Two years from Aug. 21, 2015, North
Americans will get an opportunity to observe nature's
greatest sky show.
Picture this: The day starts off bright and sunny. Then, a
bit later, you begin to notice that, although it is still
sunny, the day doesn't seem quite so bright. And still a
little while later, it almost seems like some big storm is
brewing. Then, suddenly, and without any warning, the
midsummer day turns strangely dark.
A few stars come out. Birds and animals become
confused and quickly head home to sleep. Night insects
begin to chirp. All around the horizon, there is a strange
yellow-orange glow resembling a weird sunset. And
meanwhile, up in the sky where the sun should be, there
appears instead a jet-black disk surrounded by a softly
glowing halo.
Then, just as suddenly, the sky brightens up. The stars
disappear, birds and animals awaken, and the sun
returns.
What you have just witnessed is a total eclipse of the
sun .
This total solar eclipse of 2017 will be the first time in
nearly four decades that such an event will be visible so
close to home. "Close," of course, is a relative term. But
for most Americans, this spectacular phenomenon will
occur literally in their own backyards.
Narrow zone of visibility
Contrary to popular belief, totalsolar eclipses are not
particularly rare. Astronomers predict 68 to take place
during the present century — one about every 17.6
months. On such occasions, the moon casts its dark,
slender cone of shadow (called the umbra) upon the Earth's surface.
The track traced by the moon's umbra can run for many
thousands of miles, but it's also very narrow — at most,
about 167 miles (268 kilometers) wide. For this reason, it
has been calculated that, on average, a total eclipse of
the sun is visible from the same spot on Earth only once
in about every 375 years.
In recent years, for instance, assiduous eclipse chasers
had to travel to remote locations such as Ellesmere
Island in Nunavut, Canada (2008), Easter Island (2010) or
the Norwegian archipelago of Svalbard (2015).
The long wait will soon come to an end
But Aug. 21, 2017, will mark the first time this century,
and the first time since 1979, that a total solar eclipse
will cross the contiguous (lower 48) United States
(Alaska had its turn in 1990; Hawaii in 1991). And for the
very first time, the shadow track — better known as the
"path of totality" — will sweep only over the United
States and no other country, leading some people to
refer to this upcoming event as "The Great American
Eclipse ."
Many Americans taking full advantage of this event's
close proximity to their homes will have a golden
opportunity to witness firsthand one of the most
beautiful and most exciting of nature's sky events. The
total eclipse will be seen by an estimated 12 million
people who fortuitously live within the totality path.
However, the number of people who are within just one
day's drive of the totality zone is probably around 200 million.
Not since 1970 has there been an opportunity to see a
total solar eclipse in such easily accessible and
widespread areas of the United States. There have been
a couple of limited opportunities, such as in 1972
(Quebec and the adjacent Canadian Maritime Provinces)
and 1979 (the Pacific Northwest and Northern Plains),
but the areas of visibility were either limited or
somewhat difficult to reach. And not until April 2024 will
there be another opportunity comparable to that offered
on Aug. 21, 2017.
One end of the continent to the other!
Also, for the first time since 1918, the moon's dark
shadow will sweep across the U.S. from coast to coast.
At local sunrise on this third Monday in August, the
shadow of the moon will first touch the Earth at a point
in the North Pacific Ocean, about 1,500 miles (2,414 km)
northwest of the Hawaiian Islands. Then, for 3 hours and
13 minutes, the shadow will race first east-northeast,
then east and finally southeast, along the way darkening
a narrow strip of the North American continent.
Initially, the shadow will traverse nothing but wide-open
ocean for 28 minutes. Finally, it will make landfall along
the coast of Oregon at the community of Lincoln Beach.
Traversing the United States, the total eclipse will be
visible within a path of darkness stretching from Oregon
and then on through Idaho, Wyoming, Nebraska, Kansas,
Missouri, Illinois, Kentucky, Tennessee, Georgia, North
Carolina and, finally, South Carolina.
The path will average 67 miles (108 km) in width, but
while moving through western Kentucky, about 12 miles
(19 km) northwest of the town of Hopkinsville, the
shadow's size will widen to a maximum of 71 miles (114
km).
Look quickly!
Because the moon's shadow is moving at such a
tremendous speed, totality cannot last very long in any
one place. The duration of the total phase is always
longest along the center of the shadow's path. As an
observer moves away from the center, the duration of
totality time decreases, becoming zero along the path's edges.
Right at the Oregon coastline, totality lasts 1 minute and
59 seconds, as the shadow will be traveling at more than
three times the speed of sound (2,400 mph, or 3,900 km/
h). Heading southeast along the center line, the totality
time slowly lengthens, reaching a maximum of 2 minutes
and 40.3 seconds at a spot in southern Illinois about a
dozen miles (about 20 km) southeast of the city of Carbondale.
The shadow — which, at this point, has slowed to 1,450
mph (2,330 km/h) — then begins to increase in speed,
and the duration of totality subsequently begins to
diminish. Indeed, when it arrives at the South Carolina
coastline, the duration of totality will have dropped to 2
minutes and 34 seconds. The shadow then exits out to
sea, finally leaving the Earth 75 minutes later at local
sunset in the North Atlantic Ocean, 390 miles (627 km) to
the southwest of the island nation of Cape Verde.
Notable cities inside the totality path include Idaho
Falls, Idaho; Casper, Wyoming; Grand Island, Nebraska;
Lincoln, Nebraska; Columbia, Missouri; Nashville,
Tennessee; Columbia, South Carolina; and Charleston,
South Carolina. The metropolitan areas of Kansas City,
Missouri, and St. Louis are right on the edge of the
totality path.
Warning: Never stare directly at the sun, either with your
unaided eye or through telescopes and binoculars,
without proper protection. Serious eye damage can
occur. Solar eclipse observers and astronomers use
special filters to safely observe the sun .
Weather prospects
Weather experts have performed studies to help
observers pick the best opportunity for cloudless skies
in and near the path of totality on Aug. 21, 2017.
The best chances for a clear look appear to be in the
area running from western Oregon through Wyoming,
where, on average, the odds are on the order of about 70
percent. Going farther east, the odds of clear skies
gradually diminish; from Kentucky to the Atlantic coast,
it's just about a coin toss for favorable weather: just
50-50.
Starting the week before, the National Weather Service
will provide increasingly reliable forecasts, which will
enable people to choose a location where the chances of
a cloudy sky are low.
Everyone will see at least a partial eclipse
Surrounding the dark umbra is the penumbra, or partial
shadow. It will also be conical but much larger — nearly
6,000 miles (9,700 km) in diameter. The penumbra is
simply the half shadow that lies outside every deep
shadow, whether it is cast by the moon or a house.
Wherever the penumbra falls, a partial eclipse will occur.
All of North America will be inside the penumbra,
causing a rather large partial eclipse for much of the
U.S. and portions of southwest Canada. The closer you
station yourself to the path of totality, the greater the
magnitude of the partial eclipse will be. When watching
the partial phases, observers should take precautions
while viewing the still blindingly bright sun.
A number of large cities will lie just outside the totality
zone and will see the sun cut down to an exceedingly
thin sliver of light. These include Portland, Oregon;
Omaha, Nebraska; Chattanooga, Tennessee; and Atlanta.
From these cities, a strangely weird twilight effect will
fall for a couple of minutes.
But a partial solar eclipse pales in comparison to a total
one, even when 99 percent of the sun's disk is
obscured. The great show can only begin at the moment
when the last speck of sunlight is extinguished. And
only when it's in total eclipse is the sun perfectly safe
to look at; only inside the totality path itself can you
witness the full grandeur of the eclipse. For that reason,
many people will likely journey to position themselves
within the path.
Final thoughts
Without a doubt, the "All American Eclipse" will be one
of the big news stories of 2017.
The major news networks will certainly deploy teams of
cameras and reporters along the totality path and will
follow the moon's umbra during its 90-minute sweep
across the contiguous United States. Media coverage —
at least for this one day — could be reminiscent of the
coverage accompanying the first flights of the space
shuttle .
In short, the third Monday in August 2017 will be an
amazing day. Mark your calendars!
