May's Eclipse: Flirting with Totality
At Atlanta, Georgia, fourth- and fifth-grade students safely watch "The Big Eclipse" through Mylar-filtered windows cut in cardboard boxes. Photograph by Billy G. Downs for the Atlanta Journal-Constitution.
LETS CONSIDER the basics: When
/ the Moon passes directly across the Sun, two things can happen. If the lunar disk is large enough, it will entirely cover the blinding solar photosphere, create a to-tal eclipse, and reveal the Sun's hidden at-mospheric wonders. On the other hand, if the Moon is too small, a ring of bright sunlight will remain and mask these phe-nomena from our eyes.
But in rare cases, when an eclipse just borders on being total, as it did on May 30th, these distinctions don't necessarily hold. Prognosticators of what might be seen at this spectacle played down pros-pects of glimpsing the corona and allied phenomena. Rightly so, for the real and predictable show was to be the dance of Baily's beads. These gleaming beacons are tiny patches of the solar surface that shine through deep lunar valleys just before the Moon's limb covers and uncovers the Sun's.
Yet, on May 30th it turned out that the prophets were too conservative. True, the beads flashed all around the Moon's dark disk. Geometrically it was a ring eclipse, but spiritually it was total!
For example, at Greer, South Carolina, the undersigned first spied the corona 73 seconds before central eclipse and kept it in view for the next 2m36s. This sighting was particularly surprising, for it was made at f/23 on a camera viewfinder and through a neutral-density filter that re-duced the light 100 times. At its best, the Sun's atmosphere reached outward a solar radius, and some structure appeared. The large prominence photographed on the west limb (see pages 103-105) was invisi-ble, but the small, bright one to its south was conspicuous.
At least three other observers also saw the corona in their camera viewfinders. Salvatore LaRiccia in Atlanta, Georgia, first glimpsed it 45 seconds before mid-eclipse with his f/33 system. The corona was also evident to Annemarie Franklin and Wendy Carlos as they prepared to take the spectacular picture reproduced on page 104. In an allied observation, Sky & Telescope staffer John Briggs was able to spot the corona with his unaided eye 48 seconds after mid-eclipse by carefully blocking the uneclipsed limb of the Sun with his thumb.
The solar corona may have been extra-ordinarily bright on May 30th, as suggest-ed by an unusual observation Tom Carter made at Atlanta. While projecting the Sun's image onto a white piece of paper with a 4-inch f/15 refractor, he saw "the coronal glow extending 1 or 2 are minutes from the limb of the Moon just before and after mid-eclipse and outlining that portion of the lunar limb not silhouetted against the solar disk."
BRING ON THE BEADS!
At an eclipse like this one, in which the circumference of the Moon is just a tad smaller than that of the Sun, the rims of the solar and lunar disks have nearly the same curvature. Thus, they can almost superimpose over long arcs, giving many hills and valleys a chance to produce beads. But these near-superpositions last only a few seconds, causing the panoply of beads to change rapidly, "like a fireworks pinwheel" in the words of Robert Slobins.
"At no time was there an unbroken ring," writes John W. Stewart, who ob-served from just outside Greensboro, North Carolina. This sentiment was echoed by all observers along the eclipse track in the United States. Surprisingly, prédictions had indicated that the ring would be unbroken from the Mississippi-Alabama border westward. Only in Mexico, where the Moon's diameter appeared considerably smaller than the Sun's, was an unbroken ring seen — as by Stephen Edberg's group, which set up in El Rincón, a village about 15 miles from Mexico's west coast.
Laurence Marschall, who observed from Greensboro, was particularly struck by the length of time Baily's beads remained visible, "for several minutes before and after mid-eclipse." His statement agrees with what the seven-member Sky & Telescope team saw at Greer, where the first bead appeared at the south cusp of the Sun 2m 53' before mid-eclipse. Corresponding times were 2m 47* for participants in the World of Oz expédition at Greensboro, and 2m 01$ for Larry Kelsey and Darrel
Hoff at Atlanta (where the Moon's disk was a bit smaller relatíve to the Sun's).
Predicting where the beads will appear, by knowing the profile of the Moon's edge, has become a fine art. In the bottom il-lustration on the next page, compare the locations of dips (lunar valleys) in Fred Espenak's plot for the Greensboro airport and where the beads actually appeared. Incidentally, this city lay only about 60 miles west of where storm clouds began to obliterate the eclipse track. The most easterly observation received of the ring eclipse came from Peter Grant, who set up his camera at Buffalo Springs, Virginia, near the North Carolina border.
The mysterious shadow bands (S&T: February, 1984, page 116) remained just that. While definite sightings were report-ed by Walter Mudgett and Marschall at Atlanta and Greensboro, respectively, other observers situated near them didn't notice any bands.
According to Mudgett, "The bands were copious for about 10 to 12 minutes centered on mid-eclipse. Viewed against light-colored pavement, they were of low contrast but easily observed, except when the eclipse was darkest. Two band sys-tems were present. One headed about 12° to the right of the moving axis of the Moon's shadow, while the other headed to the left by the same amount. The band centers were separated by 2Vá meters, and the bands traveled that distance in a sec-ond (both estimated values are uncertain by 30-40 percent)."
Marschall also remarked that the bands were of relatively low contrast, resembling