In just two months, a total solar eclipse will sweep across the United States. On Aug. 21, 2017, observers along the eclipse's 70-mile (113 kilometers) path will see the sun slowly vanish behind the moon, turning midday into twilight and revealing the hidden layers of the sun's atmosphere.
"Most people never get to see a total solar eclipse, so this is an amazing opportunity for Americans," Angela Speck told Space.com by email. Speck, a researcher at the University of Missouri, is a member of the American Astronomical Society (AAS) eclipse team.
Today (June 21) — which happens to be the summer solstice, or first day of summer, in the Northern Hemisphere — NASA is webcasting two news conferences featuring experts on eclipse science, as well as eclipse safety, travel and traffic information. The first briefing, which will focus on anticipated crowd sizes and traffic levels on Aug. 21, will run from 1 p.m. to 2 p.m. EDT (1700 to 1800 GMT). The second briefing will focus on eclipse science, and will go from 2:30 p.m. to 3:30 p.m. EDT (1830 to 1930 GMT). You can watch the webcasts on NASA TV or here on Space.com.
This so-called Great American Eclipse will be the first total solar eclipse to touch the continental United States since 1979, and the first to cross from coast to coast since 1918. Not until 2024 will another total solar eclipse cross the continental U.S., though that eclipse will be visible only east of Texas. [Amazing Solar Eclipse Pictures from Around the World]
For the August total solar eclipse, however, NASA estimates that most Americans live within a two-day drive of the path.
What will you see during the total eclipse?
A solar eclipse occurs when the sun appears to pass behind the moon as seen from Earth. In a total eclipse, the sun vanishes completely behind the Earth's lunar companion, while less than 100 percent of the sun's disk is hidden during a partial eclipse. Totality (the period when the sun is completely hidden) is short, lasting only a few brief minutes. But it can be quite stunning for those fortunate enough (or with foresight enough) to be within the path of totality.
Once the sun disappears, its atmosphere becomes visible. Although the hot gases that make up the solar corona are always present, the hotter central disk of the star usually outshines this outer region. During totality, however, with the disk blocked, the atmosphere's tangle of streamers and loops becomes visible.
The disappearance of the body of the sun can cause the temperature to drop. Animals may behave as though it is nighttime, and signals from radio stations can bounce through the atmosphere differently. Observers may be able to view the approaching shadow of the eclipse in the minutes before totality.
During the partial phase, when the moon appears to "take a bite" out of the sun, viewers can use pinhole cameras to watch the moon progress. But a tree will work as well, said veteran eclipse chaser and press officer for the AAS Rick Fienberg.
"During the partial phases, especially when the sun has been reduced to a crescent, look under a tree," Fienberg said. "You'll see the ground dappled with little crescent suns projected by the spaces between the leaves!"
Both Speck and Fienberg stressed that first-time viewers should not focus on shooting photographs but should instead spend the brief time of the eclipse enjoying the rare event. In addition to the corona, observers should look at the sunset colors and unusual light on the horizon, as well as the stars and planets that will be visible.
"Just take it in as much as you can," Speck said.
You're not completely out of luck if you can't make it to the eclipse path. "You will still have a partial solar eclipse if you're anywhere within North America," Fienberg said.
As long as the sun is visible, observers should view the event only through designated eclipse glasses. Observers viewing a partial eclipse will need to leave their viewing glasses on during the entire event, as the sun will always be visible.
For those in the path of totality, during the 2 to 3 minutes of totality, when the body of the sun is hidden from view, it is safe to remove your eclipse glasses.
"It is perfectly safe to view the totally eclipsed sun without any filters," Fienberg said. "In fact, if you leave your filters on, you won't see anything at all during totality."
REMEMBER: Looking directly at the sun, even when it is partially covered by the moon, can cause serious eye damage or blindness. NEVER look at a partial solar eclipse without proper eye protection. See our complete guide to find out how to view the eclipse safely.
Many eclipse enthusiasts, along with plenty of first-time viewers, have known about the Great American Eclipse for years and have made plans accordingly, as evidenced by the fact that most hotels along the path are booked. That poses a challenge for anyone hoping to make a last-minute reservation.
Campgrounds can provide another option. A day-trip to the path of totality may be an option, but because the shadow of the total eclipse is within driving distance for millions of people, NASA anticipates that traffic will be heavy. Travelers should budget plenty of extra time to reach their destinations the day of the eclipse. If you haven't made your travel plans yet, both Fienberg and Speck said you should make them now.
While the eclipse itself is certainly worth the trip, Fienberg recommended locating an eclipse party for an enhanced experience.
"Organized events will have knowledgeable experts to guide your experience and, usually, [will] provide solar-eclipse viewers that meet the ISO international safety standard," he said.
Many cities in the path are also hosting multiday festivals to accompany the eclipse. If you're planning a multiple-day trip to see the eclipse, take a look at our state-by-state guide to find other attractions near your observing location.
While some events will distribute eclipse glasses so that observers can study the sun during its gradual disappearance, if you have your own glasses, you won't have to fear missing the event. Both NASA and the AAS list several certified vendors who sell inexpensive, quality eclipse glasses, and the AAS offers a guide to help you determine if your glasses are safe for eclipse viewing. The primary qualification is that the glasses meet the SO 12312-2 (sometimes written as ISO 12312-2:2015) international standard; this information should be printed on the filters.
Weather can also throw a wrench into your plans, so be sure to check the forecast in the days leading up to the eclipse.
"Fortunately, mid-August weather is usually pretty good across most of the country," Fienberg said. "But, on average, it's better in the Northwest than in the Southeast, so many of the most die-hard eclipse chasers are setting a course for Oregon, Idaho, Wyoming or Nebraska.”
Although you may make preparations well in advance of the eclipse, sometimes things don't work out. For example, the weather may be cloudy. Bad traffic or other events may keep you from reaching your intended destination on the day of the eclipse. It is important to have a backup plan for viewing the event, such as a second site if the weather is bad, Fienberg said.
Don't worry about picking the perfect site to watch the event, he added. Locations that fall inside the shadow of the moon should be more or less equal.
"As long as one is on the path of totality — and has a clear line of sight to the sun — anywhere will do," Speck said. Line of sight depends on location; a spot surrounded by mountains may give you a smaller glimpse of the sky than a site on the plains.
Still, the closer you are to the center of the path, the longer totality will last, Fienberg said, with a maximum of about 2 minutes and 40 seconds in southern Illinois and western Kentucky.
NASA is affiliated with multiple eclipse events along the path of totality, where the sun will vanish completely behind the moon. The agency has created a map showing the locations of these events, including spots where the agency will film its eclipse program that will broadcast online and on NASA TV. Some of these events span multiple days or even the week before the eclipse.
If you aren't fortunate enough to view the complete eclipse, you might be able to catch other events hosted by the space agency at sites around the United States where only a partial eclipse occurs. Most of these occur in cities where the space agency already has a presence, including Houston and Washington, D.C.
The AAS has compiled a list of events pertaining to the eclipse on the society's website. Some events, like lectures on eclipse science and history, are already taking place around the country.
Viewers from around the world will also be able to tune in to NASA's live webcast of the total solar eclipse, with footage from multiple locations inside the path of totality.
"This will probably be the most-watched total solar eclipse in history," Fienberg said.