Astronomy

66 ASTRONOMY • JANUARY 2018

T

he 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 pro-

posed 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.

SECRETSKY

The ‘black drop’

effect of

sunspots

Can this well-known

phenomenon during

planetary transits also be

observed with sunspots?

phases, observers using tele-

scopes 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.

A black bridge,

teardrops,

and more

I got to see only one event

(t h roug h Judy ’s 16 -i nch

Dobsonian-mounted ref lec-

tor), 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

BY STEPHEN JAMES O’MEARA

A parade

of illusions

The phenomenon I had

mentioned is akin to the

“black drop” effect that observ-

ers 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 dimen-

sion to the effect.

During the eclipse’s partial

of darkness vanished and reap-

peared repeatedly before becom-

ing a solid bridge that rapidly

melted into the lunar limb.

In addition to this phenom-

enon, Chuck, Judy, and Deborah

also observed the larger sunspot

cores transforming into either a

pencil-tip or teardrop shape

before the advancing limb cov-

ered them all. Deborah

described the re-emergence of

the last sunspot core before

third contact as a mirage.

“First, the spot appeared

f lattened 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 siz-

able 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.

BROWSE THE “SECRET SKY” ARCHIVE AT http://www.Astronomy.com/OMeara.

The train of sunspots

near the center of the

Sun was visible even

through the smallest

telescopes. JOHN CHUMACK

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

Stephen James O’Meara

is a globe-trotting observer

who is always looking for the

next great celestial event.