Natural History Magazine

Skylog

May 2008

By Joe Rao

Io, one of Jupiter's moons, cruises the planet's surface.

NASA/JPL/University of Arizona

It’s not very often that Mercury is easier to see than dazzling Venus. But Mercury begins May in the midst of its best evening apparition of the year for northern observers. It’s an easy to see naked-eye object, while Venus is hopelessly hidden deep in the glow of sunrise all month. Mercury sets an hour and a half after sunset on the 1st; by the 14th it is setting nearly two hours after the Sun.

As it happens, one of Jupiter’s four largest moons is as big as Mercury, and another one is even bigger, though they are too far from us to be seen without binoculars or a small telescope. The Giant Planet’s main moons are, in descending order of size, Ganymede, Callisto, Io, and Europa. They are commonly known as the Galilean satellites, because they were first described by Galileo in 1610. (Nearly sixty smaller moons also orbit the planet, two-thirds of them discovered only in the past decade, thanks to advances in digital imaging and computer analysis.)

Sky watchers in the eastern United States and Canada who look at Jupiter on the night of May 21–22 are in for a surprise: all the Galilean satellites will be briefly hidden. From 11:51 p.m. until 12:10 a.m. eastern daylight time (EDT), Io and Callisto will be eclipsed by Jupiter’s shadow, while Europa is passing in front of the planet and Ganymede is behind it. Such a coincidence happens only about twenty-five times per century.

May Nights Out

1 and 2 Look for Mercury low above the west-northwest horizon about an hour after sunset; nearby are the Pleiades (discernable through binoculars), the V-shaped Hyades star cluster, and, seemingly part of the Hyades, the orange star Aldebaran.

3 High in the south-southwest as evening twilight ends, Saturn is set to resume its direct (eastward) motion against the background stars. The ring system is at its maximum tilt toward us for the year.

5 The Moon is new at 8:18 a.m. EDT.

6 A thin crescent Moon sits a couple of degrees above and slightly to Mercury's right, low in the west-northwest before they both set at evening twilight's end.

Joe Rao is a broadcast meteorologist and an associate and lecturer at the Hayden Planetar- ium in New York City.

11 The Moon waxes to first quarter at 11:47 p.m. EDT.

14 Mercury reaches its greatest eastern elongation, or apparent distance, from the Sun, 22 degrees. After sunset it remains in sight for nearly two hours, setting three minutes after full darkness descends.

19 The Moon becomes full at 10:11 p.m. EDT.

21 For viewers in the eastern U.S, and Canada, Jupiter briefly appears without moons, low in the southeast (see story above). The phenomenon ends at 12:10 a.m. EDT (May 22), when Ganymede pops out from behind Jupiter. Farther west, Jupiter will not rise until after Ganymede has emerged.

27 The Moon reaches last quarter at 10:57 p.m. EDT.