Astronomy - June 2015

(Jacob Rumans) #1
Pacific
Ocean

Atlantic
Ocean

Gulf of Mexico

Greatest eclipse

Path of totality

Nashville

St. Louis
Kansas City

Salt Lake City

PortlandSalem

Seattle

Charleston

Miami

Philadelphia

New York
Chicago

Detroit

Toronto
Minneapolis

Vancouver

Los Angeles
San Diego

San Francisco

Oakland

Houston

Dallas

New Orleans

Baltimore

Atlanta

Boise

Carbondale

Casper

Columbia

Columbia

Denver

Des Moines

Grand Island

Lincoln

Greenville

Jefferson City Hopkinsville

Memphis

MISSOURI

ILLINOIS

IOWA

KENTUCKY

NEBRASKA

TENNESSEE SOUTH
CAROLINA

NORTH
CAROLINA

GEORGIA

Paducah

WYOMING

KANSAS
Wichita

Idaho Falls

OREGON

IDAHO

St. Joseph
Washington D.C.

CANADA

MEXICO
300 miles

0 400 kilometers

0

56 ASTRONOMY • JUNE 2015


reason comes from the fact that Earth is
not always at the same distance from the
Sun and the Moon is not always the same
distance from Earth. The Earth-Sun dis-
tance varies by 3 percent and the Moon-
Earth distance by 12 percent.
The result is that the Moon’s apparent
diameter can range from 7 percent larger
to 10 percent smaller than the Sun. A big-
ger apparent size for the Moon and a
smaller one for the Sun equals a longer
totality. But a Moon that looks smaller and
a Sun that appears larger means that you’ll
experience a shorter time in the dark.
According to Belgian astronomer Jean
Meeus, the maximum duration of totality
from 2000 b.c. to a.d. 3000 is 7 minutes
and 29 seconds. That eclipse will occur July
16, 2186, so don’t get too anxious.
The length of totality during the August
21, 2017, eclipse won’t be nearly that long.
Its duration will vary according to your
location. Where the Moon’s umbra first
touches land, at Government Point,
Oregon, totality lasts 1 minute and 58.5
seconds. The maximum duration, 2 min-
utes and 41.6 seconds, occurs just south of
Carbondale, Illinois.


It’s all about totality
Everyone in the contiguous U.S. will see at
least a partial eclipse. In fact, if you have
clear skies on eclipse day, the Moon will
cover at least 48 percent of the Sun’s bril-
liant surface. And that’s from the northern
tip of Maine. But although our satellite
covering part of the Sun’s disk sounds
cool, you need to aim higher.
Likening a partial eclipse to a total
eclipse is like comparing almost dying to
dying. If you are outside during a solar
eclipse with 48 percent coverage, you won’t
even notice it getting dark. And it doesn’t
matter whether the partial eclipse above
your location is 48, 58, or 98 percent. Only
totality reveals the true celestial spectacles:
the two diamond rings, the Sun’s glorious
corona, 360° of sunset colors, and stars in
the daytime. But remember, to see any of
this, you must be in the path.
That said, you want to be close to the
center line of totality. The fact that the
Moon’s shadow is round probably isn’t a
revelation. If it were square, it wouldn’t
matter where you viewed totality. People
across its width would experience the same
duration of darkness. The shadow is round,

however, so the longest eclipse occurs at its
center line because that’s where you’ll
experience the lunar shadow’s full width.

Ye s , we ’r e s u r e
This event will happen! As astronomers,
some of the problems we deal with are
due to the uncertainty and limited visibil-
ity of some celestial events. Comets may

As totality was ending July 22, 2009, this imager
captured three exceptionally nice Baily’s beads,
which form when light from the Sun’s disk passes
through valleys on the Moon’s edge. BERT HALSTEAD

The path of the Moon’s umbral shadow across land begins in Oregon and ends in South Carolina. Numerous large cities lie within easy reach of the center
line. The longest duration of totality — 2 minutes and 41.6 seconds — occurs in and around Giant City State Park, in Illinois. ASTRONOMY: ROEN KELLY


Darkness crosses


the United States

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