Sky and Telescope - September 2017 - 45

Under the Stars by Fred Schaaf

Diamond of Three Rings
A total solar eclipse offers the most spectacular of jewels.

BA BA K TA FRESHI / G E T T Y IM AG ES

M

ost subscribers to Sky & Telescope
will receive this issue in late July,
well before August 21st's total eclipse
of the Sun, so I want to continue the
discussion of total eclipses I began last
month. I'll conclude with an eclipse
that brought me the most staggering
moment I've ever had in a lifetime of
observational astronomy.
Can you observe the F-corona?
What overall shape will the corona take
during the August 21st eclipse? The
ultimate visible extension of the corona
is not the gases of the Sun that glow on
their own, but the part of the corona
formed by dust particles that reﬂect
light from the Sun out to many million
miles. In fact, this "F-corona" is really
the beginning of a phenomenon that
stretches all the way to Earth's orbit and
beyond - the zodiacal light. But we can
only pick up the zodiacal light starting
many degrees away from the Sun as twilight starts to fade away.
I'm familiar with only two observers
who have knowingly seen the F-corona
extending along the ecliptic during a
total eclipse. Steve Albers did so with
the naked eye. Robert Victor, columnist for Sky & Telescope in the 1970s
and 1980s, traced the corona out to 5°
with binoculars. However, both made
these observations during the extremely
long 1991 total eclipse of the Sun - so
perhaps that gave their eyes more time
to become dark-adapted.
The diamond ring of rings. The
ﬁrst or ﬁnal piece of solar photosphere
shining through the deepest valley on
the Moon's edge creates, with the ring
band of the inner corona, the most
spectacular of all eclipse phenomena:
the diamond ring. This diamond is truly
brighter than all of Earth's diamonds
combined. But during the eclipse of February 26, 1979, in Manitoba, I got to see
a diamond wonder beyond all wonders.

The last rays of the Sun create a diamond ring as the solar
corona appears during the total solar eclipse of August 1999.

Thin clouds had created a "cloudcorona" around the Sun before totality.
The cloud-corona that most skywatchers have noticed on occasion occurs
when clouds in front of the Moon
produce a disk of blue or green bordered
by a red ring around the Moon. The
cloud-corona is an interference pattern
formed by water droplets - or, rarely,
ice needles - that are similar in size to
the wavelengths of visible light.
Back to the 1979 eclipse: As totality
began, I saw the dazzle of three Baily's
beads - but no diamond ring. After 2
minutes and 50 seconds shouts of "It's
coming back!" rang out.And then I
heard the loudest yells and cries of all:
The most briliant "star" any of us had
ever seen had appeared at the edge of
the Sun. The diamond brightened and
hung, still starlike, second after second.
What happened next I've described
best in my book Wonders of the Sky: "As
the diamond brightened, two perfect

rings of color came into view, centered
upon it. It was a two-ringed cloudcorona, but never had I seen an example
like this, for the smallness and incredible brightness of the diamond made the
two rings also the smallest, sharpest,
and most intensely deep purple-red that
I have ever seen."
But most stunning of all was the
positioning of the rings. "Since this
phenomenon was caused by ice needles,
the inner ring cut directly in front of
the Moon, with the outer, exactly twice
the radius of the ﬁrst, encircling all.
And the band of the diamond (ring),
the inner corona, was still clearly visible
and thus interlocked with the inner
cloud-corona ring." This was the most
staggering beauty I've seen in all my
years of skywatching.
¢ Contributing Editor FRED SCHAAF
has been writing about the skies above
us for more than 40 years.

s k y a n d t e l e s c o p e .c o m

* SEPTEMBER 2017

45
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Sky and Telescope - September 2017

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