[grco-staff] Eclipse Info Web ASTRONOMY MAGAZINE AUG 15 2017
- From: "John W. Goerger" <pos1@xxxxxxxxxxxxx>
- To: <grco-staff@xxxxxxxxxxxxx>
- Date: Tue, 15 Aug 2017 07:42:42 -0700
Posted by <http://cs.astronomy.com/members/Michael-Bakich/default.aspx>
Michael Bakich
on Tuesday, August 05, 2014
13
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This chart shows the paths of totality for 15 solar eclipses through 2028.
// Astronomy: Roen Kelly after Fred Espenak, NASA/Goddard Space Flight
Center
When I first wrote this blog, the event was more than three years away. Not
anymore! Each day now seems to bring a new announcement of a talk, a
workshop, or an event related to the eclipse. With tens of millions of
people headed for the zone of totality, its going to be the biggest science
event in history. In this blog I list 25 of the eclipse's important details
for our readership, the general public, and the media. Read them, and learn
about the event. But for sure plan to experience totality. You'll remember
it for the rest of your life as the greatest thing you ever saw!
1. This will be the first total solar eclipse in the continental U.S. in 38
years. The last one occurred February 26, 1979. Unfortunately, not many
people saw it because it clipped just five states in the Northwest and the
weather for the most part was bleak. Before that one, you have to go back to
March 7, 1970.
2. A solar eclipse is a lineup of the Sun, the Moon, and Earth. The Moon,
directly between the Sun and Earth, casts a shadow on our planet. If youre
in the dark part of that shadow (the umbra), youll see a total eclipse. If
youre in the light part (the penumbra), youll see a partial eclipse.
3. A solar eclipse happens at New Moon. The Moon has to be between the Sun
and Earth for a solar eclipse to occur. The only lunar phase when that
happens is New Moon.
4. Solar eclipses dont happen at every New Moon. The reason is that the
Moons orbit tilts 5° to Earths orbit around the Sun. Astronomers call the
two intersections of these paths nodes. Eclipses only occur when the Sun
lies at one node and the Moon is at its New (for solar eclipses) or Full
(for lunar eclipses) phase. During most (lunar) months, the Sun lies either
above or below one of the nodes, and no eclipse happens.
5. Eclipse totalities are different lengths. The reason the total phases of
solar eclipses vary in time is because 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 distance varies by 3 percent and the Moon-Earth
distance by 12 percent. The result is that the Moons apparent diameter can
range from 7 percent larger to 10 percent smaller than the Sun.
6. It's all about magnitude and obscuration. Astronomers categorize each
solar eclipse in terms of its magnitude and obscuration, and I dont want
you to be confused when you encounter these terms. The magnitude of a solar
eclipse is the percent of the Suns diameter that the Moon covers during
maximum eclipse. The obscuration is the percent of the Suns total surface
area covered at maximum. Here's an example: If the Moon covers half the
Sun's diameter (in this case the magnitude equals 50 percent), the amount of
obscuration (the area of the Sun's disk the Moon blots out) will be 39.1
percent.
7. Solar eclipses occur between Saros cycles. Similar solar and lunar
eclipses recur every 6,585.3 days (18 years, 11 days, 8 hours). Scientists
call this length of time a Saros cycle. Two eclipses separated by one Saros
cycle are similar. They occur at the same node, the Moons distance from
Earth is nearly the same, and they happen at the same time of year.
8. Everyone in the continental 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 Suns surface. And thats from the northern tip of Maine.
9. Its all about totality. Not to cast a shadow on things, but likening a
partial eclipse to a total eclipse is like comparing almost dying to dying.
I know that 48 percent sounds like a lot. It isnt. You wont even notice
your surroundings getting dark. And it doesnt matter whether the partial
eclipse above your location is 48, 58, or 98 percent. Only totality reveals
the true celestial spectacle: the diamond ring, the Suns glorious corona,
strange colors in our sky, and seeing stars in the daytime.
<http://cs.astronomy.com/cfs-file.ashx/__key/communityserver-blogs-component
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Only being on the center line will allow viewers to see the diamond rings
and the interval of totality between them. // Ian Wardlaw
10. You want to be on the center line. This probably isnt a revelation, but
the Moons shadow is round. If it were square, it wouldnt 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 thats where youll experience the Moons shadows
full width.
11. First contact is in Oregon. If you want to be the first person to
experience totality in the continental U.S., be on the waterfront at
Government Point, Oregon, at 10:15:56.5 a.m. PDT. There, the total phase
lasts 1 minute, 58.5 seconds.
12. The center line crosses through 12 states. After a great west-to-east
path across Oregon, the center line takes roughly nine minutes to cross a
wide swath of Idaho, entering the western part of the state just before
11:25 a.m. MDT and leaving just before 11:37 a.m. MDT. Next up is Wyoming,
where the umbral center line dwells until just past 11:49 a.m. MDT. From
11:47 a.m. MDT until 1:07 CDT (note the time zone change!), the dark part of
the Moon's shadow lies in Nebraska. The center line hits the very
northeastern part of Kansas at 1:04 p.m. CDT and enters Missouri a scant two
minutes later. At 1:19, the shadows midpoint crosses the Mississippi River,
which at that location is the state border with Illinois. The center line
leaves Illinois at its Ohio River border with Kentucky just past 1:24 p.m.
CDT. Totality for that state starts there two minutes earlier and lasts
until nearly 1:29 p.m. CDT. The center line crosses the border into
Tennessee around 1:26 p.m. CDT. Then, just past the midpoint of that state,
the time zone changes to Eastern. North Carolina has the midpoint of the
eclipse from 2:34 p.m. EDT until just past 2:38 p.m. EDT. The very
northeastern tip of Georgia encounters the center line from just past 2:35
p.m. EDT until not quite 2:39 p.m. EDT. Finally, its South Carolinas turn.
The last of the states the center line crosses sees its duration from 2:36
p.m. EDT to 2:39 p.m. EDT.
13. Totality lasts a maximum of 2 minutes and 40.2 seconds. Thats it. To
experience that length, youll need to be slightly south of Carbondale,
Illinois, in Giant City State Park. You might think about getting there
early.
14. The end of the eclipse for the U.S. is not on land. The center lines
last contact with the U.S. occurs at the Atlantic Oceans edge just
southeast of Key Bay, South Carolina. Im pretty sure the crowd wont be
huge there.
15. Cool things are afoot before and after totality. Although the big payoff
is the exact lineup of the Sun, the Moon, and your location, keep your eyes
open during the partial phases that lead up to and follow it. As you view
the beginning through a safe solar filter, the universe will set your mind
at ease when you see the Moon take the first notch out of the Suns disk.
Around the three-quarters mark, youll start to notice that shadows are
getting sharper. The reason is that the Suns disk is shrinking, literally
approaching a point, and a smaller light source produces better-defined
shadows. At about 85 percent coverage, someone youre with will see Venus
34° west-northwest of the Sun. If any trees live at your site, you may see
their leaves act like pinhole cameras as hundreds of crescent Suns appear in
their shadows.
16. This eclipse will be the most-viewed ever. I base this proclamation on
four factors: 1) the attention it will get from the media; 2) the superb
coverage of the highway system in our country; 3) the typical weather on
that date; and 4) the vast number of people who will have access to it from
nearby large cities.
17. Only one large city has a great view. Congratulations if youre one of
the 609,000 people lucky enough to live in Nashville. The city center and
parts north of it will experience 2+ minutes of totality. Unfortunately,
thats the only large city with a great view. In the tally below, column 1
lists 25 other large metropolitan areas. The second column shows the amount
of the Suns surface the Moon will cover as seen by viewers in each city.
Atlanta
97 percent
Boston
63 percent
Chicago
87 percent
Cincinnati
91 percent
Dallas
76 percent
Denver
92 percent
Detroit
79 percent
Houston
67 percent
Indianapolis
91 percent
Las Vegas
72 percent
Los Angeles
62 percent
Memphis
93 percent
Miami
78 percent
Milwaukee
83 percent
Minneapolis
83 percent
New Orleans
75 percent
New York City
72 percent
Oklahoma City
84 percent
Philadelphia
75 percent
Phoenix
63 percent
Pittsburgh
81 percent
Portland
99 percent
Salt Lake City
91 percent
Seattle
92 percent
Washington, D.C.
81 percent
Now a brief follow-up: about half of both Kansas City (pop. = 464,000) and
Saint Louis (pop. = 318,000) lie within the path of totality. Unfortunately,
the center line doesnt pass through either of them. An educated guess then,
tells me that most residents interested in the eclipse will drive 30 minutes
or so for an extra two minutes of totality.
18. A few small cities are well-placed. Heres a list of smaller
municipalities either on the center line or near it with their approximate
populations.
Carbondale, Illinois
26,000
Casper, Wyoming
58,000
Columbia, Missouri
113,000
Columbia, South Carolina
132,000
Grand Island, Nebraska
50,000
Greenville, South Carolina
61,000
Hopkinsville, Kentucky
33,000
Idaho Falls, Idaho
58,000
Jefferson City, Missouri
43,000
Paducah, Kentucky
25,000
Saint Joseph, Missouri
77,000
Salem, Oregon
157,000
19. Totality is safe to look at. During the time the Moons disk covers that
of the Sun, its safe to look at the eclipse. In fact, to experience the
awesomeness of the event, you must look at the Sun without a filter during
totality.
20. Yes, the Suns a lot bigger. Our daytime stars diameter is
approximately 400 times larger than that of the Moon. What a coincidence
that it also lies roughly 400 times farther away. This means both disks
appear to be the same size.
21. You wont need a telescope. One of the great things about the total
phase of a solar eclipse is that it looks best to naked eyes. The sight of
the corona surrounding the Moons black disk in a darkened sky is
unforgettable. That said, binoculars give you a close-up view but still at
relatively low power that you should take advantage of several times
during the event.
22. Nature will take heed. Depending on your surroundings, as totality nears
you may experience strange things. Look. Youll notice a resemblance to the
onset of night, though not exactly. Areas much lighter than the sky near the
Sun lie all around the horizon. Shadows look different. Listen. Usually, any
breeze will dissipate and birds (many of whom will come in to roost) will
stop chirping. It is quiet. Feel. A 10°15° F drop in temperature is not
unusual.
23. Maximum totality is not the longest possible in 2017. The longest
possible duration of the total phase of a solar eclipse is 7 minutes and 32
seconds. Unfortunately, the next solar eclipse whose totality approaches 7
minutes wont occur until June 13, 2132. Its 6 minutes and 55 seconds of
totality will be the longest since the 7 minutes and 4 seconds of totality
June 30, 1973.
24. The future is bright but long. The next total solar eclipse over the
continental U.S. occurs April 8, 2024. Its a good one, too. Depending on
where you are (on the center line), the duration of totality lasts at least
3 minutes and 22 seconds on the east coast of Maine and stretches to 4
minutes and 27 seconds in southwestern Texas. After that eclipse, its a
20-year wait until August 23, 2044 (and, similar to the 1979 event, that one
is visible only in Montana and North Dakota). Total solar eclipses follow in
2045 and 2078.
25. This event will happen! As astronomers (professional or amateur), some
of the problems we have are due to the uncertainty and limited visibility of
some celestial events. Comets may appear bright if their compositions are
just so. Meteor showers might reach storm levels if we pass through a thick
part of the stream. (Oh, and the best views are after midnight.) A supernova
as bright as a whole galaxy is visible now, but you need a telescope to view
it. In contrast, this solar eclipse will occur when we say, where we say,
for how long we say, and in the daytime, no less. Guaranteed!
BONUS: Facts are great, but I also posted a list of two-dozen-plus-one tips
you might find useful. Check out
<http://cs.astronomy.com/asy/b/astronomy/archive/2014/08/08/25-tips-for-the-
august-21-2017-total-solar-eclipse.aspx> Two dozen tips for the August 21,
2017, total solar eclipse.
Tags: <http://cs.astronomy.com/main/tags/?tag=Michael+Bakich> Michael
Bakich , <http://cs.astronomy.com/main/tags/?tag=solar+eclipse> solar
eclipse
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