Looking Up: Earth’s moon passing by Jupiter’s moons
The nearly full moon passes the brilliant planet Jupiter and its family of moons, the weekend of June 15. On Saturday night, the moon and Jupiter form a nearly equilateral triangle with the bright red star Antares.
Although the gibbous moon will overwhelm most of the stars with reflected sunshine, there won’t be any problem seeing Jupiter, with little trouble noticing Antares, which isn’t quite as bright.
Each side of the triangle on Saturday will be approximately 10 degrees; that’s about the length of the Big Dipper’s "bowl."
The moon arrives at full phase Monday, June 17.
While our lovely natural satellite is in Jupiter’s vicinity, you may like to take binoculars and look at the moon, and then at planet Jupiter. Compare the Earth’s moon with Jupiter’s four biggest satellites: Io, Europa, Ganymede and Callisto, which is their order in distance from Jupiter with Io being the closest.
Jupiter’s big moons appear like stars very close to the much brighter point of light, which is Jupiter. The moons may be on either side, in varying combinations; you may only find two or three. Because ordinary binoculars don’t magnify very much, the moons closest to the planet may be close to resolve or be lost in Jupiter’s glare. They might also be hidden in Jupiter’s shadow or not noticed because one is passing right between you and the planet!
Jupiter is near its closest approach to Earth, so that makes it all the easier to see the moons with binoculars. Still, it’s better to brace the binoculars so they are steady as a rock, or use a tripod. If it wasn’t for being so near bright Jupiter, the moons would be faintly visible — around +5th magnitude — to the unaided eyes.
A well-mounted telescope, even a small one magnifying at least 40x will show them much better. It’s even possible to see the moons as very tiny discs, not points of light, with at least 150x, or more with a well-aligned telescope, when the air is extremely steady.
Imagine the famed Italian astronomer Galileo Galilei, who was the first astronomer to train a telescope on the stars. He discovered Jupiter’s four biggest moons in 1609 and charted their positions from night to night as they orbited. The sight was extraordinary, and helped bolster the theory that the planets — including Earth — revolved around the Sun, rather than the everything revolving around the Earth.
The four biggest moons of Jupiter are often called the "Galilean" moons because Galileo discovered them. There are actually a lot more; in fact, NASA reports Jupiter may have at least 79 moons. Of these, 53 are named. Jupiter also has a faint ring system of particles, not nearly as prominent as Saturn’s. There’s also a man-made satellite orbiting Jupiter: NASA’s Juno probe.
No other moons in the Jupiter system were known until 1904 when a tiny, 127-mile wide moon was discovered and later given the name Himalia.
The Galilean moons are all larger than Earth’s moon, which is 1,089.4 miles wide. Europa is 1,939.7 miles wide; Ganymede, 3,273.5; Callisto, 2,995.4; and Io, 2,263.8. Dwarf planet Pluto, by the way, is only 738.4 miles in diameter.
Sky and Telescope Magazine, and its website (www.skyandtelescope.com), has a chart to identify which moon is which on a particular date or hour, when observing Jupiter in a telescope. During the course of a night, you can watch as the moons shift positions.
Keep looking up!
Peter Becker is the managing editor at The News Eagle in Hawley, PA. Notes are welcome at email@example.com. Please mention in what newspaper or website you read this column.