Aug. 21 (UPI) -- Monday's total solar eclipse is expected to break records, with about 220 million people expected to watch.
Some believe the event will inspire the largest temporary migration in human history. With camera-equiped smartphones now ubiquitous, it's also likely to become the most documented event ever.
When and where
The moon's shadow will make landfall in Depoe Bay, Ore., at 10:17 a.m. PDT. The eclipse will move west and south across the United States, tracing what's called the path of totality. By 2:47 p.m. EDT, the moon's shadow will move across Charleston, S.C., and out into the Atlantic Ocean.
The shadow's trip across the country will last just 90 minutes. The path will intersect 14 states, including Oregon, Idaho, Montana, Wyoming, Nebraska, Kansas, Missouri, Iowa, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina.
Inside the 70-mile wide path of totality, the solar eclipse will be experienced as full, or total. The moon will block the entirety of the sun. Only the sun's atmosphere, its corona, will appear around the edge of the moon's silhouette.
Outside the path of totality, the solar eclipse will be experienced as a partial eclipse. The farther way one moves from the path of totality, the smaller the percentage of the sun that will be darkened by the passing moon.
The difference between a partial eclipse and a total solar eclipse is substantial. When the last 1 percent of the sun is eclipsed by the moon, the shadow becomes roughly 10,000 times darker.
How to watch
For those traveling to the path of totality, avoiding clouds and crowds will be the name of the game.
The National Park Service has compiled a list of all the national parks, monuments, trails, preserves and battlefields through which the path of totality passes. However, space could be at a premium.
"The national parks are going to be overrun with tourists," Henry "Trae" Winter, organizer of NASA's Eclipse Soundscapes project, told UPI.
Winter and his colleagues, who will be in Nebraska to watch the eclipse, are aiming to watch from an empty parking lot.
"Hopefully next to a diner with really good pie," Winter said.
Those unable to make it to a diner parking lot within the path of totality will have plenty of ways to watch online. NASA will be live streaming the event from three airplanes, 11 spacecraft, 57 high- altitude balloons and several ground telescopes. All of the streams can be accessed on NASA's live-stream page.
NASA's mobile phone apps will also help viewers find a live stream of the event. In fact, a live stream of the eclipse will be shown on almost any platform where NASA has a presence, including Facebook, UStream and YouTube.
The online observatory Slooh will also be streaming the eclipse live from a telescope in Stanley, Idaho.
What is a solar eclipse
Solar eclipses occur when the moon passes in front of the sun. Of course, the moon regularly passes in front of the sun, but because the moon orbits Earth at an angle relative to the equator, the moon's shadow casts out into space, either above or below Earth. The moon's path has to pass in front of the sun at just the right point in its orbit to cast a shadow on Earth's surface.
Furthermore, the moon's orbit isn't a perfect circle. It's an ellipse. At its pedigree, its shortest distance from Earth, the moon is 220,000 miles away. At its apogee, the moon is 250,000 miles away. If it is too far away from Earth when it passes across the sun, the moon is too small to black the entirety of the sun's face, creating what's known as an annular eclipse.
Eclipses aren't as rare as some may think. Either a partial or total solar eclipse happens more than 1.5 times per year, and a total eclipse occurs about every 18 months.
What is rare is the chance of the moon's shadow being cast in any single location, much less any single location on dry land. Earth's surface is 70 percent water. The majority of eclipses see the moon's shadow cast upon open ocean. The chance of seeing a total eclipse in a single location is about once a lifetime -- which explains why so many people are en route to the path of totality.
The last time a total solar eclipse was visible from the continental United States was Feb. 26, 1979. The moon's shadow darkened Washington, Oregon, Idaho, Montana and North Dakota. The wait between Monday's total solar eclipse and the next one won't be nearly as long.
The sun will once again by blocked entirely by the moon on April 8, 2024. The path of totality will move north-northwest from Texas across Oklahoma, Arkansas, Missouri, Illinois, Kentucky, Indiana, Ohio, Pennsylvania, New York, Vermont, New Hampshire and Maine.