Mark your calendars: in almost exactly two years, North America will get its best chance to witness a total eclipse of the sun since 1979.

While total solar eclipses are not that rare, happening about every 18 months, they often occur in far off, sparsely inhabited parts of the globe, meaning only the most dedicated of eclipse watchers may take the time, trouble and travel to observe them.

They have made the effort to position themselves in Ellesmere Island in Nunavut, Canada in 2008; Easter Island in the Remote Pacific in 2010; and the far northern Svalbard Islands of Norway this year.

That's what has sky watchers excited about the upcoming Aug. 21, 2017 celestial event, the first instance since 1979 in which the path of the total eclipse will cross the contiguous 48 states.

Not only that but the path of the eclipse's shadow will be visible in the United States but nowhere else on the globe.

An estimated 12 million people live within the totality path which, although thousands of miles long, is less than 200 miles wide.

While some previous eclipses moved across North America—in particular, parts of Canada in 1972 and the U.S. Pacific Northwest and Northern Plains in 1979—the 2017 event will sweep from coast to coast right across the heart of the United States.

The eclipse shadow, which will first appear over the Pacific Ocean, will make landfall on the Oregon coast, moving through that state at around 2,400 mph then across Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and, finally, South Carolina before moving off into the Atlantic Ocean.

The length of time the sun remains totally eclipsed by the moon will vary, reaching its peak at a spot in southern Illinois where it will last for 2 minutes and 40 seconds.

Astronomers warn eclipse watchers, whether using binoculars, telescopes or just the naked eye, to never stare directly at the sun without special precautions such as using special filters.