Update: Watch the eclipse live

A total solar eclipse, which has been described as a once in a lifetime event - will slide across the U.S. on Monday, August 21.

The totality, with the moon completely blocking out the rays of the sun, will first strike on the West Coast, at Lincoln Beach, Oregon, at 1:16 p.m.

It will then move across the country in a southeasterly direction along a 70-mile wide path of totality across parts of Oregon, Idaho, Wyoming, Montana, Nebraska, Iowa, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina.

The lunar shadow will leave the U.S. at 4:09 p.m.

The duration of the total solar eclipse will vary by where in the path of totality an observer might be. Its longest duration will be near Carbondale, Illinois, where the sun will be completely covered for two minutes and 40 seconds.

Beyond the 70-mile-wide path of totality, observers in the continental U.S., including those in Pennsylvania, will be treated to a partial solar eclipse. For them, it will look the moon is taking a bite out of the sun's disk.

Observers in Pennsylvania will see the moon covering about 75-80 percent of the sun.

In Pittsburgh, the partial eclipse will begin at 1:10 p.m., hit its maximum coverage at 2:35 p.m. and end at 3:55 p.m.; in Harrisburg, 1:14 p.m., 2:38 p.m. and 3:56 p.m.; and in Philadelphia, 1:21 p.m., 2:44 p.m. and 4:01 p.m.

Never look at a solar eclipse without proper eye protection.

Although they seem to be sold-out everywhere, solar-viewing glasses are recommended for anyone planning to view the total solar eclipse, and those glasses should meet the international safety standard ISO 12312-2.

However, according to American Astronomical Society, "the marketplace is being flooded by counterfeit eclipse glasses that are labeled as if they're ISO-compliant when in fact they are not." The society compiled a list of suppliers and their products that are verified as meeting the standard: American Paper Optics (Eclipser), APM Telescopes (Sunfilter Glasses), Baader Planetarium (AstroSolar Silver/Gold Film), Celestron (EclipSmart Glasses and Viewers), DayStar (Solar Glasses), Explore Scientific (Solar Eclipse Sun Catcher Glasses), Lunt Solar Systems (Sunshade SUNglasses), Meade Instruments (EclipseView Glasses and Viewers), Rainbow Symphony (Eclipse Shades), Seymour Solar (Helios Glasses), Thousand Oaks Optical (Silver-Black Polymer and SolarLite) and TSE 17 (Solar Filter Foil).

Many eclipse-viewing events across Pennsylvania will have supplies of glasses that meet the recommended safety standards on a first-come-first-served basis. Here's a list of those eclipse-viewing events.

A pinhole camera is another device that will render safe the viewing of a solar eclipse, total or other. It's basically two surfaces, one with a pinhole through its center that projects the image of the sun onto the second. NASA's Jet Propulsion Laboratory has developed step-by-step instructions for making a pinhole camera, which is illustrated in this PennLive.com video.

Viewers around the world will be provided a flood of images captured before, during and after the eclipse by 11 spacecraft, at least 3 NASA aircraft, more than 50 high-altitude balloons, and the astronauts aboard the International Space Station, each offering a unique vantage point for the celestial event. It will be available through NASA's Eclipse 2017 website.

NASA EDGE will join forces with the NASA Heliophysics Education Consortium, the University of Southern Illinois Carbondale, and Lunt Solar Systems to air a 4-hour 30-minute live webcast of the total solar eclipse from outside Saluki Stadium at Southern Illinois University Carbondale in Illinois. The webcast will begin airing at 11:45 a.m. During the Megacast, NASA EDGE will be tracking the eclipse as it starts in Oregon and makes its way across the country ending in South Carolina. Lunt Solar Systems will be providing high resolution and stunning imagery of the eclipse in three different wavelengths of light: Hydrogen-Alpha, Calcium-K and white light. Viewers will experience a scientific balloon launch from inside Saluki Stadium, observe several science demonstrations, learn how they can become citizen scientists, and engage with subject matter experts through social media.

More about the total solar eclipse of 2017