2017

North American Solar Eclipse

Eclipses occur when one celestial body passes in front of another, as viewed from a particular location. Solar or lunar eclipses are the kinds of eclipses most familiar to people, because they occur frequently enough, and are often dramatic enough, that they are memorable (or sometimes portentous) events.

A lunar eclipse occurs at Full Moon, when the Sun, Earth, and Moon line up (in that order), and the Moon passes exactly behind the Earth from the Sun and thus through the Earth’s shadow. The Moon’s orbit is tilted relative to the Earth’s orbit around the Sun, however, so the Moon only occasionally passes precisely through the Earth’s shadow. Most months at Full Moon, the Moon passes a little above or a little below the Earth’s shadow and, sadly, there is no eclipse.

A solar eclipse occurs at New Moon when the Sun, Moon, and Earth line up (in that order), and the Moon passes exactly between the Earth and the Sun. Again, when the geometry is just right (rarely), the Moon’s shadow can fall on the Earth. It is an incredible cosmic coincidence that the apparent angular size of the Moon in the sky is almost the same as the apparent angular size of the Sun (the Sun’s diameter is about four hundred times larger than the Moon’s, but the Moon is about four hundred times closer to the Earth!). The result is that very rarely the Moon’s disk can completely cover the Sun’s disk in the sky, resulting in a total solar eclipse.

Path of the Moon’s shadow across the United States during the August 21, 2017, total solar eclipse.

Total solar eclipses are rare—any particular spot on Earth experiences one only every 370 years, on average. Some people, including many astronomers, are “eclipse chasers,” though, traveling to the predicted path of the Moon’s shadow to view or take scientific data during these rare celestial events. The element helium, for example, was discovered in 1868 in the Sun’s extended atmosphere, or corona, which is often much more visible during a solar eclipse.

During the August 21, 2017, total solar eclipse, the Moon’s shadow swept across the United States, from Oregon to South Carolina. Astronomers on the ground and in special airborne observatories used the opportunity to study the Sun’s corona and magnetic field with newer, more sensitive instruments. However, the real legacy of the “Great American Eclipse” may be the fact that millions of people were educated and profoundly inspired by the awe-inspiring sight of the solar corona during those fleeting few minutes of totality.

View of the corona (the Sun’s outer atmosphere) from Madras, Oregon, during the 2017 eclipse.