This story appears in the August 2017 issue of National Geographic magazine.
On August 21, 2017, the moon will cast a shadow that will take a little over an hour and a half to traverse North America, plunging some areas into darkness for as much as two minutes, 41 seconds.
Total solar eclipses occur only when the moon is directly between the Earth and the sun—a rare occasion given the moon’s slightly tilted orbit. This summer’s eclipse—stretching from Salem, Oregon, to Charleston, South Carolina—will be the first total solar eclipse visible in the continental U.S. in 38 years.
But be advised: Never look directly at the sun except during totality, in the umbra, without proper eye protection.
THE DARKEST ARC
The sun will be totally obscured
by the moon in the approximately
70-mile-wide shadow called the
umbra. Outside this track, in the
penumbra, viewers should see a
partial eclipse.
UMBRA
Sun coverage
Portion of sun
covered by moon
Coronal streamer
Ionized gases stream outward
along magnetic lines of force in
dramatic displays that change
shape as the corona does.
Umbra
The shadow cast when the sun
is entirely blocked by the moon
what you
might see
EARTH
Penumbra
When the sun is partially
blocked by the moon
The sun’s corona—the
dramatic, outermost
atmosphere made of plasma
and superheated to an
unknown temperature—is
believed to be several million
degrees Fahrenheit. It’s visible
to the naked eye only during a
total solar eclipse, inside the
umbral shadow.
Coronal loop
On especially active areas of the
sun, plasma forms glowing arcs that
follow looping magnetic field lines.
MOON
PROMINENCE
Pinkish red arcs of plasma at the
edge of the solar disk may look
small, but they can be dozens of
times larger than the Earth.
what you might see
The sun’s corona—the dramatic, outermost atmosphere made of plasma and superheated to an unknown temperature—is believed to be several million degrees Fahrenheit. It’s visible to the naked eye only during a total solar eclipse, inside the umbral shadow.
Coronal streamer
Ionized gases stream outward along magnetic lines of force in dramatic displays that change shape as the corona does.
Umbra
The shadow cast when the sun is entirely blocked by the moon
EARTH
Penumbra
When the sun is partially blocked by the moon
Coronal loop
On especially active areas of the sun, plasma forms glowing arcs that follow looping magnetic field lines.
MOON
PROMINENCE
Pinkish red arcs of plasma at the edge of the solar disk may look small, but they can be dozens of times larger than the Earth.
Sun Illustration is not to scale. THE SUN IS 400 TIMES as large as THE MOON AND 400 TIMES FARTHER FROM Earth than shown.
THE DARKEST ARC
The sun will be totally obscured by the moon in the approximately 70-mile-wide shadow called the umbra. Outside this track, in the penumbra, viewers should see a partial eclipse.
Sun coverage
Portion of sun covered by moon
ON MAP, LOCAL DAYLIGHT SAVING TIME GIVEN.
Eclipse map adapted with permission: https://www.exploratorium.edu/eclipse