Looking Deeper
This enhanced-color image of Saturn's rings was taken in 1981 by the Voyager 1 spacecraft, which was then 1.6 million kilometers from the planet. All the labeled portions of the ring system, except ring F, are visible from Earth. Controversy surrounds the possibility that Earthbound observers may have seen ring F on several occasions in the 20th century. Click on the image for the complete photo.
Sky & Telescope illustration; image courtesy NASA/JPL.
In a high-quality planetary telescope of at least 6- or 8-inch aperture on a night of excellent seeing, the rings become more interesting. Near the outer edge of the A ring is the extremely thin
Encke Division, an extreme test for any telescope. On a night when the seeing was so steady I could use 450x on my 12.5-inch reflector, I detected the Encke Division for the first time — a complex blur of threadlike details during moments when the air was perfectly steady.
The rings also contain thin, grayish minima in brightness. Julius Benton, the Saturn section coordinator for the Association of Lunar and Planetary Observers (ALPO), claims that as many as 12 are detectable with large telescopes, "of which only about four show any real recurrent visibility from observing night to observing night." Interestingly, Benton claims these brightness minima are known to vary somewhat in both prominence and location.
Ring C, the crepe ring or dusky ring, can be either difficult or easy to make out. Many have seen it without knowing it. Evidence of the C ring is easy to spot when the rings' shadow on the ball appears on their outside. At such times the duskiness you see against the planet just inside the B ring is the semitransparent C.
Changes in the belts and zones become apparent, even obvious, to regular Saturn-watchers — one of the benefits of long-term study. The larger and better your scope the more likely you are to see enough detail to note changes in it.
A rare storm, first seen in September 1994, appeared as a white, arrowhead-shaped feature near Saturn's equator. Such storms are generated by an upwelling of warmer air, like a terrestrial thunderhead. This storm extended about 8,000 miles (12,000 kilometers) east-west. The white clouds are ammonia ice crystals that form when an upward flow of warmer gases shoves its way through Saturn's frigid cloud tops. South is up.
Courtesy R. Beebe, D. Gilmore, L. Bergeron, STScI, and NASA.
Spots and other markings occasionally appear amid the belts and zones. Major white eruptions happen about every 30 years (once per Saturnian year). Lesser bright and dark spots appear more commonly. To predict when a spot will be back at the same location, remember that Saturn's equatorial parts rotate once in about 10 hours 14 minutes. Higher latitudes rotate more slowly, in about 10 hours 38 minutes.
Colors change on Saturn too, but only subtly. The best way to pin them down is to note the relative brightnesses of different areas as seen through red, green, and blue filters. Oddly, the two ends (ansae) of the ring system sometimes appear to differ slightly in color. Using red and blue filters, see if one end looks brighter than the other in either color of light.
Acquired in November 2000 by the
Hubble Space Telescope, this image shows Saturn with its rings open to almost their greatest extent. Most of the features visible here can be detected in large amateur telescopes — though not nearly as plainly and sharply as seen here. South is up.
Courtesy NASA and the Hubble Heritage Team.
Overall, I find that a yellow filter sharpens up the whole planet a trace, probably by suppressing the differences in atmospheric turbulence at the far ends of the spectrum. (Red and blue images quiver and shimmer out of phase with the yellow near the middle of the spectrum. This is the same effect that causes the bright winter star Sirius to twinkle in vivid colors.) A light green filter may slightly improve contrast in the planet's belts and zones.
Saturn enthusiasts interested in long-term observing programs can learn more about the ALPO Saturn Section where observation forms are available online.
