What you need to know about the eclipse
About 31 million people live in the path of totality of the eclipse, from Mexico to Texas, Arkansas, Kentucky, Ohio, New York and Maine.
Severe storms threaten visibility for 16 million in Texas, Oklahoma, Arkansas and Louisiana.
The cities and towns where the total eclipse will be seen are packed with the arrival of tourists.
Scientists are preparing experiments "completely different from how we usually do them," according to NASA.