Annular eclipse
Because the Earth's distance
from both the Moon and Sun varies, their respective apparent sizes in our sky
also vary. The Earth can come as near to the Sun as 91.5 million miles (called
"perihelion") and be as far as 94.5 million miles away (called "aphelion").
The difference between these
two extremes amounts to just over 3 percent.
The situation in regard
to the Sun is not nearly as critical as it is with the Moon. The Moon can be
as close as 221,463 miles from the Earth (called "perigee") or as far away as
252,710 miles (called "apogee"). That amounts to a difference of 12.4 percent.
When the Moon is at, or
near perigee, it is large enough to completely cover the Sun. If the Moon is
at or near apogee during an eclipse, however, then it will appear smaller in
angular size as compared to the Sun.
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On such occasions, the umbra
does not reach as far as the surface of the Earth. Instead, people on the part
of the surface toward which the umbra points find themselves inside the "anti-umbra."
As a consequence, when we see the Moon passing centrally/directly over the Sun
it fails to cover it completely; a bright ring, or annulus of sunlight remains
visible.
This is the type of eclipse
that will be visible to those who are situated within the narrow track of the
anti-umbral shadow on June 10 (or June 11 for those west of the International
Date Line).
In fact, at the midpoint
of this so-called annular eclipse path the event will nearly be a total eclipse.
At that spot, in the middle of nowhere over the Pacific Ocean, the tip of the
Moon's umbra comes within just 800 miles (1,300 kilometers) of touching the
Earth's surface. If it were somehow possible to push the Moon just 800 miles
closer to Earth, a total eclipse of the Sun would be briefly visible for an
instant from this watery locale; the disks of both the Sun and Moon would appear
exactly the same size.
Night and day
As it is, the remaining
bright ring of sunlight will itself appear exceedingly thin, since the Moon's
apparent diameter will be just 0.4 percent smaller than that of the Sun's.
Yet the difference between
even an exceedingly thin ring of sunlight like this and a total eclipse can
be literally like night and day.
In a total eclipse, the
onset of totality is usually accompanied by what might best be described as
an "explosion of darkness" as the Moon's dark umbral shadow rushes toward the
observer. Some of the brightest stars and planets may pop into view. Visible
around the limb (edge) of the darkened Sun are tongues of hydrogen gas, called
prominences, rising out into space for tens of thousands of miles. The delicate,
pearly white corona or halo of the Sun also becomes visible.
An annular eclipse offers
none of this. The remaining ring of sunlight still visible at maximum eclipse
is bright enough to overpower the glow of both the corona and prominences.
Though they do not attract
the wide following of total eclipses, annular eclipses are still spectacular
in their own right. The unusual ring aspect of the upcoming annular eclipse
for instance, has its own very special appeal.
Where to see it
For those in the annular
eclipse zone about 20 miles to the south of Puerto Vallarta on the Pacific coast
of Mexico, the darkening of the sky just prior to, and during the maximum phase
of the eclipse should be rather impressive. Darkness will not come suddenly
as with the start of a total eclipse, but rather observers will see a gradual
diminution of the skylight. Light intensities will become rather similar to
those encountered when the Sun shines through thin cirroform cloudiness; yet
shadows will be as clear as in full sunlight.
When the annular phase is
at maximum effect an eerie twilight will fall over the surrounding landscape,
almost like the approach of a thunderstorm on a hazy, sultry day.
This annular eclipse occurs
at sunset. So the fire-ringed Sun may be distorted by atmospheric refraction,
and changed in color from a hot white to a deep orange, finally dropping below
the horizon resembling a horseshoe with pointed tips, may make this eclipse
one of the most unusual and photogenic ever seen.
Not until the year 2012
will there be another chance to view a sunset annular eclipse from North America.
Joe Rao serves as an
instructor and guest lecturer at New York's Hayden Planetarium. He writes about
astronomy for The New York Times and other publications, and he is also an on-camera
meteorologist for News 12 Westchester,
New York.
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