Astronomy - 06.2019

64 ASTRONOMY • JUNE 2019

O

n July 2, 2019,

a total solar

eclipse will be

visible over parts

of Chile and

Argentina just before sunset.

Its partial phases will be vis-

ible throughout much of South

and Central America. While

totality is usually the draw, I’d

be interested in reports of tele-

scopic observations of the dark

lunar limb during the partial

phases (using an approved

solar filter, of course), espe-

cially under less-than-perfect

atmospheric seeing. Although

it may sound counterintuitive,

an unstable atmosphere can

reward observers with some

fascinating optical phenomena.

A ripple effect

Occasionally, during the par-

tial phases of the August 2017

total solar eclipse, the Moon’s

following limb (edge) not only

rippled like waves but also

sported mysterious wing tips at

its north and south extremities.

This effect was in turn mirrored

by a bright collar of light that

ran along the outer edge of the

lunar limb. The latter phenome-

non is a well-known brightness-

contrast illusion. If you stare at

a black object (the New Moon)

long enough against a bright

background (the Sun’s disk), a

portion of the eye’s retina will

become fatigued, creating a

negative (bright) afterimage

that involuntary eye movements

carry beyond the borders of the

black crescent. Early telescopic

observers mistakenly believed

this bright collar was due to the

refraction of sunlight through

a lunar atmosphere.

The Moon’s rippling outer

edge was an auxiliary phenom-

enon. Under excellent seeing

conditions, the Moon’s limb

appears irregular under mag-

nification because of all the

peaks and valleys along the

limb that we see in profile. But

heat shimmer magnified and

warped these features and set

them in motion — an effect

similar to that seen along an

ocean horizon or a hot road

under mirage conditions.

What I didn’t expect to see

was the contrast illusion rip-

pling in sync with the mock

irregularities along the dark

lunar crescent.

I would be most interested to

hear if any of the black “wave

crests” along the undulating

limb appear to “rip free” and

vanish, like what happens with

terrestrial mirages. Watching

the partial phases as the eclipse

nears the horizon may just do

the trick.

The wing tips at the cusps

were an unexpected phenom-

enon. I’m guessing they, too,

were a mirage effect, coupled

with the black drop effect.

Usually associated with transits

of Venus, the black drop occurs

at second and third contacts,

SECRETSKY

Partial eclipse

oddities

when the black disk of Venus

may appear to elongate toward

the inner edge of the Sun’s disk.

Perhaps solar limb darkening

added the perceived extension

of the lunar limb. I’m not sure,

but it’s a working theory that

you may help to solve during

the next solar eclipse.

Lunar eclipses, too

Wavelike irregularities can

also be seen along the edge

of Earth’s shadow during the

partial phases of a lunar eclipse,

as Leonor Ana Hernandez of

La Mancha, Spain, observed

the night of January 21, 2019.

Hernandez went to the La Hita

Observatory in Toledo, Spain,

to view the event, during which

she focused on the advance of

the curved shadow and associ-

ated visual phenomena.

Hernandez likened the

advancing shadow to a “sea

breaking gently on the shore of

the beach.” She watched as the

shadow deformed as a “channel

of darkness, opened like a dike,

and overflowed toward Mare

Frigoris. That plume of dark-

ness held for a while until the

false terminator advanced to

reach Aristarchus.” Hernandez

says she was “burning with

emotions” as she sketched the

eclipse and made some draw-

ings in pastel color on black

paper to capture “what my eyes

and my heart saw [and felt] in

this eclipse of ‘ice and fire.’ ”

As always, send your

observations and thoughts

to [email protected].

BY STEPHEN JAMES O’MEARA

Ta ke a cl o s e r l o o k

at the dark edge.

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

The author made this

sketch during the partial

phases of the August

2017 total solar eclipse.

It shows a rippling effect

along the lunar limb and wing

tip at the cusps, blunting the

solar crescent. STEPHEN JAMES O’MEARA

Mountains and valleys near the south pole of the Moon are visible in this image of a partial solar eclipse taken from space by the Solar

Dynamics Observatory spacecraft October 7, 2010. SDO

Leonor Ana Hernandez, an amateur

astronomer and artist from La Mancha,

Spain, displays her pastel drawings of

the January 2019 total lunar eclipse.

LEONOR ANA HERNANDEZ

Stephen James O’Meara

is a globe-trotting observer

who is always looking for the

next great celestial event.