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The ‘black drop’ effect of sunspots

Can this well-known phenomenon during planetary transits also be observed with sunspots?
OMearaStephen
The Great American Eclipse has come and gone, and many observers have already shared their impressions of totality with me. And now I’d like to pass those exciting stories on to you.

A sunspot train

Observers using safely filtered binoculars and telescopes enjoyed views of a sunspot “train” with roughly a dozen umbral (dark inner) cores stretching some 140,000 miles (225,000 kilometers) along the Sun’s equator. Another sunspot group near the Sun’s eastern limb joined the train — a pleasant surprise for an eclipse during a solar minimum.

At the Oregon Star Party (OSP), which lay in the path of totality, the plethora of spots gave observers several opportunities to witness an optical phenomenon I had proposed might be visible during the partial phases. During a talk the night before the main event, I had asked observers for their help searching for it. And on eclipse day, several of them took on the challenge and reported success.
omearamoon
The train of sunspots near the center of the Sun was visible even through the smallest telescopes.
John Chumack
A parade of illusions

The phenomenon I had mentioned is akin to the “black drop” effect that observers have reported during transits of Mercury and Venus. Torbern Bergman first noticed it during the 1761 transit of Venus from Uppsala, Sweden, reporting that a dark “ligature” (resembling a narrow bridge) joined the silhouette of Venus to the inky background of the sky beyond the Sun. Imagine the black drop as gum on a hot pavement that clings to a shoe — until it breaks free as the shoe lifts.

Astronomers still debate the cause of the effect. Theories include atmospheric turbulence, aberrations in optical systems, and eye-brain visual deceptions. In the Proceedings of the International Astronomical Union Colloquium, No. 196 (2004), Jay Pasachoff, Glenn Schneider, and Leon Golub demonstrated how the Sun’s limb darkening is a principal culprit. Now observations made at OSP during the 2017 solar eclipse add yet another dimension to the effect.

During the eclipse’s partial phases, observers using telescopes viewed the black drop effect whenever the dark lunar limb encountered a sunspot’s umbral core. Several observers saw the effect independently using telescopes with apertures ranging from 6 to 16 inches.

Judy and Chuck Dethloff, my wife, Deborah Carter, and Richard Just all saw the effect as the Moon covered the length of the sunspot train and the eastern group before totality, and uncovered them afterward. As the Moon passed over the spots, it created a parade of illusions mostly near the center of the Sun’s disk — away from any limb darkening.

omeara3pic
The photographs at left and right show the approach and disappearance of a sunspot near the lunar limb during the 2017 solar eclipse. The middle image has been altered with software to show what the author saw visually.
Stephen James O'Meara
A black bridge, teardrops, and more

I got to see only one event (through Judy’s 16-inch Dobsonian-mounted reflector), but it was dramatic enough to convince me of the effect’s reality. As the Moon’s limb approached the core of one spot, a thin ligament appeared like a narrow bridge that joined the spot’s core and the Moon’s advancing silhouette. Over the course of seconds, this thread of darkness vanished and reappeared repeatedly before becoming a solid bridge that rapidly melted into the lunar limb.

In addition to this phenomenon, Chuck, Judy, and Deborah also observed the larger sunspot cores transforming into either a pencil-tip or teardrop shape before the advancing limb covered them all. Deborah described the re-emergence of the last sunspot core before third contact as a mirage.

“First, the spot appeared flattened above the dark limb of the Moon separated by a narrow gap,” she said. “Darkness from the spot then dripped into the lunar limb as the spot transformed into a teardrop before separating from the lunar limb and appearing as a normal spot.”

It’s important to note that the observers deemed the atmosphere steady during many of these events, with occasions of imperfect seeing. So these effects were observed under stable air, through sizable telescopes, and mostly near the center of the Sun’s disk. Is it possible, perhaps, that a sunspot’s penumbra serves equally well as limb darkening to help cause the effect?

As always, if you observed similar phenomena, send your reports to sjomeara31@gmail.com.

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