1. What's the big deal?
Solar eclipses aren't rare: a total eclipse -- when the entire sun is blocked by the moon -- is visible from someplace on earth every 18 months or so. The last total solar eclipse visible in the lower 48 states was February 26, 1979.
What makes August 21, 2017, so special is that it will be first eclipse in 99 years that's visible from the Pacific to the Atlantic. The last time there was a coast-to-coast total eclipse was June 8, 1918, when the moon's shadow, or umbra, crossed from Washington to Florida.
The next solar eclipse that touches such a wide swath of the United States isn't until 2024, when it crosses from Texas to Maine.
2. What's happening?
There are several kinds of solar eclipses, all of which occur when the moon passes between Earth and the sun. Unlike partial eclipses of the sun, which are fairly common, a total eclipse involves a series of coincidences:
1. The moon must in its "new" phase, when the dark side is facing Earth. The moon casts a shadow during every new phase, but because the moon's orbit around the Earth is slightly tilted -- about 5 degrees -- the umbra usually misses the Earth.
2. To account for this slight tilt, the moon must be at one of two specific spots in its orbit, when an imaginary line would intersect the sun, moon and Earth. These points in the moon's orbit, when the umbra hits the Earth, are called nodes.
3. The sun and the moon must appear to be the same size in the sky. The sun is 400 times the size of the moon and 400 times farther away. They achieve the same 'angular' size only when a new moon is at the point in its elliptical orbit when it's closest to Earth.
3. How can I see it?
Most people in the U.S. will see only a partial solar eclipse, with the moon covering about 80 percent of the sun. That's not bad, and certainly better than along the Mexican border and in New England, where coverage will be roughly 65 percent.
But at the urging of anyone who has seen a total eclipse, millions of people are expected to travel from their homes in partial-eclipse territory to points along the "path of totality" -- the 70-mile-wide track of the moon's shadow across the earth's surface.
Moving an average of 1,600 mph, the shadow will cross the northern coast of Oregon at 10:15 a.m. and cut across the country to South Carolina in about 94 minutes, passing through Charleston at around 2:49 p.m. Along the way, the path of totality will pass through Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia and North Carolina.
Of course, the closer you are to the center of the path of totality, the greater the moon's coverage. But even then, "totality' will last no more than 2-3 minutes, growing shorter as you move toward the edge of the path, where it might last just a few seconds. Totality is shortest along the path at the beginning and toward the end of the eclipse.
Wherever you are, be ready: Eclipses travel at three times the speed of a commercial airliner, according to space.com, so they're here and gone pretty quickly -- one reason why experienced viewers warn against attempting to photograph a total eclipse.
4. What will I see?
People who have witnessed a total eclipse are often at a loss for words, although "life-changing" isn't an uncommon way to describe the experience. "It brings people to tears," Rick Feinberg, press officer for the American Astronomical Society, told Space.com. "It makes people's jaw drop."
A total solar eclipse is, as NASA puts it, "a rare opportunity to look directly at the sun’s vast, striking outer atmosphere," or corona, which appears as white jets and streamers that radiate light from the edges of the pitch-black moon. Shadow bands or shadow snakes -- "thin, wavy lines of alternating light and dark" -- can be seen undulating on plain-colored surfaces before and after totality.
It's also possible to see the moon's shadow approaching on the ground -- an effect known as the "cone of darkness" -- as totality nears and to see it move away as it diminishes. That can quiet the chirping of birds, trigger streetlight and draw nocturnal animals out of their lairs.
Viewers who take a moment to look at the horizon during a total eclipse will see what appears to be a 360-degree sunset, caused by sunlight outside of the path of totality.
5. What do I need?
A solar eclipse is unlike anything you've laid your eyes on and potentially more dangerous. Unless you are in the path of totality, when the moon completely blots out the sun's rays, protective measures are required - and we're not talking ordinary sunglasses, which typically reduce sunlight by just 50-60 percent.
The best way to eliminate the risk of permanent eye damage is to invest in a special-purpose filter, such as eclipse glasses or a handheld solar viewer. In the wake of increased complaints of online sales of bogus eclipse viewers, the American Astronomical Society has put together a list of vendors whose eclipse glasses and handheld viewers meet the ISO 12312-2 international safety standard.
With August 21 quickly approaching, there's a good chance that if you haven't already purchased a good solar filter, you're out of luck.
You could check with your local library. The STAR Library Network has distributed more than 2 million pairs of eclipse glasses to some 7,000 libraries and bookmobiles in all 50 states
Or, check out the American Astronomical Society's instructions on how to make a pinhole or optical projection device. Another DIY possibility is a sun funnel, a rear-screen projection that's perfect for groups of people gathered to watch the eclipse together.