By Randy Gener
A total solar eclipse happens somewhere on Earth about once every 18 months. But because Earthâ€™s surface is mostly ocean, most eclipses are visible over land for only a short time, if at all. The total solar eclipse of Aug. 21, 2017, is different â€“ its path stretches over land for nearly 90 minutes, giving scientists an unprecedented opportunity to make scientific measurements from the ground.
When the Moon moved in front of the Sun on Aug. 21, it completely obscured the Sunâ€™s bright face. This happens because of a celestial coincidence â€“ though the Sun is about 400 times wider than the Moon â€” the Moon on Aug. 21 was about 400 times closer to us, making their apparent size in the sky almost equal.
A total solar eclipse â€” in which the moon completely obscures the sun â€” crossed the United States on a 70-mile-wide ribbon of land stretching from Oregon to South Carolina. Throughout the rest of North America â€” and even in parts of South America, Africa, Europe and Asia â€” a partial eclipse was visible.
The last total eclipse in the contiguous United States occurred on Feb. 26, 1979. The last total eclipse that crossed the entire continent occurred on June 8, 1918.
Eclipseâ€™s shadow as seen from above the Earth
In fact, the Moon appeared slightly larger than the Sun to us, allowing it to totally obscure the Sun for more than two and a half minutes in some locations. If they had the exact same apparent size, the total eclipse would only last for an instant.
The hair on the back of my neck stood up and I felt different things as the eclipse reaches totality. Itâ€™s been described as peaceful, spiritual, exhilarating, shocking. If youâ€™re feeling these things, donâ€™t worry, youâ€™re experiencing the total eclipse of the sun!
A total solar eclipse presents a rare opportunity to observe the corona and chromosphere, the two outer most layers of the sunâ€™s atmosphere. Under normal circumstances, the bright yellow surface of the sun, called the photosphere, is the only feature we can observe. But during an eclipse, the moon blocks out that intense light, allowing scientists to observe the much dimmer solar atmosphere.
Sunrise through solar arrays
On July 26, 2017, a member of the Expedition 52 crew aboard the International Space Station took this photograph of one of the 16 sunrises they experience every day, as the orbiting laboratory travels around Earth. One of the solar panels that provides power to the station is seen in the upper left.
The stationâ€™s solar arrays produce more power than it needs at one time for station systems and experiments. When the station is in sunlight, about 60 percent of the electricity that the solar arrays generate is used to charge the stationâ€™s batteries. The batteries power the station when it is not in the Sun
The moonâ€™s rough, craggy terrain influences what we see on Earth during a total solar eclipse. Light rays stream through lunar valleys along the moonâ€™s horizon and form Bailyâ€™s beads, bright points of light that signal the beginning and end of totality.
The moonâ€™s surface also shapes the shadow, called the umbra, that races across the path of totality: Sunlight peeks through valleys and around mountains, adding edges to the umbra. These edges warp even more as they pass over Earthâ€™s own mountain ranges.
For more information about the AUG. 21, 2017 total solar eclipse, visit eclipse2017.nasa.gov.
For NASA, the eclipse provides a unique opportunity to study the sun, Earth, moon and their interaction, because of the eclipseâ€™s long path over land coast to coast.
The eclipse will reveal the Sunâ€™s outer atmosphere, called the corona, which is otherwise too dim to see next to the bright Sun. Though we study the corona from space with instruments called coronagraphs â€“ which create artificial eclipses by using a metal disk to block out the Sunâ€™s face â€“ there are still some lower regions of the Sunâ€™s atmosphere that are only visible during total solar eclipses.
Because of a property of light called diffraction, the disk of a coronagraph must block out both the Sunâ€™s surface and a large part of the corona in order to get crisp pictures. But because the Moon is so far away from Earth â€“ about 230,000 miles away during the eclipse â€“ diffraction isnâ€™t an issue, and scientists are able to measure the lower corona in fine detail.
NASA is taking advantage of the Aug. 21, 2017, eclipse by funding 11 ground-based science investigations across the United States. Six of these focus on the Sunâ€™s corona.