Astro Bob: Jupiter absolutely dazzles this fall
The solar system's largest planet really wants your attention.
Jupiter has the eastern sky all to itself this fall. It's really a head-turner. There are couple reasons for its brighter-than-normal appearance.
First, the planet reached opposition Sept. 26, when it was closest to Earth for the year. This happens about every 13 months, when Jupiter and Earth line up alongside one another on the same side of the sun. Astronomers call it "opposition" because from our perspective, the sun and planet appear in opposite ends of the sky. At sunset, Jupiter rises in the east and remains visible the entire night.
Like all the planets, Jupiter's orbit is an ellipse with one end closer to the sun and the other end more distant. The close point is called perihelion from the Greek peri meaning "near" and Helios, the Greek god of the sun. Jupiter reaches perihelion once every 12 years, the time it takes to circle the sun.
When opposition happens around the time of perihelion, Jupiter and Earth get especially close. This year is one of those times because perihelion is just a few months down the road in January 2023. The planet hasn't been this close since 1963 and won't be again until October 2129.
That's one reason the planet blazes. The other is its location. It shines from Pisces the Fish, one of the dimmest constellations of the zodiac. There are no bright stars nearby to compete, so the gas giant rules the roost. If you're into specifics, the planet shines at magnitude -2.9 (brighter than any point of light in the sky except Venus) and its disk measures about 1/30 the width of a full moon.
I know that sounds small, but it's big for a planet. You'll notice right away if you point a telescope at it. Even with a magnification of 20x Jupiter's shape is obvious. Depending on the night, from 2 to 4 of its brightest moons — Io, Europa, Ganymede and Callisto — will also be visible. The reason the number changes is because on some nights, one or more of these delightful orbs is passing in front of or behind the planet or temporarily eclipsed by Jupiter's shadow.
If you have a 4-inch or larger telescope you can see the shadows the moons cast on Jupiter's cloud tops during shadow transits. Ganymede, the biggest of them at 3,274 miles across (half again as big as our moon), casts the largest shadow.
You can identify the moons and also get a list of eclipses, shadow transits and occultations (when a moon passes behind Jupiter) online at Sky & Telescope's Jupiter's Moons calculator. Times shown are UT or Universal Time. The site also shows the offset between UT and your time zone. Subtract that number from UT if negative (or add if positive) to convert to your local time.
Don't own a telescope? Jupiter's satellites are also visible through 10x binoculars as tiny "stars" in a tight bunch on either side of the planet. Focus sharply and hold your instrument as steady as possible. I often buttress my pair against the roof of my car.
NASA's Juno probe , which has been photographing the planet since 2016, will make an extremely close flyby (322 miles / 518 km) of Europa on Sept. 29 at 4:36 a.m. CDT. We may get to see the photos — expected to be the best yet of its cracked, icy surface — as soon as this weekend. More information here .
Finding Jupiter is easy. It lights up the eastern sky around 8 p.m. local time in evening twilight and looks like a super-bright "star." Rising higher through the night, the planet passes due south about 12:30-1 a.m., when it reaches peak elevation. The Great Square of Pegasus, fall's best known asterism, shines a fist-and-a-half directly above the planet.
Jupiter will shine brightly throughout the fall, so you'll have lots of time to bask in its light or study it in a telescope.
Jupiter has no solid surface. It's all atmosphere and clouds in a riot of colors and forms when viewed through larger telescopes or better, by spacecraft. Clouds are contained in alternating dark bands and bright "zones" whisked along at different wind speeds in excess of 400 miles an hour (620 kph) and in alternating directions. Storms shaped like spinning tops abound. The biggest, the Great Red Spot, has been shrinking in recent decades, but it's still big enough to swallow the Earth and then some.
Through your telescope, look for two pale gray, parallel bands across the center of the planet. These are the North and South Equatorial Belts. Between them, like a nice egg salad between slices of dark rye, you'll spy the brighter Equatorial Zone.
Additional thinner bands are visible on calm nights when the air settles. I like to use a magnification of 75x to 100x for clear views of the belts and zones. Jupiter rotates rapidly, once in 9.9 hours. If you hang with the planet for a while you'll actually be able to see new features rotate around to the front side.
To spot the Great Red Spot (GRS), which has a pretty red-orange color this season, a 4-inch or larger scope with a magnification of 100-150x will do. The best and easiest time to view the GRS is when it's front and center on Jupiter, or as astronomers would say, when it transits the planet's central meridian. You'll find those times at Transit Times of the Great Red Spot at skyandtelescope.org . Just click the Initialize to Today button for a list of local transit times. It's best to observe within an hour of either side of transit. On Wednesday night, Sept. 28, the GRS transits around 9:30 p.m. CDT. Perfect!
No matter how you observe Jupiter, whether with the naked-eye, binoculars or telescope, let it guide you to the sky this autumn season.