LEAD: NASA scientists and astronomers are already planning for the first total solar eclipse for the United States in 38 years.
1. On August 21, 2017, the moon will pass between the sun and Earth in an alignment that will cast the moon's shadow onto Earth.
2. A dark shadow of the moon, 170 miles wide, will sweep across the U.S. over the course of one-and-a-half hours.
3. People in cities lying within the narrow path of the shadow (red line in the video) will experience an eerie sense of twilight as day turns to night and back to day again within roughly 2-2.5 minutes.
TAG: Solar astronomers will use the solar eclipse to study the outer atmosphere of the sun.