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Anomalous appearances: how will your observations of the Mercury transit stack up to the historical record?

Amateur astronomers around the globe are making preparations to observe this month's transit of Mercury (see p. 48, this issue). Passages of the tiny planet across the face of the Sun have never stirred as much popular interest as the far rarer transits of Venus, those spectacular events that have inspired expeditions to far-flung corners of the globe for several centuries. However, sufficient attention has been paid to transits of Mercury that many anomalous appearances have been reported over the years.

These oddities have long been the subject of speculation and even heated debate. Many astronomers have tried to explain the mysterious "black-drop effect" frequently seen when Mercury's disc touches the inner edge of the Sun at second contact (near the beginning of a transit) or at third contact (near the end of a transit). (See S&T: Jan. 2012, p. 73.) But other strange visual effects have sparked interest as well. For example, the anomalies observed during the Mercury transit on November 5, 1868, were the subject of a detailed account in the Monthly Notices of the Royal Astronomical Society written by the celebrated British amateur astronomer William Huggins, who is chiefly remembered today for using a spectroscope to determine the chemical composition of stars and nebulae. A wealthy dealer in silks and linen with a keen interest in the sciences, Huggins used the proceeds from the sale of his family's lucrative textile business to build a well-equipped observatory on Upper Tulse Hill in south London. Its centerpiece was a telescope that would still be the envy of most amateur astronomers a century and a half later--an 8-inch refractor featuring a superb objective lens made by the renowned American optician Alvan Clark.

Huggins enjoyed decent daytime seeing during the November 1868 transit. "The Sun's edge was a little tremulous from atmospheric agitation," he recounted in his notes, "but the solar surface was so well defined that the bright granules of which it is composed could be distinctly seen."

Shortly after the entire disc of Mercury was silhouetted against the brilliant backdrop of the Sun's surface, Huggins noticed an even brighter, sharp-edged halo surrounding the black dot:

The breadth of the luminous annulus was about one-third of the planet's apparent diameter. The aureola did not fade off at the outer margin, but remained of about the same intensity throughout, with a defined boundary. The aureola was not sensibly coloured, and was only to be distinguished from the solar surface by a very small increase of brilliancy.

There were a few corroborating observations of the bright halo surrounding miniscule Mercury that day. Observing with the 12.7-inch refractor at the Royal Observatory at Greenwich, E. J. Stone reported: "With power 137, a ring of light was clearly visible around the disc of Mercury. It extended to a distance of nearly a semi-diameter." But scores of other observers failed to see any trace of the halo, and even Stone was cautiously skeptical, writing: "I am of the opinion that it arose from mere contrast."

Similar effects had been reported for more than a century. First described by the French astronomer Francois de Plantade in 1736, the luminous ring was documented again by his countryman Honore Flaugergues during the transits of 1786, 1789, and 1799. To the German astronomer Johann Hieronymus Schroter, the halo was a pale, almost ghostly, object that was "scarcely brighter than the surface of the Sun."

A few observers interpreted the halo as sunlight refracted by a dense, distended atmosphere surrounding Mercury. Most, however, compared it to the bright band bordering the limb of the airless Moon that had been reported during the partial phases of solar eclipses, a phenomenon that Astronomer Royal George Biddell Airy had dismissed in 1864 as "strictly an ocular nervous phenomenon." In the 1881 edition of his classic observing handbook Celestial Objects for Common Telescopes, the Reverend Thomas William Webb wrote off the halos as "deceptions from the violent contrast and the fatigue of the eye." To the French astronomer Camille Flammarion, they called to mind the illusory bright aura that he repeatedly saw surrounding the shadow cast by a hot air balloon onto sunlit prairies during his many ascents.

The halo is a striking example of a phenomenon first described by the Austrian physicist Ernst Mach in 1865. Mach noted that the eye-brain combination invariably exaggerates contrasts at the borders of adjacent extended surfaces of differing brightness. Observational astronomy is rife with examples of these "Mach bands," notably the "Terby White Spot," a spurious bright feature often seen bordering the intensely black shadow cast by Saturn's globe across the planet's rings (S&T: May 2014, p. 54).

But in addition to the bright halo, Huggins witnessed an even more curious phenomenon:

Almost at the same moment that I first perceived the surrounding annulus of light, I noticed a point of light nearly in the centre of the planet. This spot had no sensible diameter with the powers employed [120x and 240x], but appeared as a luminous point ... I kept it steadily in view until that part of the planetary disc, where the point of light was situated reached the Sun's limb, I then ceased to see it.

Like the bright halo, points of light and diffuse bright patches were frequently reported during transit events. Most dispatches described features that were centrally located, although in some instances they were offset towards the edge of the planet's tiny black disc.

The fact that a host of observers, generally of equal skill and experience, and equipped with telescopes of comparable size and quality, saw nothing unusual while their colleagues were reporting these curious appearances was certainly troubling, and a matter of much discussion. Suggestions that erupting volcanoes or intense auroral displays might account for lights rivaling the solar photosphere in brightness were deservedly taken with a grain of salt. Far more plausible were the explanations involving internal "ghost" reflections from the surfaces of the telescopes' objective and eyepiece lenses, which lacked modern antireflection coatings.

In 1850, the Reverend Baden Powell, Professor of Geometry at Oxford University and father of the founder of the Boy Scouts, suggested that optical diffraction was responsible. Eighty years later the French astronomer and optician Andre Couder was able to reproduce the luminous spot in his laboratory at Meudon Observatory in Paris. While photographing black circles projected against a brilliant background, Couder was able to record the bulls-eye pattern of the bright Airy disc at the center of the circle, surrounded by faint diffraction rings. A slight misalignment of the optical elements displaced the spot toward the edge of the circle. Even with perfectly aligned lenses, when the background was not uniformly illuminated (to mimic the darkening that occurs near the limb of the Sun), the spot was not concentric.

Despite Couder's convincing experiments, more than mere optical effects seemed to be at play. Observing the November 8, 1881, transit of Mercury through a 4.75-inch refractor equipped with a Herschel wedge at the Sydney Observatory, Australian astronomer Lawrence Hargrave saw a central bright spot very distinctly three minutes after ingress. However, he soon realized that it would disappear "on looking steadily at it." Reports like Hargrave's led the late William Corliss, who compiled several catalogs of astronomical anomalies, to conclude that "something akin to those optical illusions where grey images appear out of nowhere amid geometrical designs" might be involved.

The upcoming transit of Mercury will be an opportunity to glimpse these strange, elusive anomalies once again. The knowledge that they're strictly in the eye of the beholder shouldn't rob them entirely of the ability to evoke a sense of wonder. Through them, we're visually connected to our observing forebears.

Contributing Editor Thomas A. Dobbins has observed most reported phenomena on the planets, both real and illusory.
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Title Annotation:Mercury Transit
Author:Dobbins, Thomas
Publication:Sky & Telescope
Geographic Code:4EUUK
Date:May 1, 2016
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