Studying the Sun's atmosphere with the total solar eclipse of 2017
A total solar eclipse happens somewhere on Earth about once every 18 months. But because Earth’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 – its path stretches over land for nearly 90 minutes, giving scientists an unprecedented opportunity to make scientific measurements from the ground.
When the Moon moves in front of the Sun on Aug. 21, it will completely obscure the Sun’s bright face. This happens because of a celestial coincidence – though the Sun is about 400 times wider than the Moon, the Moon on Aug. 21 will be about 400 times closer to us, making their apparent size in the sky almost equal. In fact, the Moon will appear 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 eclipse will reveal the Sun’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 – which create artificial eclipses by using a metal disk to block out the Sun’s face – there are still some lower regions of the Sun’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 Sun’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 – about 230,000 miles away during the eclipse – diffraction isn’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 Sun’s corona.