November 24, 2014 | Last updated on May 18, 2023
Italian astronomer Galileo Galilei discovered Jupiter’s four brightest moons in 1610. Because of his find, you’ll often hear them called the Galilean satellites. Observers frequently see them passing in front of or disappearing behind the planet. But every six years near Jupiter’s equinox, the orbital planes of these four moons tilt almost edge-on to Earth, and less common phenomena occur: The satellites eclipse and occult each other. Astronomers call these occurrences mutual events.

The next equinox occurs February 5, so sharpen those pencils. Ample opportunities exist through August 2015 to sketch the phenomena. To help you plan, the Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE) provides the dates, times, and types of mutual events for Jupiter’s current apparition on their website at In addition, Astronomy mentions a few each month in “The Sky this Month.”

During a mutual occultation, two satellites approach each other and merge to form a brighter oblong image. That image then either dims as it rotates during a partial occultation or becomes circular during a total or annular occultation. It brightens once more just before the satellites separate and move away from each other.

The January 12, 1991, mutual eclipse and occultation of Io (I) by Europa (II).
This series of sketches shows the January 12, 1991, mutual eclipse and occultation of Io (I) by Europa (II). It also shows Callisto (IV) in the same field of view. The sketcher used a 10-inch reflector.
John Rogers
When observing a mutual eclipse, you can’t see the shadow approaching the second moon, so there’s no warning of its beginning other than a dimming of the eclipsed satellite. Ephemerides may be off by a few minutes, so start your observation early. Once the event begins, estimate the brightness of the satellite being eclipsed at three stages: when it starts to decrease, when it stabilizes, and when it begins to increase. The easiest way to estimate a moon’s magnitude is to compare it to similarly bright nearby stars viewed through your scope.

Although you can observe both types of events through a small telescope, you’ll have difficulty noticing a change in brightness less than 20 percent during an eclipse with an aperture under 6 inches.

I’ve found the key to a skillful sketch sequence is preparation. Practice gauging brightnesses beforehand by making magnitude estimates of variable stars, and then formulate a game plan. Incorporating a voice recorder for notes frees up time you can use for sketching.

historical sketches showig mutual events of Jupiter's satellites in early 1932
These historical sketches show mutual events of Jupiter’s satellites in early 1932. Drawings come from Memoirs of the British Astronomical Association. For the top image, the sketcher used an 8-inch refractor; for the image at lower left, the sketcher employed a 12 1/4-inch reflector; and the third image was made through a 9-inch refractor.
Courtesy The British Astronomical Association
Prepare your brightness value estimates ahead of time by using a nearby Galilean moon for comparison. Identify satellites that will be in the vicinity to prevent unexpected distractions and to guarantee that you keep your eye on your target moon.

Try to complete a sketch every few minutes during the event. Record the times you began each sketch and the times for all brightness estimates within six seconds. Express them in Universal Time if you plan to share them.

The IMCCE uses visual observations and sketches for research and has additional material on its website. I encourage you to participate and also suggest forwarding your reports to organizations such as the Jupiter sections of the Association of Lunar and Planetary Observers and the British Astronomical Association. Both groups promote the study of mutual events.

Take advantage of this once-every-six-years opportunity. You’ll find watching the dance of planetary satellites fascinating, and you’ll create memories you actually can show people.