Thursday, August 17, 2017

Total Eclipse of the Sun

On Monday, August 21, 2017, all of North America will be able to see an eclipse of the sun. Anyone within the path of totality from Salem, Oregon to Charleston, South Carolina can see a total solar eclipse. The path of totality is where the moon will completely cover the sun making the sun’s corona visible.

The shadow of the moon enters the United States near Lincoln City, Oregon, at 9:05 a.m. PDT. Totality begins in Lincoln City, Oregon, at 10:16 am PDT. The total eclipse will end and hour and 2 minutes later in Charleston, South Carolina, at 2:48 p.m. EDT, though the lunar shadow will linger in the United States until 4:09 p.m. EDT. A partial eclipse will be visible throughout the United States. Here in Virginia we will see a partial solar eclipse where the moon covers about 85% of the sun's disk.

Looking directly at the sun is unsafe except during the brief total phase of a solar eclipse (“totality”), when the moon entirely blocks the sun’s bright face, which will happen only within the narrow path of totality and only during the window of complete coverage which is about 2 minutes 40 seconds. Otherwise you must protect your eyes and vision. The only safe way to look directly at a partially eclipsed sun is through special-purpose solar filters, such as “eclipse glasses or hand-held solar viewers.

Homemade filters or ordinary sunglasses, even  dark ones, are NOT SAFE for looking at the sun; they transmit thousands of times too much sunlight. Eclipse glasses and handheld solar viewers must be verified to be compliant with the ISO 12312-2 international safety standard for such products. However, there have been reports in the news of large numbers of fake solar glasses. It's not enough today to just look for the ISO 12312-2  certification, as many vendors have started printing fake glasses with ISO 12312-2  certifications. At this point, if your have not purchased solar glasses from a vendor from the approved list of the American Academy of Ophthalmology or NASA it’s probably too late and you should protect your eyes and view the eclipse on NASA’s web site or through a pinhole projector as we did when we were kids.

As described on NASA’s web site, a convenient method for safe viewing of the partially eclipsed Sun is pinhole projection. You simply pass sunlight through a small opening (for example, a hole punched in an index card) and project an image of the Sun onto a nearby surface (for example a piece of printer paper). See the NASA website for full instructions. I will be inside viewing the eclipse on-line. There is also lots of fun information about science data gathering that will take place during the eclipse.

Do not look at the  partially eclipsed sun through an unfiltered camera, telescope, binoculars, or other optical device- even if you are wearing eclipse glasses. The magnification will damage the eclipse glasses and damage your eyes. Be safe.


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