You've probably already seen Jupiter without even knowing it. It's that brilliant "star" in the southeastern sky that clears the trees around 9:30-10 p.m. local daylight time. Two fists to its right is fainter Saturn. Both planets light up the dim constellation Capricornus the Sea-goat like a Christmas tree.

When Earth and Jupiter pair up on the same side of the sun at opposition they're at their closest. This year that distance is 373 million miles or 600 million kilometers. Jupiter oppositions occur about every 13 months. Contributed / Bob King
When Earth and Jupiter pair up on the same side of the sun at opposition they're at their closest. This year that distance is 373 million miles or 600 million kilometers. Jupiter oppositions occur about every 13 months. Contributed / Bob King

Jupiter will be at opposition later tonight (August 19-20) and closest to the Earth for the year at a distance of 373 million miles (600 million kilometers). Every 13 months or so, Earth and Jupiter line up together on the same side of the sun. Because Earth is in the middle with the sun and Jupiter on opposite sides, this special circumstance is called opposition.

From our perspective, the sun and Jupiter appear at opposite ends of the sky. When the sun sets in the west, Jupiter rises in the east and shines all night.

Jupiter's darkest belts are visible in small telescopes, but a larger instrument will reveal many more. Clouds are arranged in parallel bright zones and dark belts. The Great Red Spot lodges in a "hollow" just south of the South Equatorial Belt (SEB). This photo was taken on Saturday, August 14, 2021. Contributed / Christopher Go
Jupiter's darkest belts are visible in small telescopes, but a larger instrument will reveal many more. Clouds are arranged in parallel bright zones and dark belts. The Great Red Spot lodges in a "hollow" just south of the South Equatorial Belt (SEB). This photo was taken on Saturday, August 14, 2021. Contributed / Christopher Go

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Being close, Jupiter is also extra bright. At magnitude -2.9 it's only about one magnitude fainter than Venus right now. Venus is typically only visible during twilight, which dilutes its brilliance, whereas Jupiter blazes in a dark sky.

It's also positively gigantic. It would take a "necklace" of 11 Earths to reach from one side of the planet's disk to the other. If you have a pair of 10x binoculars and focus them sharply on Jupiter you can actually make out the shape of its glaring globe, which is currently almost one arcminute (1/30 the the apparent diameter of the full moon) across.

We'll really get an eyeful on Saturday night, August 21, 2021, when the full moon lines up below Jupiter. Don't miss it! Contributed / Stellarium
We'll really get an eyeful on Saturday night, August 21, 2021, when the full moon lines up below Jupiter. Don't miss it! Contributed / Stellarium

Depending on circumstances, four of Jupiter's brightest moons are visible in the same instrument. They look like tiny stars in a line very close to the planet. I say "depending" because one moon or another regularly passes in front of or behind Jupiter. For instance, last night (August 18) I saw only one moon, Callisto, in my binoculars. The other three were much closer to Jupiter and swamped by its glare.

While binoculars are useful for Jovian moon observation a telescope is the way to go. And you don't need a big one. A 3-inch (90 mm) refractor or 4.5-inch reflector will show all the moons whenever they're visible. You'll also see shadow transits, when moons pass in front of Jupiter and cast inky black shadows on its cloud tops. Jupiter also eclipses and occults (hides) the moons. This constant hum of activity make the solar system's biggest planet a joy to observe every clear night.

On Thursday, August 19, three moons will be visible around Jupiter until 11:10 CDT, when Io will reappear off the west (right) edge of the planet. This diagram of the moons is set for around 10 p.m. CDT. From 8:52-11:10 p.m. CDT Io's shadow will be visible transiting Jupiter's disk. Contributed / Stellarium
On Thursday, August 19, three moons will be visible around Jupiter until 11:10 CDT, when Io will reappear off the west (right) edge of the planet. This diagram of the moons is set for around 10 p.m. CDT. From 8:52-11:10 p.m. CDT Io's shadow will be visible transiting Jupiter's disk. Contributed / Stellarium

To find out what moons are currently visible and when to expect eclipses, shadow transits and the like, use the Jupiter's Moons tool. When you call up the page, the time shown is the current time (on the 24-hour clock), but you can change it to whatever time you like. The site uses UT or Universal Time, basically the same as Greenwich Time. To convert to your local time, subtract 4 hours for Eastern; 5 for Central; 6 for Mountain and 7 for Pacific.

For example, if you're on Central Daylight Time and it's 18:30 UT, August 19, your local time is 13:30 or 1:30 p.m., August 19. If the time is 1:30 UT, August 20, that converts to 8:30 p.m. the prior evening, August 19.

If for any reason you find time conversions confusing, I suggest you download the free program Stellarium for Mac or Windows. After you've installed it and selected your location, search for Jupiter using the magnifying glass on the left-side pop-out menu. Then zoom in with the mouse scroller to see exactly where the moons are at that moment. Unfortunately, Stellarium doesn't offer a list of events. For that, you'll need to return the aforementioned site. Or for $2.99 you can download the Sky & Telescope app JupiterMoons (for iOS) or pick up the free Moons of Jupiter for Android.

Jupiter displays dark belts and light zones of ammonia ice clouds which sweep around the planet at speeds around 325 mph. Other wind-driven features abound, such as the Great Red Spot, multiple small oval storm cells, and curly festoons. North is up and east left. The planets rotates from east to west — or left to right — in this image. Contributed / Damian Peach with labels by Bob King
Jupiter displays dark belts and light zones of ammonia ice clouds which sweep around the planet at speeds around 325 mph. Other wind-driven features abound, such as the Great Red Spot, multiple small oval storm cells, and curly festoons. North is up and east left. The planets rotates from east to west — or left to right — in this image. Contributed / Damian Peach with labels by Bob King

Jupiter is all clouds with no solid surface. Its rapid rotation of just under 10 hours, combined with fierce winds that can gust up to 900 miles an hour (1,450 km/hour), have stretched and ordered the clouds into a series of alternating dark belts and bright zones. Zones are where clouds of ammonia ice crystals are rising. Belts are sinking clouds tinted various shades of brown and red from sulfur, carbon and other compounds.

Astrophotographer Damian Peach prepared this montage showing how much the Great Red Spot (GRS) has shrunk since the late 1800s. The cause is still unclear. The elongated dark spots along the northern edge of the NEB are cyclonic storms called barges. Contributed / Damian Peach, 1891 from Lick Observatory
Astrophotographer Damian Peach prepared this montage showing how much the Great Red Spot (GRS) has shrunk since the late 1800s. The cause is still unclear. The elongated dark spots along the northern edge of the NEB are cyclonic storms called barges. Contributed / Damian Peach, 1891 from Lick Observatory

The thickest, darkest belts are the North and South Equatorial Belts (NEB and SEB) that straddle the pale yellow Equatorial Zone (EZ). Right now, the NEB appears distinctly red in 8-inch and larger scopes. The Great Red Spot (GRS) looks pink to my eye and requires a 6-inch telescope magnifying around 100x or higher to see. It's not only on the pale side but considerably smaller than it was decades ago as shown in the accompanying montage.

Astronomers aren't sure what's causing the shrinkage, but the spot's size has always been variable since it was first seen in the 17th century. Its color also changes from brick red to pale pink, salmon or even yellow-brown depending on the balance of chemicals whirling around its interior. The best time to spot the Red Spot (excuse the pun) is when it transits Jupiter's meridian and appears squarely in view. Click here to find out those times and then plan your observing session accordingly.

Jupiter's rapid spin means that you can watch the GRS and other features shift from east to west in as little as an hour. While you're doing so take note of the planet's shape. Since it's made of gas and not rock like the Earth its spin flattens the globe into a slightly oval shape easily visible to the eye.

Jupiter's bright and fun to watch in any instrument you choose — naked eye, binoculars or telescope. With smoky skies so common over the U.S. and Canada right now it's also one of the few sights you can count on seeing!

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.