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Skywatch: Get up early to see Jupiter’s moons eclipse each other
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Skywatch: Get up early to see Jupiter’s moons eclipse each other

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Jupiter is one of my favorite telescope targets. Under modest magnification, it presents a sizeable disk with prominent dark cloud belts straddling its equator. Plus, its four largest moons (discovered by Galileo in 1610) can cast shadows on the planet, disappear into the planet’s shadow, and transit its face.

A subtler, rarer event is when Galilean moons eclipse one another. This can only happen when the Galilean moons’ orbits (which lie nearly in Jupiter’s equatorial plane) are nearly edge-on to the sun, every 5.93 years (half of Jupiter’s orbit). During these “Jovian mutual moon seasons,” dozens of events can happen over the span of most of a year.

This year, the angles are right, and mutual events are going on until November. Unfortunately, Jupiter currently rises only a few hours before sunrise, creating a narrow window in time when such events can be observed in the pre-dawn hours, before morning twilight intervenes.

Since a moon rarely passes through the center of another moon’s shadow, what’s seen from earth is a subtle dimming of the eclipsed moon which lasts for several minutes. The eclipsed moon usually isn’t hidden by the eclipsing moon itself, however, since the earth and sun lie in different directions from Jupiter’s point of view.

Jupiter is easy to spot, as the brightest star-like object in the pre-dawn, rising just south of east around 5:00 AM. Here are the events to look for this month: 4/12: Callisto (appearing closest to Jupiter) dims for 22 minutes starting at 5:51 a.m.; 4/25: Europa (appearing second closest to Jupiter) dims for 5 minutes starting at 6:06 a.m.; 4/29: Callisto (appearing second closest to Jupiter) dims for 14 minutes starting at 6:13 a.m. Of these three events, the second will show the most noticeable change in brightness.

Next column: The largest constellations.

Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 732-6663 or


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