A photo-illustration of the optical illusion known as Mach bands, seen on Saturn. Credit: Astronomy: Roen Kelly according to Ethan Chappel
About every 15 years, Earth passes through the plane of Saturn’s rings, causing them to nearly disappear from view, not to mention generating a host of other interesting phenomena. The next such apparition will take place in March 2025, although Saturn will unfortunately be too close to the Sun (just 9.5° away) for us to enjoy it.
This year, however, the rings narrowed to their minimum on June 25, tilted just 1.9° from the edge. That’s when I discovered a fascinating optical phenomenon at play, one that might make you think twice about whether what you’re seeing is real.
On the morning of June 9, I was showing my wife, Deborah Carter, Saturn in bright twilight through my 3-inch Tele Vue refractor. As I described the planet and its appearance near the edge of the rings (2.0°), she identified several features, including the narrow rings on both sides of the planet, the shadow of the planet on the rings, and vice versa. . None of us, however, could see the rings pass in front of the globe.
After a prolonged study of the ring’s shadow with direct vision, I looked away to one of the rings’ ansae: the bright “handles” that appear on either side of Saturn where the rings arch around the planet. Suddenly and fleetingly, I saw what appeared to be the full extent of the rings cutting across the face of the planet. They appeared bright against the planet’s equatorial belt and hugged the northern edge of the ring’s shadow, though I was suspicious.
Near the center of the planet, the rings appear very foreshortened, much thinner than they are at the loops. Additionally, the shading effects of the ring particles cause the projected section of the ring to appear darker in the center than at the loops. What I saw must have been an illusion.
Indeed, I later remembered an observation made by the American astronomer Edward Emerson Barnard. On October 26, 1891, Barnard used the Lick Observatory’s 12-inch refractor to study Saturn when the rings were tilted only 1.6° off the edge. Even at magnifications of 150x, 175x, and 500x, he reported that the rings were invisible. He was struck, however, by an important visual effect: “Looking at the black trace on the ball, and then glancing at the sky near the sides of the planet, I could, apparently, see for a moment the rings as a faint line of light in the dark sky. I became convinced beyond any doubt that it was an optical phenomenon.
Barnard believed that the faint line of light was an afterimage of the ring’s shadow. “I mention this,” he wrote, “because it might mislead an observer, who would think he had glimpsed the real ring. Perhaps this same phenomenon has relevance to other types of astronomical observations.”
With these words in mind, I observed Saturn again on the mornings of June 18 and 19, following the planet until late twilight. Although my observation and Barnard’s were different, both concerned the shadow of the ring. After staring at the ring’s shadow for an extended period of time and then looking away to the side of the planet, I was able to create a similar, albeit fleeting, optical illusion that exaggerates the edge of the bright equatorial zone next to the ring. shadow of the ring. This phenomenon is known as the Mach band or Mach effect. It is an optical illusion in which the edge of a bright object next to a dark object appears even lighter and vice versa.
Saturn’s rings can also be seen projected a short distance across the planet’s face, where they taper like pincers. I have found that the best time to see this aspect of the ring is during bright twilight, when the contrast between the planet and the sky is at its lowest. I call it “ghosting” because the apparition through a small telescope looks like a ghostly apparition.
Saturn’s rings opened from around 2° in June this year to between 4.5° and 5° in October. So it should now be possible to resolve Saturn’s ring across the planet with sufficient magnification and excellent viewing conditions. This gives us a wonderful opportunity to observe the visual appearance of the tightly opened ring before it begins to close in early December. When we lose Saturn to the Sun’s glare in February 2025, the rings will be less than 3° from the edge. I will be very interested to know what you see or don’t see before this time. The real challenge will begin when Saturn re-emerges in the morning twilight in early May 2025 and the rings open again to 2°.
Be sure to send observations with time and date, telescope used, magnifications and state of the atmosphere to sjomeara31@gmail.com.
