A sentinel in the autumnal sky, Jupiter marks its closest opposition since 1963.
Have you yet been asked about that "really bright star" in the eastern sky yet? I've already fielded a half-dozen eager queries. Of course, it's Jupiter, the biggest planet of all, and its brilliance is attracting a lot of attention this fall.
Jupiter reaches opposition on September 26th just 591 million kilometers (367 million miles) from Earth, the closest they'll pair for the year. Opposition distances vary depending on where the planet happens to be in its orbit when opposition comes around. The closer perihelion and opposition dates align, the closer the two planets will draw together and the brighter and larger the gas giant will shine.
This go-round, Jupiter lines up with Earth just four months shy of its January 21, 2023, perihelion. It hasn't been this close since the October 1963 opposition and won't be again until October 7, 2129. That's why it appears exceptionally large (49.9″ across) and bright (magnitude –2.9). But that's only half the story.
After digging coal in the mines of Sagittarius and Capricornus for the past couple of years, Jupiter has risen to the celestial equator in Pisces. From latitude 40° north it now beams from an altitude of 50° at meridian passage with minimal atmospheric extinction. The mighty orb also dominates a large region of the sky with few bright stars. Fomalhaut (magnitude 1.2) and Saturn (+0.5), located 35° and 44° away, respectively, make only feeble competitors. Taken together you can see why Jupiter's having a moment in 2022.
Here's a fun naked-eye project. Now through late November, Jupiter moves westward in retrograde motion below the Great Square of Pegasus across the faint stars of Pisces. Can you detect its movement in a day? Two days? Fortunately, we have an excellent measuring stick: Alpha (α) Andromedae, better known as Alpheratz, and Gamma (γ) Pegasi, or Algenib. The pair form the eastern side of the Great Square. On September 21st, Jupiter will appear almost exactly below the two stars. A night or two later, you should be able to detect a slight misalignment. Let us know your result.
For a much more difficult catch try spotting either Ganymede (magnitude 4.4 at opposition) or Callisto (5.4), the planet's two bright outer moons, without optical aid. At maximum elongation this apparition Ganymede stands off about 5.6′ from the glaring planet and Callisto 10.5′. Pick a time when either or both reach maximum elongation using Sky & Telescope's Jupiter's Moons tool. Then hide Jupiter behind a roof line, power pole, mountain top, or tree trunk and use averted vision till it hurts. Confirm your observation with binoculars after you're sure you've spotted them.
The Jupiter's Moons calculator also displays daily eclipses, occultations, transits, and shadow transits. An eclipse occurs when a moon passes into Jupiter's shadow; an occultation takes place when a moon slips behind Jupiter from our perspective. The moons also transit or pass in front of the planet and trail shadows on its cloud tops called shadow transits. These events are so much fun to watch. To make sure I don't miss the juicy ones (shadow transits are my favorite) I run the simulations, write down the dates and times, and then tuck that information into my eyepiece case.
Last year, the Earth crossed the orbital plane of the four Galilean moons. Since the bright moons orbit closely within Jupiter's equatorial plane, we watched them shuttle back and forth on either side of the planet in pretty much a straight line. This year, the orbits are no longer coplanar, and the moons trace out narrow ellipses around the Jovian globe. When near Jupiter, they can appear remarkably out of line, producing all manner of improbable configurations. One night a few weeks ago, they enclosed the planet in an attractive trapezoid. The quartet will still occasionally align in a tidy line but only when all lie at or near maximum elongation.
Besides the four Galilean moons, it's possible to see a fifth Jovian satellite, Himalia. At opposition it glows feebly at magnitude 14.5 about 23′ north of Jupiter, close enough to be swamped by the planet's glare. Better to wait until early November when the moon stands off about 45′ to the west-southwest of Jupiter. Greatest elongation of about 1° occurs in early December, when Himalia will be a few tenths of a magnitude fainter. You can pinpoint the moon's position by using the SkySafari app, Guide 9.1, the Minor Planet Center's Natural Satellites Ephemeris Service, or go to the JPL Horizons System site. With the latter two you can create your own ephemeris, then hand-plot the moon's location on paper or in Stellarium. Planetary photographers can shoot for close-in Amalthea (magnitude 13.9) and fainter but more distant Elara (magnitude 16.5).
If you like zebras you'll love Jupiter. The planet presents a wealth of detail through the telescope including its famous dark stripes called belts that alternate with paler zones. Even an 80-mm (3-inch) scope will show the two most prominent belts, the North Equatorial Belt (NEB) and South Equatorial Belt (SEB). This season, I've observed that the NEB appears noticeably narrower and darker than the SEB on the hemisphere it shares with the Great Red Spot (GRS), while on the opposite side of the globe, the two look more alike.
Between the two belts lies the bright Equatorial Zone (EZ), home to looping, gray festoons that are visible at magnifications of 150× or more. A #80A blue filter will reduce glare and enhance the contrast of the reddish belts and festoons as well as assist in finding the Great Red Spot. Although the GRS is an attractive orange-red, it doesn't exactly jump out at you. Wait until the Spot appears front and center — within an hour on either side of Jupiter's central meridian — and use 100× or more. You find the times of its meridian passage at Sky & Telescope's Great Red Spot Transit Times calculator.
Shrinkage is one reason for the Spot's decreased visibility. When Voyager 2 flew by in 1979, it recorded a width at around 25,000 kilometers, or about twice as big as the Earth. It's now closer to 1.3 times as large. A monster Jovian hurricane with winds reaching 680 kilometers per hour (420 miles per hour), the GRS has persisted for hundreds of years.
On nights of good seeing you can explore beyond the equatorial belts, the prominent South Tropical Zone (STZ), and North Temperature Zone (NTZ) in search of the North Temperate Belt (NTB) and others. Both polar regions appear ashen gray and are frequently bounded by partial belts. Given its fast rotation period of 9.9 hours, there's always something new coming over the Jovian limb, so it makes sense to revisit the planet more than once each night you're out. Jupiter's clouds and weather are subject to continual change. Storms blow up and fade. Belts darken or disappear. Even the Great Red Spot's color can be unpredictable. What's the forecast for Jupiter tonight? Go have a look!