The big events in astronomy are wonderful to watch and read about, but it’s the small stuff that makes up the bulk of sky watching. Case in point. Tonight the moon will pass between the planet Jupiter and the bright star Aldebaran in Taurus the Bull. The actual passage happens in dark skies for eastern European and Asian observers. By the time it’s dark in the U.S., the moon will have moved on to the east, and the trio will form an eye-catching triangle.
If you have a small telescope, you have a chance to see Jupiter’s four brightest moons in an odd configuration. We often see them in a nearly straight line on either side of the planet, but tonight only Io and Europa will be aligned in that fashion. Callisto floats above the planet’s south polar region, while Ganymede hovers near the north pole.
These interesting, non-linear arrangements happen because Jupiter’s axis tilts 3.3 degrees (Earth’s is 23.5). Presently, the planet’s north pole is tipped in our direction. Since the four bright moons all orbit about Jupiter’s equator, the tilt causes the more distant ones – Callisto and Ganymede – to miss or nearly miss the planet when they pass directly in front of it.
By the time those moons move far enough to either side of Jupiter, the difference caused by the planet’s axial tilt isn’t as obvious and once again, all four moons appear approximately in a line. The closer-in moons Io and Europa always pass in front of the planet from our perspective on Earth, because the tilt isn’t enough to shift them above or below Jupiter’s disk. Every six years however, Earth’s orbital plane exactly lines up with Jupiter’s equator and all four moons parade back and forth in straight lines.
One last note. The moons regularly pass in front of or in back of Jupiter during their orbits. Tonight, Ganymede will be occulted or covered up by the planet starting at 9:16 p.m. CST. The event can be easily observed in any telescope. Go out about 10 minutes before and watch as Ganymede and Jupiter get closer and closer until the moon sticks to the planet’s limb like an earring.
If Jupiter were occulting a star, which is a point of light, it would disappear very quickly. Ganymede will take a minute or two because it has real girth despite its stellar appearance. You can find lots of fun occultations and other Jupiter moon events at Sky and Telescope’s online Jupiter’s Moons site.