www.astrosociety.org/uitc
No.
17 - Spring 1991
©
1991, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco,
CA 94112
The
Upcoming Total Solar Eclipse!
by
Andrew Fraknoi, Astronomical Society of the Pacific
On
11 July 1991, people in certain parts of Hawaii, Mexico, and Central America
will have a chance to see one of nature's most spectacular sights — a
total eclipse of our Sun. And tens of millions of other people across the Americas
will see a partial solar eclipse. This event is already being called the "eclipse
of the decade,'' and for good reason! This is the first total eclipse since
1979 to cross any part of the North American continent, and, if you're in the
right place, it will be one of the longest eclipses in this century.
Eclipses are unique
astronomical events, occurring when the Earth, Moon, and Sun are exactly aligned
in space. A solar eclipse occurs when our Moon passes in front of the Sun, casting
a shadow across a portion of the Earth's surface. From some vantage points on
the Earth, the Moon will only seem to block a piece of the Sun, and the Moon's
shadow will not be completely dark. This is called a partial eclipse. But
when the alignment is just right, observers in a few lucky locations on the Earth
will see the Moon appear to cover the entire disc of the Sun. The Moon's shadow
will be completely black, and "darkness will come across the skies.'' This is
called a total eclipse.
The size of the
Moon's shadow that we see depends on the distances, and sizes, of both the Moon
and the Sun. By the time the Moon's dark shadow reaches our planet during a
total eclipse, it is only about 200-300 kilometers wide. As the Earth rotates
on its axis, that dark shadow seems to sweep across our planet, creating a path
of total darkness. Observers must be within that path, called the path of
totality, at the right time, to see a total solar eclipse. Observers on
either side of this path — almost 2,200 miles on either side — will
see only the partial eclipse.
The shadow cast
by our Moon 384,400 kilometers away would be similar in size to the shadow of
a marble on a tennis ball 2 meters away. On this scale, the Sun would be a sphere
7 meters (23 feet) in diameter almost 750 meters away.
As the Moon orbits
our planet, it sometimes comes between the Earth and Sun; sunlight falls on the
side of the Moon not visible from Earth, and we say that the Moon is in its "new''
phase. Solar eclipses can only occur at the new Moon, but not every new Moon results
in a solar eclipse. The Moon's monthly orbit around the Earth is tilted slightly,
and it can pass as much as 5 degrees above or below a line between the Earth and
Sun. In that case, the Moon's shadow will miss the Earth, and no eclipse will
be seen. Thus, a solar eclipse is possible only when the Moon is exactly on that
Earth-Sun line, and in the new phase.
Not every solar eclipse
is a total eclipse! About 35 percent of the time, the Moon does not completely
cover the Sun, and we see a partial eclipse. Since the Moon orbits the Earth on
an elliptical path, and not along a perfect circle, its distance from the Earth
varies. Occasionally, solar eclipses will occur when the Moon is at its farthest
from Earth, and it will not completely cover the disc of the Sun. Instead, the
Moon will appear to pass across the middle of the Sun, leaving a ring of bright
sunlight around it. This is called an annular eclipse, and these occur about 37
percent of the time. So only 28 percent of all solar eclipses are total.
The July event
is special for another reason, too. Because the path of totality for a total
solar eclipse is so small, any particular place on Earth is treated to this
special event on average only once in 360 years! And the path of the Moon's
shadow for this eclipse is much more accessible to more people than any recent
or near-future event. Mexico City lies directly in the path of totality, and,
if the weather is clear, millions of people in that populated area could witness
the eclipse.
And finally, this
eclipse promises to last as long as seven minutes in some place — far
longer than average. Indeed, the 1991 total solar eclipse will be the third
longest eclipse since 1898, and the longest until the year 2132! Most eclipses
last only three to four minutes. (The longest a total eclipse can last is approximately
seven minutes, 40 seconds.)
If you are lucky enough
to be in the path of totality, you will see the Sun slowly covered by the Moon.
This partial phase lasts up to an hour or more, depending on where and when you
see the eclipse. During most of the partial phase, the dimming of sunlight is
hardly noticeable; when seen through proper equipment, the Sun will appear to
have a "bite'' taken out of it. Then, as the Moon slides totally in front of the
Sun, the magic really begins! The sky will darken. The Moon's black disc will
look almost like a hole in the sky, and peeking around the black disc should be
a number of pinkish solar prominences, gigantic explosions of hot hydrogen gas
that boil off from the Sun in spectacular arching shapes.
At the last instant
before the Moon completely covers the Sun, observers may behold two fascinating
sights. First, the last bit of sunlight streaming past the Moon might seem to
break into "beads,'' (known as Bailey's Beads after the English astronomer Francis
Bailey, who first described their appearance in 1836). Bailey's Beads arise
from sunlight passing through lunar valleys in the Moon's uneven, cratered surface.
The beads will last a few seconds, and wink out as the Moon glides further in
front of the Sun. And finally, just as the last bead is visible, light from
the Sun's outer atmosphere will start to be visible around the Moon, creating
a glowing ring of light. This is called the "Diamond Ring Effect.''
The most dramatic
part of the total eclipse is being able to see the Sun's corona, a ghostly
aura streaming away from the Sun in all directions. Total eclipses are the only
time most people ever get to see this delicately-hued layer of the Sun's outer
atmosphere. Indeed, we on Earth are very lucky to live on the only planet in
the solar system with a moon the right size and distance to just cover the Sun's
disc, but not cover its corona as well. Our Moon is about 400 times smaller
than the Sun, but it is also about 400 times closer, too, so it appears to be
the same size in the sky.
The corona is
the Sun's outermost atmospheric layer, with extremely hot gas (reaching temperatures
near a million degrees) flowing off many millions of miles into space. The corona
is usually outshone by the Sun's visible layer, called the photosphere,
which is one million times brighter.
The darkness across
the land in the path of totality during a total solar eclipse will be comparable
to a night with a full moon, but slightly more eerie and wonderful. Many people
have reported that some animals become quiet and even go to sleep, thinking that
night has fallen!
The eclipse makes
the daytime sky dark enough for stars and planets to be seen; during the 11
July event, Mercury, Venus, Mars, and Jupiter will all be visible in the darkened
sky from the eastern Pacific Ocean and Mexico. And the bright stars of the constellations
Gemini, Canis Major, Canis Minor, Orion, and Auriga should be visible, too!
Away from the path of totality, the sky will not get significantly darker and
the surrounding stars and planets will still be invisible.
On Thursday morning,
11 July 1991, the eclipse will start at sunrise roughly 12,00 miles to the west-southwest
of Hawaii. The total phase of the eclipse starts at 7:30 a.m. over the main
island of Hawaii, when the Sun is about 22 degrees high in the sky to the east-northeast.
For the next 70 minutes, the Moon's shadow will glide across the Pacific Ocean,
heading east-southeast towards Baja California. The path of totality crosses
the Gulf of California a little before noon, local time, and arches to Guadalajara,
Mazatlan, and Mexico City, before turning southeast to cross Central America,
Columbia, and Brazil around sunset.
 |
| The
path of totality crosses the Pacific Ocean, Mexico, and Central and northern
South America. A partial eclipse will be visible north and south of these
areas. (From F. Espenak, Predictions for the Total Solar Eclipse of 1991,
NASA) |
Table
1 lists approximate local times for major cities across the U.S. and Canada,
when the partial eclipse will start, when it reaches maximum coverage of the Sun's
disc, what percentage of the Sun will be obscured, and when the eclipse ends.
A more complete list of times for other locations can be found in Joe Rao's book,
Your Guide to the Great Solar Eclipse of 1991, available from the
ASP.
Ancient societies
in China believed that solar eclipses were caused by a magical serpent that swallowed
the Sun. During an eclipse, people would make a tremendous din, banging pots and
pans, screaming and yelling, to frighten the serpent into releasing the Sun. Since
the eclipses lasted only minutes, the people were always successful; the Sun always
returned to the sky. Other stories from Native Americans and Western Europeans
tell of the dread caused by eclipses, thought to indicate anger of the gods, plagues,
death, and evil. But solar eclipses are not really dangerous, as long as you
take the proper precautions to observe them!
Remember, it is
the Sun, and not an eclipse, that can cause damage to the eye. People
should never stare at the Sun, at any time, whether there is an eclipse
or not. The Sun is very bright — more than 400,000 times brighter than
the full Moon — and looking at it will cause pain. But it is not just
this visible radiation that causes eye damage. The Sun also emits invisible
ultraviolet and infrared radiation, and these rays can seriously burn the sensitive
tissue of the retina, causing blindness. These rays cannot be usually felt until
it is too late to save the eyesight.
If you are lucky
enough to be in the path of totality, you can safely observe and photograph
the Sun when it is totally eclipsed. No filters are necessary. The only danger
comes from continuing to view the Sun when the total phase is over, and the
bright limb comes back into view.
The next major total
solar eclipse after 11 July 1991, visible over a large part of North America,
will occur on 21 August 2017; the path of totality will stretch from British Columbia
in the Northwest to the Carolinas in the Southeast, but the total phase will only
last approximately two minutes, 40 seconds. Between 1991 and 2017, a number of
partial and annular eclipses will also be visible. In May 1993, a slight partial
eclipse will be visible in the Pacific Northwest and western Canada. Observers
across the continental United States will have an excellent opportunity to see
an annular eclipse on 10 May 1994, which promises to pass directly over a path
running from Southern California to New York. And in February 1998, a partial
eclipse will be visible from much of the Southeastern United States; the path
of totality for that eclipse will pass over Columbia and Venezuela.
The safest way to
observe the Sun, and watch the progress of the partial phase of the eclipse, is
to not look directly at the Sun at all. Instead, project the Sun's image onto
a piece of white paper, using a pinhole camera (see Activity
#1) . Binoculars may also be used to project the Sun's image; they need to
be supported on a tripod, one lens should be covered, and a sheet of white paper
held behind the eyepiece. Focus the binoculars to achieve a sharp image. Do
not look directly at the Sun through the binoculars! To observe the Sun safely
before and during the partial phase of the eclipse, most of its light and its
harmful ultraviolet and infrared rays must be filtered out. Safe filters include:
- Using two layers
of fully exposed and developed black-and- white film (color film will not
provide adequate protection).
- Using a special
metal-coated piece of plastic, typically aluminized Mylar, certified for viewing
the Sun. Double-sided coatings greatly reduce the possibility of filter defects.
Two suppliers are:
- Thousand
Oaks Optical, Box 248098, Farmington, Ml 48332-8098, 313-353-6825.
- Roger Tuthill,
Inc., Box 1086-A, Mountainside, NJ 07092, 800- 223-1063.
- Rectangular
welder's glass, shade No. 14. Less dense shades (lower numbers) are not suitable
for direct solar observation. Welder's glass will also produce a green-colored
image.
Many filters that
are sometimes recommended for observing the Sun are not safe for direct-eye viewing,
including a piece of glass blackened by soot from a candle, sunglasses, photographic
neutral-density filters, and solar filters that operate at the eyepiece
of telescopes or binoculars. Eyepiece filters are especially dangerous; they are
placed at the point where the Sun's light is most concentrated and magnified.
They can crack or even explode.
Illustration
caption
| LOCATION |
START |
MAXIMUM |
COVERAGE |
END |
| Atlanta |
2:30 p.m.
EDT |
3:31 p.m.
|
28% |
4:29 |
| Chicago |
1:39 p.m.
CDT |
2:16 p.m. |
13% |
3:01 |
| Dallas |
12:56 p.m.
CDT |
2:11 p.m. |
51% |
3:23 |
| Denver |
11:44 a.m.
MDT |
12:50 p.m. |
37% |
1:56 |
| Honolulu |
6:31 a.m.
HST |
7:29 a.m. |
96% |
8:35 |
| Los Angeles |
10:12 a.m.
PDT |
11:28 a.m. |
69% |
|