MENDOTA – For many Americans, Aug. 21 will be a once-in-a-lifetime opportunity to see one of the greatest displays in nature: a total solar eclipse.

Partial solar eclipses — those in which the moon blocks a portion of the sun during the daytime hours — aren’t particularly uncommon. Total solar eclipses, in contrast, are far less common, especially for people who don’t have the means or interest to travel to obscure corners of the globe to witness them.

What makes this year’s eclipse special, then? It’s the path it’s taking across the United States, making it visible in or near populated areas where tens of millions of people can experience it.

National Geographic has called this year’s event the “best total solar eclipse in a century” because it is the first to cross the United States from coast to coast since 1918.

Prime viewing spots are located along a narrow strip that crosses 12 states, from Oregon to South Carolina. People along this path will see a total eclipse, in which the moon fully blocks out the sunlight for a period of time, briefly turning day into night.

Millions more, though, will witness a near-total eclipse that is almost as spectacular. Because of its prime path across North America, most people in the United States will be able to see at least a 75-percent eclipse. NASA has created an interactive map at eclipse2017.nasa.gov that can give you all the details.

In northern Illinois, approximately 90 percent of the sun’s surface will be covered by the moon at around 1:20 p.m.

Because of the unique nature of this year’s eclipse, many people are expected to travel to get a better view of the unusual heavenly phenomenon.

Even months before the eclipse, hotels in some areas were reportedly sold out as sky gazers made reservations well in advance to ensure they had the best view of the event.

Others are planning parties to mark the occasion, while many observatories and educators are holding special events in conjunction with the eclipse.

Being something that happens so rarely in the United States, the total eclipse is a great way to teach and raise excitement around science and the natural universe.

While solar eclipses have existed ever since the moon was formed — which scientists say happened several billion years ago — humans’ understanding of the eclipse is more recent.

Below are answers to some common questions about eclipses.

What Causes an Eclipse?

Eclipses are caused by the alignment of the sun, Earth and moon. A solar eclipse occurs when the moon’s path falls between Earth and the sun, blocking out sunlight for a period of time.

How Often Does a Total Eclipse Happen?

On average, a total solar eclipse occurs somewhere on Earth every 18 months. The timing for a total eclipse over any specific spot, though, can vary dramatically depending on how the cosmic bodies align. According to Science.com, a ballpark estimate of 400 years is a reasonable average for any given point, but some spots on Earth can take up to 3,600 years between totalities.

Can I Watch it Online?

Yes. If the skies are cloudy or your job keeps you working at a desk, NASA is planning a two-hour-long live broadcast during the eclipse from the Newseum in Washington, D.C. It can be seen at nasa.gov or on cable television’s NASA Channel.

How Long Will The Eclipse Last?

The Aug. 21 total eclipse will last no more than 2 minutes, 43 seconds, and perhaps considerably shorter depending on the viewing location. The longest eclipses have a duration over 7 minutes.