(CNN) On August 21, the sun will disappear for a short time.

For a swath of the country from Portland, Oregon, to Charleston, South Carolina, it will feel like someone just turned off the sun in the middle of the day.

Even if you live elsewhere in the US, a portion of the sun will partially disappear.

It is being called the "Great American Eclipse." And you can mark it on your calendar down to the millisecond.

It's been 99 years since a total solar eclipse crossed the country from the Pacific to the Atlantic. The total solar eclipse on June 8, 1918, crossed the States from Washington to Florida.

During the celestial event, the moon will pass in between the sun and the Earth, appearing to block the sun for almost an hour and a half.

Kansas City, Nashville and Charleston are some of the cities that will have a good chance of seeing the sun totally covered.

Space enthusiasts are already starting to get excited for the eclipse, some counting down the days, while others are booking hotels for the big moment.