Annular Solar Eclipse

On the late afternoon of May 20th, most of the United States and Canada will have an opportunity to watch an eclipse of the Sun.  Nearly everywhere it will be a partial solar eclipse—but for those living in a narrow band across the southwestern part of this country, the event will be seen as an “annular eclipse.”  In either case, solar eclipses are relatively rare in any given location, and if skies are clear that day it should not be missed!

Except those living along the Eastern Seaboard, readers across the country (and Canada) should mark the late afternoon of Sunday, May 20th, on their calendars.  Anywhere west of a line running through the middle of New York State, Pennsylvania, Virginia, the extreme western portions of North and South Carolina, and northwestern Georgia, just before sunset the Moon will take a bit out of the edge of the Sun.  The further west you live, the more of the eclipse will be seen and the bigger the bite taken out of the Sun before it sets.  The entire eclipse will be visible in the western third of the U.S.

For those lucky enough to be situated in a narrow zone about 150 miles wide stretching from the panhandle of Texas all the way to the California-Oregon border, an annular eclipse will be seen.  A total solar eclipse is Nature’s grandest spectacle (just ask anyone who has seen one!).  It occurs when the Moon completely blocks the Sun’s disk, causing darkness to fall, the stars to come out and the temperature to drop dramatically—all in an event lasting only minutes.  (It’s one the many “coincidences” found in astronomy that the Sun is 400 times bigger in apparent size than the Moon, but the Moon at it’s average distance is 400 times closer!)  Since the Moon’s orbit is elliptical, at times it’s somewhat further away from us—enough so that its disk is a little too small to completely block the Sun. This results in an annular eclipse in which a ring (or annulus) of the Sun is seen shining around the Moon.  Its duration will range from about 1 minute to 4 1/2 minutes, depending on location.

Observers must exercise care in watching this event.  Looking at the Sun through even the darkest pair of sunglasses will not protect you.  They may filter its visible light, but harmful ultraviolet and infrared rays will still get through.   One traditional recommendation is looking through a #14 welder’s filter.  Better are an inexpensive and safe pair of “Eclipse Shades” similar in construction (only!) to the 3-D glasses being used in theaters, or a hand-held “Eclipse Viewer,” both of which are available from Rainbow Symphony at: http://www.rainbowsymphony.com/soleclipse.html.  For up-close views of the eclipse, binoculars and telescopes are recommended.  However (and here especially), proper filters are an absolute must to avoid serious eye damage or blindness!  And the filters must be of the type that fit over the front of a telescope or both binocular lenses and NOT over their eyepieces.  Many inexpensive small telescopes (especially imported ones) have an eyepiece filter marked “Sun” on it.    These are absolutely deadly and often crack due to the intense heat being focused by the telescope itself.  Stopping most of the light and heat of the Sun before it enters an instrument is the only safe and sane way to view it directly. One extensive source for optical solar filters is Thousand Oaks Optical at: http://thousandoaksoptical.com/ (click on the “solar filters” icon).  Another safe but not as effective way to view the eclipse is to project the Sun’s image through a pinhole in a piece of cardboard and onto a white sheet of paper.  The projected image will be quite small, but depending on the hole size and how far back the screen is placed it will show a black chunck taken out of the Sun by the Moon. A similar scheme called “eyepiece projection” is sometimes used with telescopes themselves but has been known to crack eyepieces if prolonged too long.  And here, as any time a telescope is being used for solar viewing, its finder scope must be capped since it can also cause damage!

— James Mullaney

Former assistant editor at Sky & Telescope magazine & author of eight books on stargazing.