See the full moon last night? If you were out past 9:30 you might also have noticed Jupiter shining below it. As Venus sets in the west, Jupiter rises in the east to carry the flame. The solar system’s largest planet by far, Jupiter is rewarding in many ways. Binoculars show its four brightest moons, while a small 3 to 6-inch telescope reveals parallel bands of brownish-red clouds that stripe the globe, the Great Red Spot and more. This week, it gets a “new moon” as the planet passes very close to the star Nu (ν) Librae.
To date, astronomers have discovered 69 moons orbiting this giant planet more than 10 times the diameter of Earth. Of those, only four are bright enough to see in most amateur telescopes: Io, Europa, Ganymede and Callisto. They’re called the Galilean moons because they were first seen by Galileo in the winter of 1610 through his 1.5-inch (37mm) telescope that magnified just 20 times.
On the night of May 8-9, Jupiter will line up with the Earth on the same side of the sun, and they’ll be closest for the year. Just like a full moon, Jupiter will lie opposite the sun in the sky, rise around sunset and remain visible all night. That’s the reason Jupiter is said to be “at opposition.” Not long before opposition and for some time after, Jupiter moves backwards in the sky, toward the west. Outer planets normally travel from west to east in the sky as they orbit the sun, but around the time of opposition they stop and back up in reverse, moving east to west instead. It’s called retrograde motion.
Of course planets don’t really stop and turn around. It’s just a trick caused by our own planet’s motion. Around opposition, the faster Earth catches up to and then passes slower-orbiting Jupiter. As we do, the planet appears to slow down, stop and then “drive in reverse.” The same thing happens when you pass a car on the freeway. You pull into the left lane and slowly sidle up to the other car. For a moment, you’re neck-in-neck, and then the other car appears to slide slowly backwards as you pass. Earth laps all the outer planets because they orbit the sun more slowly than we do. And it always happens shortly around opposition, when Earth zooms that other planet and soon leaves it behind.
Unlike a freeway, the planets travel in near-circular orbits, so some months after opposition, as Earth turns away from the more distant planet, Jupiter starts moving east again.
Over the next few nights, Jupiter’s retrograde travels will bring it up alongside the 5th magnitude star Nu (ν) Librae. While it won’t line up with the planet’s Galilean satellites, the star will be close enough to masquerade as a “5th moon.” You’ll be able to see several moons and the star in a pair of binoculars, but the view will be clearer and more satisfying in a small telescope.
Either way, if you watch over several nights you’ll witness Jupiter traveling backwards in retrograde as our swifter planet blows by.
There’s just one more little event I want you know about. Tonight, the Moon will occult (cover up)the 4th-magnitude star Gamma (γ) Librae around 11:30 EDT (12:30 CDT, 10:30 MDT, and 9:30 PDT) for observers across much of the United States, western Canada, and northern South America. Click here for a map and table of cities listing the times when the star disappears and then reappears on the opposite limb of the moon.