"During those brief moments when the moon completely blocks the sun's bright face … day will turn into night, making visible the otherwise hidden solar corona (the sun's outer atmosphere)," according to NASA's Eclipse website. "Bright stars and planets will become visible as well. This is truly one of nature's most awesome sights."
But in order to see this awesome natural sight, skywatchers need to know how to view the eclipse safely. In an effort to inform the public on this topic, an information guide on safe viewing has been written up and released by NASA, along with the American Astronomical Society (AAS), the American Academy of Ophthalmology, the American Academy of Optometry and the National Science Foundation.
Eye protection for looking at the sun
Looking directly at the sun without eye protection can cause serious eye damage or blindness. But there are ways to safely observe the sun. During a partial solar eclipse, people often use pinhole cameras to watch the progress of the moon across the sun's surface (pinhole cameras are easy to make at home). This is an "indirect" way of observing the sun, because the viewer sees only a projection of the sun and the moon.
To view the sun directly (and safely), use "solar-viewing glasses" or "eclipse glasses" or "personal solar filters" (these are all names for the same thing), according to the safety recommendations from NASA. The "lenses" of solar-viewing glasses are made from special-purpose solar filters that are hundreds of thousands of times darker than regular sunglasses, according to Rick Fienberg, press officer for the American Astronomical Society (AAS). These glasses are so dark that the face of the sun should be the only thing visible through them, Fienberg said. Solar-viewing glasses can be used to view a solar eclipse, or to look for sunspots on the sun's surface.
But beware! NASA and the AAS recommend that solar-viewing or eclipse glasses meet the current international standard: ISO 12312-2. Some older solar-viewing glasses may meet previous standards for eye protection, but not the new international standard, Fienberg said.
Buy Eclipse Glasses by American Paper Optics on Amazon.com
Buy Rainbow Symphony Eclipse Glasses on Amazon.com
"Manufacturers that meet this standard include Rainbow Symphony, American Paper Optics and Thousand Oaks Optical," according to the information sheet on safe eclipse viewing. (Click any of the company links to find out how to purchase eclipse glasses). "Homemade filters or ordinary sunglasses, even very dark ones, are not safe for looking at the sun."
Fienberg said some manufacturers are making solar-viewing glasses with plastic frames, rather than the traditional paper frames. While these may look like regular sunglasses, do not be mistaken. Sunglasses are never a substitute for solar-viewing glasses. Fienberg said some people may even try to view the sun through two or three pairs of sunglasses in an attempt to replicate the protective power of real solar-viewing glasses; however, even multiple pairs of sunglasses will not protect your eyes from sun damage.
Telescopes, cameras, binoculars and other optical devices need their own solar filters. Solar-viewing glasses are not a substitute for a proper solar filter on magnification devices. Never view the disk of the sun through a telescope, binoculars or camera without a proper solar filter. Solar-viewing glasses are not powerful enough to protect your eyes from magnified sunlight. Even if you are wearing solar-viewing glasses, viewing the disk of the sun through a magnification device will result in serious eye damage if the device is not equipped with a proper solar filter, according to the viewing safety sheet.
"The concentrated solar rays will damage the filter and enter your eye(s), causing serious injury," according to the safety recommendations. "Seek expert advice from an astronomer before using a solar filter with a camera, a telescope, binoculars, or any other optical device."
Fienberg said there is no need for skywatchers to use a telescope during the eclipse, but a pair of binoculars can be helpful during totality. But, as per the recommendations, do not attempt to look at the disk of the sun through binoculars, even with solar-viewing glasses.
The safety sheet offers these tips regarding solar filters/eclipse glasses/solar viewers:
• Always inspect your solar filter before use; if scratched or damaged, discard it. Read and follow any instructions printed on or packaged with the filter. Always supervise children using solar filters.
• Stand still and cover your eyes with your eclipse glasses or solar viewer before looking up at the bright sun. After glancing at the sun, turn away and remove your filter — do not remove it while looking at the sun.
• Do not look at the uneclipsed or partially eclipsed sun through an unfiltered camera, telescope, binoculars or other optical device.
Safety during totality
Now that you have some general information about how to view the sun safely, here are NASA and the AAS's recommendations for how to safely view the total solar eclipse with the naked eye. Again, these tips come from NASA's safety information sheet here.
Viewers who are looking at the eclipse with solar-viewing glasses will be able to see when the sun's face is completely obscured by the moon (because, once again, the only light that can penetrate these solar-viewing glasses is the light from the sun's disk). Viewers will be able to observe the moon creep slowly over the sun's disk and eventually cover the sun entirely.
In the moments before totality, viewers looking through their solar-viewing glasses will see a crescent of light from the sun growing thinner and thinner as the moon progresses over its face. In the last few seconds just before the disk of the sun is entirely covered by the moon, the crescent will break up into a series of small dots of light that look like beads on a string (typically there are about three to eight such dots, according to Fienberg). These are called Baily's beads (after Francis Baily, the British astronomer who discovered them). Once the last bead disappears, the face of the sun has been covered by the moon, and totality has begun. [Solar Eclipses: An Observer's Guide (Infographic)]
"If you are within the path of totality, remove your solar filter only when the moon completely covers the sun's bright face," according to the official safety information sheet.