About 1 hour after the sun sets, the eye-catching celestial duo will be visible in the southern sky, roughly halfway up from the horizon to the point directly overhead (called the zenith). The moon, which will be 2 and a half days past first-quarter phase — 75 percent illuminated by the sun — will be situated just above and to the left of Jupiter, a distance of roughly 2 degrees.
To gauge how wide 2 degrees is, consider that the moon is a half-degree wide. However, because of an optical illusion, the moon actually appears twice as big as it actually is. So while the separation between the moon and Jupiter should equal four moon widths, when you see them in the sky on Saturday evening, the two will seem much closer — to some, perhaps less than half the predicted "four moons wide" distance! [The Brightest Planets in the June's Night Sky (and How to Find Them)]
Even without the moon, Jupiter alone readily attracts attention; it's the brightest "star" for the time that it's above the horizon (it currently sets at around 2:45 a.m. local daylight time), first coming into view in the southern sky during the early stages of twilight. Jupiter, at magnitude minus 2.2, outshines everything in the night sky except Venus and the moon. (The lower the magnitude, the brighter the object.) As twilight fades, Jupiter is soon joined by the bright stars of late spring and early summer. When you sight the moon and Jupiter, for instance, don't overlook the bluish 1st-magnitude star Spica, located more than 10 degrees — or a little more than the width of your fist held at arm's length — far to the lower left of the moon and Jupiter.
In a telescope, Jupiter is a prime attraction. It's best observed during early evening, when it's still high and its image reasonably calm. It's still a great big belted ball showing tantalizing glimpses of detail. Admittedly, it appears about 8 percent smaller than it did at opposition in early April, but don't let that stop you. The quirks of atmospheric seeing, which can change from minute to minute, often have a bigger effect than the planet's apparent size on what a telescope will show on Jupiter. And its four bright moons are always performing.
In fact, if you look at Jupiter with a small telescope, or even steadily held binoculars, on Saturday evening, you'll see two of those big satellites on one side of Jupiter: Ganymede and Callisto. For viewers in the eastern U.S., Ganymede initially will appear very close to Jupiter, but then, as the hours pass, it will begin to noticeably move away from it.
And at 11:16 p.m. EDT, a third satellite will appear: Io. This moon was in transit — that is, crossing in front of the disk of Jupiter — and now will be emerging back into view as the transit comes to an end. And at 12:36 a.m. EDT on Sunday, a fourth satellite will come into view: Europa. This moon will be emerging from Jupiter's shadow, coming out of eclipse on the opposite side of Jupiter from the other three moons.
So, on Saturday, while you enjoy the sight of Jupiter near to our own moon, keep in mind that with only slight optical aid, you'll be able to see up to three of Jupiter's moons as well. They look like small stars, but two of them are actually larger than our own moon. And it is indeed possible to watch them change their positions relative to one another from hour to hour and night to night.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for Verizon Fios1 News, based in Rye Brook, New York.Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.