Two jovian moons cross each other’s paths from our point of view, appearing to merge in the telescope eyepiece.
At 5:35 A.M. CDT, Io and Callisto are just minutes away from a mutual event in which Io occults Callisto, passing in front of the larger moon. Ganymede (not shown) lies west of Jupiter. Credit: Stellarium/James Hastings-Trew/NASA Cassini/Oleg Pluton/John van Vliet/PDS (CC-BY-SA)
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October 13: Vesta at opposition
Observers in the U.S. Midwest and farther west have a great opportunity this morning to watch one Galilean moon pass in front of another. Io crosses in front of Callisto, occulting it from our point of view between about 5:39 A.M. and 5:44 A.M. CDT. This occurs in bright twilight along the East Coast, making the observation challenging. If you do want to try from this time zone, make sure to put away your optics at least several minutes before sunrise from your location, which might differ from the time given for our standard location below.
Around the time of the event, Jupiter is some 36° high in the east in the Midwest, to the upper right of the bright star Regulus in Leo. The gas giant is unmissable, shining brightly at magnitude –1.9. Zoom in with a telescope around 5:30 A.M. CDT and you’ll see Ganymede alone to Jupiter’s west, with Europa close to the planet on the eastern side, followed by Callisto just west of Io (farthest from the planet). Keep your eye on those last two moons, as they will close in and appear to merge for visual observers. If you’re an experienced astrophotographer, try capturing the event on camera and see what detail you can get!
For evening observers or those without large scopes, the Moon passes 0.4° south of Antares at 5 P.M. EDT. Look for the pair in the southwestern sky after sunset, where they linger for roughly two hours after the Sun disappears beneath the horizon. The Moon now sits just to the lower left of the red giant star Antares and passes close to 3rd-magnitude Tau (τ) Scorpii as well. No telescope is required to enjoy this view.
Sunrise: 7:10 A.M.
Sunset: 6:21 P.M.
Moonrise: 11:28 A.M.
Moonset: 8:23 P.M.
Moon Phase: Waxing crescent (19%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 10 P.M. local time from the same location.
Alison Klesman is senior editor of Astronomy magazine. She holds a Ph.D. in astronomy and has studied a variety of topics, from minor planets to supermassive black holes.