I just know you’ve seen it already. That big, pale yellow “star” climbing the southeastern sky at nightfall. That’s Jupiter! It’s on a seesaw with the other bright evening planet, Venus. As Venus settles into the west, Jupiter takes over in the east.
On Monday night, Jupiter reached opposition to the sun, meaning it appears “opposite” the sun in the sky: when the sun sets, Jupiter rises and shines the entire night until the sun rises the next day. Opposition occurs when both Earth and Jupiter are paired up on the same side as the sun. Faster-moving Earth laps the giant planet every 13 months. Just like runners who pass each other in a race, the two planets are closest for a short time before Earth moves on and their separation increases.
This week, Earth and Jove are just 409 million miles apart. Last October, when Earth and Jupiter sat on opposite sides of the sun, the planet was 200 million miles farther away. Skywatchers like things that are close because they appear brighter and bigger than usual. That’s certainly true of Jupiter, and the reason we eagerly dust off our telescopes and get out to see what the solar system’s biggest planet has to offer. Yes, it IS big. You could hollow Jupiter out and fill it more than 1,300 Earths! Seen another way, it would take a popcorn string of 11 Earth to reach across Jupiter’s globe.
This spring and early summer, Jupiter moves west across Libra in retrograde motion. Outer planets normally travel eastward across the sky as they orbit the sun, but for a time before and after opposition, they appear to slow down and turn around to the west. “Appear” is the operative word here. Of course, Jupiter didn’t suddenly change its mind and reverse direction. Earth laps the planet, and just like when you pass a car on the freeway, the vehicle appears to slow down and slide backwards, the same way Jupiter does the same when Earth zips by.
Jupiter spends its time retrograding through Libra as it gets closer and closer to that constellation’s brightest star, Zubenelgenubi (zu-ben-el-ge-NOO-bee) Planet and star come closest twice: first in early June and then even closer on August 15-16. On that date, the two will be a smidge more than one full moon diameter apart.
Zubenelgenubi happens to be a challenging double star for naked eye observers with a faint 5th magnitude companion (α1 Librae) sitting off to the northwest of the star. If you’re into testing your vision, Jupiter will point you there.
While steadily-held binoculars will show up to four of Jupiter’s moons, they’re much easier to spot in a 3 to 6-inch telescope. You don’t need fancy eyepieces or high magnification either. Sometimes one or another of the moons won’t be visible. That’s because it’s temporarily hidden from view either in front or behind the planet.
When a moon moves behind Jupiter, it’s blocked from view or occulted. When it passes in front, a moon casts a shadow on the planet’s ammonia-ice clouds that looks like the most exquisite black dot in a 4-inch or larger telescope. These events are called shadow transits. Finally, a moon can move into Jupiter’s shadow and be eclipsed.
Since each of Jupiter’s four bright moons — Io, Europa, Ganymede and Callisto — orbit about the giant planet with a different period, they’re always making new and interesting arrangements worth your while to check out whenever it’s clear.
Jupiter itself is all gas and clouds without a shred of solid surface. Heat from within the planet’s core rises through the atmosphere and creates powerful winds that draw the planet’s clouds into long parallel belts and zones, giving it a striped appearance. Belts are where the air is descending and warming; zones mark where the air is rising and cooling. Enormous storm cells ranging from a couple thousand miles across to the monster Great Red Spot dot the planet. They’re particularly plentiful in the polar regions as revealed in remarkable new images taken by NASA’s Juno spacecraft.
A 3-6-inch scope will easily show the two most prominent stripes, the North and South Equatorial belts (NEB and SEB). Others, including the South and North Temperate belts, appear using higher magnifications (100-200x) when the air is steady and the planet looks sharp and clear through the eyepiece.
Following are some indispensable websites to help you find out the layout of Jupiter’s moons from any location any night of the year and when to see the Great Red Spot. When using the Red Spot calculator, the time shown will be when the Spot is squarely in view. It’s still well placed up to an hour before and and after that time. If you’re interested in more detailed lists of what to look for on the planet based on the size of your scope, please check out my Sky & Telescope article.
As the solar system’s largest planet, with active weather and dancing moons, Jupiter always makes for a rewarding view.