Astro Bob: See Saturn at its biggest and brightest of the year
On Aug. 14, Saturn and the Earth will be shy of a billion miles apart — as close as they get in 2022.
At a distance of 823.7 million miles (1.3 billion km) it might seem incredible we can see Saturn at all. Yet there it is, shining bright and true in the southeastern sky at nightfall. Even light, traveling at 186,000 miles a second, takes 80 minutes to get from there to here. That's why it feels slightly miraculous that a small refracting telescope magnifying 40x is all you need to reveal its signature rings.
Every 378 days on average, Earth and Saturn line up together on the same side of the sun at opposition. This year, opposition occurs Sunday, Aug. 14, when the two planets will be at their closest for the year.
Earth is the faster planet, so it passes directly between Saturn and the sun on that day. From our perch, we look out and see Saturn directly opposite the sun in the sky, rising in the east about the same time the sun sets in the west.
Planets move slowly to the east (to the left if you face south) as they revolve around the sun. Each year, Saturn slides about 12° eastward through the zodiac constellations. That's the amount of sky covered by your fist held horizontally at arm's length. Even though Earth is the speedier planet, Saturn moves, too. We need extra time to catch up with the ring king, the reason oppositions occur about two weeks later each year.
Right now, the second largest planet shines from Capricornus the sea-goat. If you'd like to picture where Saturn will be in a year, measure off a fist to its lower left. That will bring you to Aquarius. Both constellations have no bright stars, one of the reasons Saturn stands out so well this season.
Another is that its rings are tipped into view. Through a telescope they look as solid as a dinner plate. But the main rings are mostly composed of trillions of individual moonlets ranging from the width of your pinkie fingernail to the size of a guitar. They're made of 99.9% water ice, which is an excellent reflector of light.
Similar to Earth, Saturn's axis is tipped 26.7°. During its 29-year orbit of the sun we first see the south face of the rings fully open and tipped toward us. Then the rings gradually close and become edgewise. In smaller telescopes they completely disappear from view and the planet appears noticeably fainter.
Soon after, the north face opens up and gradually reaches its maximum tilt about 7 years later. In August, Saturn's rings are tipped 14° (about halfway) and closing. They'll be edge-on again during the northern hemisphere fall equinox in March 2025.
A moment ago, I wrote that Saturn moves to the east as it circles the sun. That's true except around the time of opposition. As Earth laps the planet at that time, it appears to reverse direction and move west or "backwards." The same thing happens on the freeway when you pass a car in the left lane. As you zoom past, the car appears to move backwards from your perspective. Called retrograde motion, you can see it for yourself by comparing Saturn's changing position relative to nearby stars over the next few weeks.
Saturn comes into its own through a telescope. The rings look so much like the photos, first-time observers will sometimes kid that they must be fake. At low magnification a single ring shows. But if you increase the power to 125x or better, that ring divides into two: the outer A-ring, separated by a narrow, dark gap called Cassini's Division from the wide, bright B-ring. In 6-inch or larger instrument, a third, translucent C-ring glows subtly between the B-ring and Saturn's butterscotch globe.
Saturn has 82 known moons, more than any other planet in the solar system. Of those, four or five are visible in smaller scopes. The brightest and easiest to spot is Titan at magnitude 8.5, followed by Rhea (9.8 mag.), Tethys (10.3) and Dione (10.5). Titan is half again as big as our moon and massive enough to retain a thick atmosphere. To know exactly where to look for each of them, use Sky & Telescope's Saturn's moons tracker .
Let's hope for nights ahead so we can enjoy Saturn's heavenly crawl, its remarkable rings and the nightly dance of its satellites.