Those few times I've been up at dawn this season, I've enjoyed watching Jupiter and Saturn slowly climb the morning sky. The two giants are now easily visible in the southeast at the start of twilight about an hour and a half before sunrise. On Tuesday, May 11, Jupiter finally cleared the trees at my house, allowing a first look in the telescope.
There's always a moment of excited anticipation when bringing the planet into focus again after a long hiatus. With no solid surface, Jupiter is all clouds and weather. Just as changing weather patterns alter Earth's external appearance from space, so too at the fifth planet. The Great Red Spot changes colors. New storms form and dissolve. Even prominent cloud belts can disappear and reappear.
When I pointed a 6-inch telescope at the planet on May 11, the planet's two most prominent
"stripes" immediately jumped out — the North and South Equatorial Belts (NEB and SEB). Both straddle either side of Jupiter's equator. The northern belt was dark and obvious, while its southern counterpart appeared paler and thinner.
Jupiter displays a series of alternating dark belts separated by bright zones. Gas descends in the belts, where temperatures are warmer, and wells up in the chillier zones. The biggest zone is the Equatorial Zone (EZ). Normally white or pale yellow, this season much of it is veiled by a yellow-orange haze.
Zones appear pale because of upwelling cirrus-like clouds made of ammonia ice crystals. In the belts, as the air descends and warms, the bright ammonia ice evaporates, which allows us to gaze into the darker clouds below. Exactly what colors them is unknown, but their rusty hues may be due to compounds of carbon, sulfur and phosphorus that form at these lower, warmer altitudes.
Wind speed and direction varies with latitude at Jupiter. Near the poles it barely blows but increases to over 300 miles an hour (500 kph) within the prominent belts and zones on either side of the equator. The winds zig-zag from west to east and back again as you pass from one zone or belt to the next.
The Great Red Spot (GRS) is a persistent, hurricane-like storm that has been observed since the mid-17th century, while the white ovals are smaller storms with shorter lifetimes. Through a small 4-inch to 6-inch telescope, ovals are really tough to see, but the orange-red GRS is visible at magnifications of 100x and higher. The best time to see it is when it faces us square-on during transits. To find transit times for your location, click here. You have about two hours centered on the transit time to see the Spot best. . For all its heft, the planet rotates rapidly, taking only about 9.9 hours to make a complete spin.
Jupiter's four brightest moons — Io, Europa, Ganymede and Callisto — are visible in even the smallest telescopes with magnifications as low as 25x. Since each revolves around the planet with a different period, ranging from 1.8 to 16.7 days, they form a new arrangement every clear night. Watching them is a delight, like seeing a solar system from afar, with mighty Jove sitting in for the sun. To identify which moon is which you can download the free Stellarium program, select your location and then set the time to when you want to see Jupiter. Or you can go just online and use Sky & Telescope's Jupiter's Moons finder.
As Jupiter rises later and becomes better placed for viewing, we'll explore more things you can see there with a small telescope in a future article.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.