— -- Wherever you are in the United States on Aug. 21, you may want to look up and watch as the sun goes dark, with the moon casting a shadow across the country.
Any astronomer in the U.S. would suggest that you take advantage of this rare opportunity but not to do so without proper eye protection. And sunglasses won't do.
The brief total phase of a solar eclipse -- when the moon entirely covers the sun's beaming face -- will occur on Aug. 21 and will last for no longer than 2 minutes and 40 seconds, according to NASA.
The 70-mile-wide path of totality will sweep across portions of 14 U.S. states: Oregon, Idaho, Wyoming, Montana, Nebraska, Iowa, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina. NASA estimates more than 300 million people in the country could directly view the total solar eclipse on Aug. 21.
Meanwhile, a partial solar eclipse will be visible in every U.S. state on Aug. 21. In fact, everyone in North America — as well as people in parts of South America, Africa and Europe — will see at least a partial eclipse on that day, according to NASA.
There's no health risk to simply being outside during an eclipse. But the only safe way to look directly at the uneclipsed or partially eclipsed sun during this astronomical event is through special-purpose solar filters.
Eclipse glasses and handheld solar viewers
Any people who plan to view the eclipse with just their eyes must obtain a pair of eclipse glasses or handheld solar viewers, which must meet an international safety standard.
NASA recommends checking the safety and authenticity of eclipse glasses or handheld solar viewers to make sure they meet all the following:
Do not use homemade filters.
Ordinary sunglasses — even very dark ones — should not be used as a replacement for eclipse-viewing glasses or handheld solar viewers.
With counterfeit eclipse glasses hitting the market, NASA and the American Astronomical Society suggest that consumers purchase products only from a list of verified brands and vendors to ensure safe viewing.
The American Astronomical Society says a number of manufacturers have certified that their eclipse glasses and handheld solar viewers meet the ISO 12312-2 international standard for such products. For more information on reputable vendors, click here.
Some eclipse glasses and handheld solar viewers come with warnings that users shouldn't look at the sun through them for more than three minutes at a time and that they should be discarded if they are more than three years old. Be sure to read and follow any instructions printed on or packaged with the item.
According to the American Astronomical Society, you must cover your eyes with your eclipse glasses or solar viewer before looking at the sun. After looking at the sun, turn away and then remove your filter. Do not remove it while looking at the sun.
Outside the path of totality, you must always use safe eclipse glasses or a solar viewer to look directly at the sun.
Remember to always supervise children using eclipse glasses or handheld solar viewers.
"Looking at the sun without eclipse glasses or solar viewers can cause eclipse blindness or retinal burns," said Nirav Shah, the director of the Illinois Department of Public Health. "Homemade filters or ordinary sunglasses, even very dark ones, are not safe for looking at the sun."
If you normally wear eyeglasses, the American Astronomical Society says to keep them on and put your eclipse glasses on over them or hold your handheld viewer in front of them.
Over 6,800 libraries across the country are distributing safety-certified glasses for the Aug. 21 eclipse, with many collaborating with scientists to hold viewing events and activities before and during the event. For a list of participating libraries, click here.
If you plan on watching the eclipse through a camera, telescope, binoculars or any other optical device, the American Astronomical Society says to buy a solar filter to place on the end of the lens. Do not look at the uneclipsed or partially eclipsed sun through any of these without a solar filter.
Similarly, do not look at the sun through any of these while wearing eclipse glasses or a handheld solar viewer. The concentrated rays could damage the filter and cause serious injury to your eyes, according to the American Astronomical Society.
If you are within the path of totality, remove the solar filter from your optical device only when the moon completely covers the sun's face. Replace the filter as soon as the sun begins to reappear to watch the remaining partial phases of the eclipse.
Outside the path of totality, you must always use a safe solar filter to look directly at the sun through an optical device.
The American Astronomical Society advises to always inspect your solar filter before use. If the filter is scratched, punctured, torn or otherwise damaged, discard it. Be sure to read and follow any instructions printed on or packaged with the filter.
As long as filters aren't damaged in any way, the American Astronomical Society says you may reuse them indefinitely.
The American Astronomical Society recommends seeking expert advice from an astronomer before using a solar filter with a camera, telescope, binoculars or any other optical device.
A number of manufacturers have certified that their solar filters for cameras, telescopes and binoculars meet the ISO 12312-2 international standard for such products, according to the American Astronomical Society. For more information on reputable vendors, click here.
Remember to always supervise children using solar filters.
Pinhole projection, other safe viewing methods
There are alternative methods for safely viewing the partially eclipsed sun. One convenient technique is pinhole projection: passing sunlight through a small opening and projecting the resulting image of the sun onto a nearby surface. And you don't need any special equipment.
Follow the American Astronomical Society's instructions on how to create a pinhole projection during the partial phases of a solar eclipse using just your hands:
Remember that pinhole projection doesn't mean looking at the sun through a pinhole. Rather, you project sunlight through the hole onto a surface, such as a wall or the ground, and you look at the solar image on that surface.
Pinhole projection is not useful for observing the totality phase of a solar eclipse because the projected image will be too faint to see. The American Astronomical Society says it is perfectly safe to look directly at the eclipsed sun during totality.
For other safe viewing methods via projection, click here.