If you plan to be in the central or eastern portion of South America or the western or southern part of Africa on Friday, Sept. 22, you will be treated to a partial solar eclipse.
This event will be the by-product of an annular or ring eclipse of the Sun, so called because the Moon's disk will be too small to completely cover the Sun's disk, a circumstance due to the fact that the Moon will be a bit farther from Earth than average. Although this will be a rather striking spectacle, in essence, this is really nothing more than just a fancy partial eclipse.
Partial eclipse: The Moon covers only part of the Sun.
Total eclipse: The Moon covers the entire disk of the Sun along a narrow path across the Earth.
Annular eclipse: The Moon is too far from Earth to completely cover the Sun. A thin ring of the Sun's disk surrounds the Moon.
The panoply of striking phenomena seen during a total eclipse such as the solar corona and prominences and the dramatic darkening of the sky accompanied by the appearance of some of the brighter stars and planets, will not be seen.
Rather, at maximum, sky watchers will see a "penny atop a nickel" effect, with the Sun mimicking a blazing ring of light rimming the dark silhouette of the Moon (creating the so-called "annulus" or ring effect).
What to expect
The path of annularity, from where the entire silhouette of the Moon's disk will appear against the Sun's brilliant disk (creating a "annulus" or ring effect) averages 180 miles in width. Virtually the entire path lies over the Atlantic Ocean, sliding from northeastern South America southward across the South Atlantic toward the Cape of Good Hope.
It is only at the very beginning of the eclipse path that land-based observers get an opportunity to view the ringed Sun.
No part of this eclipse will be visible from North America or Europe, but favorably situated observers within the eclipse track in Guyana, Suriname, French Guiana and a very small segment of adjacent Brazil will see the rising Sun turn into a blazing "ring of fire." The track of the so-called "negative shadow" or "anti-umbra" of the Moon will pass almost directly over the capital cities of Paramaribo, Suriname, and Cayenne, French Guiana. Also within the eclipse track is Kourou, the European Space Agency's launch site in French Guiana. All these places should see the ring effect for at least five minutes. Georgetown, Guyana (pop. 250,000) lies right on the northern limit of the eclipse track.
After the track moves out over the Atlantic, it never touches land again, passing a few hundred miles to the west of the Cape of Good Hope as well as the South Atlantic Islands of Saint Helena and Ascension. It is over the South Atlantic Ocean that the annular eclipse reaches its maximum: the apparent diameter of the Moon's disk appearing roughly 6½ percent smaller than that of the Sun. Here, the duration of annularity will last 7 minutes 9.3 seconds.
Where to see it
The associated partial phases will be visible in varying extent over most of Brazil (except the westernmost part), the eastern half of Bolivia, all of Paraguay, Uruguay, most of Chile (except its northern part) and virtually all of Argentina.
In Africa, anywhere south and west of a line running roughly from Laayoune in the Western Sahara to Quelimane in Mozambique will be able to see a part of the Moon's dark silhouette cross in front of the Sun. The Moon's penumbral shadow will also cut across the lower half of Madagascar, with locations south of the capital city of Antananarivo getting a brief glimpse of the eclipse near sunset. The closer you are to the path of the annular eclipse, the greater the amount of coverage of the Sun by the Moon.
In South America, all of Guyana, Suriname, French Guiana and the adjacent part of northern Brazil, will see at least 80 percent of the Sun's diameter eclipsed by the Moon. For all of these regions an unusual "counterfeit twilight" effect may be noticed for a few minutes around the time of mid-eclipse.
Local circumstances for a number of cities within the visibility zone of the eclipse can be found here.
This information is based on data, which appears in the 2006 Observer's Handbook of the Royal Astronomical Society of Canada, and was calculated by Fred Espenak of the NASA/Goddard Space Flight Center, Greenbelt, Maryland.
Once again it needs repeating: to look at the Sun without proper eye protection is dangerous. Even if you are in the path of the annular eclipse you will need to protect your eyes.
By far, the safest way to view a solar eclipse is to construct a "pinhole camera." A pinhole or small opening is used to form an image of the Sun on a screen placed about three feet behind the opening. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the Sun onto a white card. Just be sure not to look through the binoculars or telescope when they are pointed toward the Sun!
A variation on the pinhole theme is the "pinhole mirror." Cover a pocket-mirror with a piece of paper that has a ¼-inch hole punched in it. Open a Sun-facing window and place the covered mirror on the sunlit sill so it reflects a disk of light onto the far wall inside. The disk of light is an image of the Sun's face. The farther away from the wall is the better; the image will be only one inch across for every 9 feet from the mirror. Of course, don't let anyone look at the Sun in the mirror.
Acceptable filters for unaided visual solar observations include aluminized Mylar. Some astronomy dealers carry Mylar filter material specially designed for solar observing. Also acceptable is shade 14 arc-welder's glass, available for just a few of dollars at welding supply shops.
Unacceptable filters include sunglasses, color film negatives, black-and-white film that contains no silver, photographic neutral-density filters, and polarizing filters. Although these materials have very low visible-light transmittance levels, they transmit an unacceptably high level of near-infrared radiation that can cause a thermal retinal burn. The fact that the Sun appears dim, or that you feel no discomfort when looking at the Sun through the filter, is no guarantee that your eyes are safe.
Basic Sky Guides
- Full Moon Fever
- Astrophotography 101
- Sky Calendar & Moon Phases
- 10 Steps to Rewarding Stargazing
- Understanding the Ecliptic and the Zodiac
- False Dawn: All about the Zodiacal Light
- Reading Weather in the Sun, Moon and Stars
- How and Why the Night Sky Changes with the Seasons
- Night Sky Main Page: More Skywatching News & Features
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.