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Exploring the solar system: when seeing isn't believing: this bright feature in Saturn's rings is merely an illusion.

On March 14, 1889, an intriguing announcement was issued by the Central Bureau for Astronomical Telegrams in Kiel, Germany, the international clearing house for announcing discoveries and transient events. It reported that Belgian astronomer Francois Terby had detected a strange new feature in the rings of Saturn.

An experienced planetary observer, Terby was widely admired for his studies of Mars and Jupiter. On the evening of March 6th that year, he was observing Saturn through the 8-inch refractor at his private observatory in the city of Louvain. Using magnifications of 150* to 280*, he noticed a white region bordering the black shadow cast on the rings by Saturn's globe. Extending across the bright A and B rings, this feature remained stationary for more than an hour until clouds intervened. Six nights later Terby saw the bright boundary again in precisely the same location.

A flurry of confirming observations were soon reported, some by astronomers using telescopes of only 3 or 4 inches aperture. Many observers saw Terby's spot as a bright strip with parallel edges, while to others it looked elliptical or D-shaped, with the flat side of the D's semicircle defined by the shadow's edge.

William Brooks, the director of the Smith Observatory in Geneva, New York, reported that Terby's spot "comes out almost conspicuously in the 10-inch equatorial [refractor] at times." Visible with all magnifying powers from 80* to 450*, it was usually most conspicuous with a power of 150*. "To me the brilliancy of the white region appears variable," Brooks wrote. "Fluctuations of light at irregular intervals of a few minutes have been detected by careful scrutiny."

Despite such corroborating testimony, the Terby White Spot's lack of motion caused many to immediately question its reality. British amateur astronomer William Noble argued that "the white spot on Saturn's ring can have no objective existence," because "the rotation of the ring must carry the luminous patch round and, instead of remaining persistently in contact with the shadow of the ball, it must travel right round the planet."

Andrew Ainslie Common presented compelling evidence that the Terby White Spot is an optical illusion. He reported that when he used a wedge-shaped neutral density filter to gradually attenuate the brightness of Saturn's image, "the ring disappears at the same time all round, whereas if the white spot had any real existence (i.e. if it were really brighter than other parts of the ring), it would have remained visible after the other portions of the ring had been cut out by the dark wedge."

Common attributed the Terby White Spot to an effect of the stark contrast between the sunlit surface of the rings and the adjacent black shadow of the globe. He noted that an observer can create a spurious, narrow band of increased brightness on any part of the rings (or even the globe itself) simply by placing Saturn partially out of the field of view so that its image is in contact with the eyepiece's field stop.

Another British amateur, J. E. Drower, offered further damning evidence. "I have got a precisely similar appearance to that described by Dr. Terby," he wrote, "by making a small model of Saturn with a ball and cardboard ring. The latter I nicked on the edge and bent slightly convex until I got a shadow of the ball projected upon it by a strong light."

The Terby White Spot is a striking example of an optical phenomenon first described by the Austrian physicist and empirical philosopher Ernst Mach in 1865. Mach noted that the human eye-brain combination exaggerates the contrast between light or dark areas at the borders of adjacent extended surfaces of differing brightness. Contours and borders dominate human visual perception because edges are accentuated by these features, known as "Mach bands."

In recent years I've encountered remarks posted in online discussion groups alleging that the Terby White Spot must have some basis in reality because it appears in many webcam images of Saturn, often with an uncanny similarity to its appearance in sketches by visual observers. Nothing could be further from the truth. The unsharp masking and wavelet sharpening image-processing algorithms mimic very closely the edge enhancement that occurs in the eye-brain's neural network.

Saturn will reach opposition on May 10th this year. For about 10 days before and after that date, the shadow cast by the globe on the rings will disappear as the Sun, Earth, and Saturn line up. The globe's shadow will reappear on the following (eastern) side of the ring in late May, and will be widest at quadrature on August 9th, when Saturn is 90[degrees] from the Sun. On late spring and early summer evenings Saturn will be well placed in the southwestern sky, affording an opportunity to see the Terby White Spot and answer the old question: "Who are you going to believe, me or your own lyin' eyes?"

Thomas A. Dobbins
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Title Annotation:OBSERVING
Author:Dobbins, Thomas A.
Publication:Sky & Telescope
Geographic Code:1USA
Date:May 1, 2014
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