From Oregon to South Carolina, a total solar eclipse on Monday will darken the sky as it arcs across the contiguous United States.
A total solar eclipse is when the moon moves between the sun and Earth, an occurrence that lasts up to three hours from beginning to end. Monday's total solar eclipse is particularly rare because it's the first time in 99 years that the path of totality exclusively crosses the continental United States from coast to coast. It's also the first continent-wide eclipse to be visible only from the United States since 1776.
The last time the contiguous United States saw a total solar eclipse was Feb. 26, 1979, when the path of totality crossed the Pacific Northwest. ABC News' Frank Reynolds anchored a special report on the celestial phenomenon at the time and pledged that the network would cover the next total solar eclipse in 2017.
“So that’s it -- the last solar eclipse to be seen on this continent in this century. And as I said not until Aug. 21, 2017, will another eclipse be visible from North America. That’s 38 years from now. May the shadow of the moon fall on a world in peace. ABC News, of course, will bring you a complete report on that next eclipse 38 years from now,” Reynolds said before signing off.
You must be in the path of totality to witness a total solar eclipse. NASA estimates more than 300 million people in the United States could potentially view the total solar eclipse in its entirety.
However, a partial solar eclipse will be visible in every U.S. state. In fact, everyone in North America, as well as parts of South America, Africa and Europe, will see at least a partial eclipse, according to NASA.
The path of totality for the Aug. 21 solar eclipse is a 70-mile-wide ribbon that will cross the United States from west to east, sweeping over portions of 14 U.S. states: Oregon, Idaho, Wyoming, Montana, Nebraska, Iowa, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina.
The moon's shadow will start to eclipse the sun over the West Coast just after 9 a.m. PT. From there, the moon's shadow will speed across the country and leave the East Coast just after 4 p.m. ET.
The exact times for partial and total phases of Monday's eclipse vary depending on your location.
With millions of people expected to pour into the select U.S. cities located within the path of totality, law enforcement, emergency personnel and hospitals there are on high alert.
"It's all hands on deck," Kentucky's Madisonville Police Chief Wade Williams told ABC News. "We're kind of throwing everything at it."
The state of Oregon alone is anticipating a million visitors Monday, causing some local hospitals to cancel elective surgeries and call in extra help for the emergency rooms. Some cities even preemptively declared a state of disaster, a move that allows them to call in the National Guard to help direct the large crowds if needed.
"If a police department in a certain area is overwhelmed and they need us to help come and set up traffic control check points, we're ready to do that," Oregon National Guard spokesperson Leslie Reed told ABC News.
More than than 17,000 cars and SUVs have already been rented for Monday at Oregon's Portland International Airport, a number it normally hits over an entire week.
In northeast Georgia, the mountain town of Blairsville is expecting to host up to 200 percent more people than the number of residents who live there. The town's hotels are fully booked and camping sites are sold out.
"There is no reason to panic," Blairsville-Union County Chamber of Commerce tourism director Tobie Chandler told ABC News. "We are not going to run out of gas, we are not going to run out of groceries. We just need to enjoy this event."
The American Red Cross has set up resources along the path of totality to help keep people safe during Monday's rare celestial event.
"One thing is we really encourage folks to have in their cars an emergency go kit and that should include things like water, non-perishable foods, a flashlight with batteries and an envelope with cash," Josh Lockwood, regional CEO for the American Red Cross Greater New York region, told ABC News.
Authorities warned that people might have to look for landlines in some areas to make emergency phone calls, as some small towns may not be able to handle the large number of cellphone calls.
ABC News' Steve Osunami, Gina Sunseri and Ariella Weintraub contributed to this report.