The plump moon will stroll by Jupiter tonight and Wednesday night making the planet particularly easy to find. Binoculars and small telescopes will show all four moons tonight but you’ll seek them in vain during part of tomorrow night. This illustration and the one below were created with Stellarium.
OK, this is weird. Normally I write about things that might nudge you outside for a look-see but this latest happening is the exact opposite. It’s a total non-event … in a sense. Tomorrow night (Weds) between 11:43 p.m. and 1:29 a.m. Central time none of Jupiter’s four bright moons will be visible alongside the planet. The solar system’s father of the sky will be hard pressed to find his children. Just for a little while.
Jupiter in binoculars tonight (Tues.) around 10 o’clock. I’ve listed the order of its four brightest moons from left to right.
Normally you can see at least a moon or two on either side of the planet. Indeed tonight all four should be visible in a pair of steadily-held binoculars and through any small telescope. Tomorrow night’s a different show. For nearly two hours Ganymede and Europa will be in front of the Jupiter and camoflaged from view. You’ll need a modest telescope to see them against the pale clouds of Jupiter’s equator assuming the air is steady. Io will be tucked behind the planet’s edge and then emerge into eclipse, invisible in the planet’s shadow. Moon #4, Callisto, will also be in eclipse. If you’re used to seeing those familiar little "stars" alongside big, bright Jupiter you might find the moonless view oddly disturbing.
The disappearance of Jupiter’s moons is a rare event that happens only about 20 times per century. The last time was May 21 but that one was only visible in the eastern U.S. and for a very brief time. Observers across the U.S. will witness Wednesday night’s. Plan your outing well since the show will be over at 1:29 a.m. when Io slides out of Jupiter’s shadow and back into sunlight.
A modest telescope at high magnification will show the moon Ganymede and perhaps Europa "hiding" in front of Jupiter at the start of the moonless period. This illustration and the one below are courtesy of Chris Marriott and SkyMap software.
Ready for more? Users with modest-sized telescopes (6 to 10 inches) can enjoy watching the moons Ganymede and Europa and their shadows dot the planet during the moonless time. If you stay up late, you’ll see Europa move off the planet’s face and shine again in a dark sky. How fine a thing it is to watch this solar system in miniature move with the precision of a classic Swiss watch.
In this view at 1:45 a.m. Central time, both Ganymede (G) and Europa (E) and their shadows are visible through a telescope. The easiest shadow to see is Ganymede’s since it’s the biggest.