Jupiter is front and center at nightfall May evenings. The planet is the first “star” to show in the sky at dusk, when it appears in the southeastern sky a fist above fainter Spica in the constellation Virgo. Tonight (May 11), if you have a modest-sized telescope, say a 4 to 6 incher, you can see two of Jupiter’s moons cast their shadows on the planet at the same time.
When a satellite casts a shadow on its mother planet, astronomers call it a shadow transit because the moon’s shadow transits or moves from one side of the disk to the other. Jupiter’s moons take time to orbit the planet, so transits usually last several hours. The shadows look like inky black dots or pinpricks against the brighter clouds that circulate about the planet. Ganymede, the largest of Jupiter’s four brightest moons, throws the biggest shadow. Europa, the smallest, casts the tiniest.
Single shadow transits are fairly common, but two happening simultaneously not so much. Three is rare. Io and Europa will cast their shadows on either side of Jupiter tonight for only a few minutes for skywatchers in the eastern half of the Americas and western Europe. For the Americas, this happens during evening twilight and early nightfall; for Europe in the early morning hours of May 12.
The sun will still be up and Jupiter too low for a good view from western North America.
Europa’s shadow makes its first appearance at 6:40 p.m. CDT at the eastern edge of the planet and departs the disk at 9:07 p.m. Io begins its trek at 9 p.m. and exits Jupiter at 11:11 p.m. That means that the two black dots will be visible simultaneously for only about 7 minutes — from 9:00 to 9:07 p.m. !
Will it be clear? Will the seeing be good? While both shadows are visible at a magnification of around 70x, higher power will make them easier to see provided the air is steady. Use the highest magnification that provides the sharpest view.
While observers in the western states will miss the double event, you’ll at least get to see Io’s shadow soloing across the great planet. Below is a table summarizing the events.
6:40 p.m. Shadow of Europa comes into view
9:00 p.m. Shadow of Io
9:07 p.m. Shadow of Europa exits
11:11 p.m. Shadow of Io exits