On Monday, Aug. 21 at 1:31:35 p.m., the moon will obscure 99.1 percent of the sun from view for people living in Sewanee. Just 26 miles northeast, the moon will block the sun completely. The entire United States, including Alaska and Hawaii, will experience at least a partial solar eclipse, with 21 million people living in the path of the rare spectacle of the total eclipse.

The last total solar eclipse touching the United States occurred in 1979. It has been 99 years since the entire country had the opportunity to view a solar eclipse. In Sewanee, starting at 12:01 p.m., the moon will begin blocking a small then gradually increasing portion of the sun’s disk until only a small sliver is visible. For those in the path of the 70-mile-wide total eclipse shadow, the moon will block the sun entirely for about two minutes, depending on the location. Dunlap is the nearest town in the path of totality, and much of Savage Gulf State Park will experience a total eclipse.

Three circumstances need to coincide for a total solar eclipse to occur. The moon needs to be in the new moon or dark moon phase; this happens approximately once a month when the moon is between the earth and sun. Usually, though, because the moon’s orbit is tilted, the shadow the moon casts in the new moon phase is above or below the earth; for a total solar eclipse, the new moon’s shadow must fall directly on the earth, which only occurs twice a year. And finally, the moon, whose orbit is elliptical not round, must be at or near the point closest to the earth so the moon appears sufficiently large to block the sun entirely; this occurs every 27 and one-half days.

The three conditions necessary for a total solar eclipse only coincide every 18 months. In any given location, the phenomenon only recurs every 375 years!

But rightfully eager eclipse viewers should use caution.

Do not view the eclipse without approved eyewear. It’s never safe to look at the sun. The sun’s intense light can damage the retina causing permanent scotoma or ‘blind spot’ in the central vision. Normally people squint or blink when looking at the sun. During an eclipse the moon obscuring part of the sun makes the sun’s brilliance easier to look at, but the sun’s rays are every bit as dangerous.

Regular sunglasses don’t block enough light. Eclipse viewers need eyewear blocking all but 0.003 percent of the visible light. Welders goggles meet the standard, but eclipse glasses cost as little as $1. Look for NASA approved eclipse eyewear manufacturers—American Paper Optics, Rainbow Symphony, Thousand Oaks Optical, and TSE 17. The American Astronomical Society also sanctions Baader Planetarium eclipse glasses.

The first thing those viewing the eclipse will see is a crescent shape beginning to obscure the sun. Bits of light called Bailey’s beads appear around the edges of the crescent because of the moon’s cratered and creviced rough surface.

Right before totality, the last glimpse of light from the sun gives the image a diamond ring effect with a glittering jewel at the apex.

The Space Science Institute distributed two million pairs of eclipse glasses to 4,800 public libraries. The Franklin County Public Library and Coffee County Manchester Public Library will host eclipse programs offering free eyewear beginning at 10 a.m. on Aug. 21.

Sewanee Elementary School has a variety of activities planned. All teachers, students, and staff will wear black in honor of the eclipse and don safety glasses to view the rare event.

The next total solar eclipse will begin over the Southern Pacific then pass across Chile and Argentina. The next total solar eclipse touching the continental United States won’t occur until 2024.