The What and Why

The sky begins to darken noticeably, even though it is daytime. The air cools, and a breeze picks up. Confused birds start heading back to their nests. Shadows start to become eerily sharp, and crescents of light can be seen in the shadows of trees. These are just some of the signs you may notice during a solar eclipse.

- Deborah Pasachoff

The Facts about TSEs

While the Sun is approximately 400 times larger than the Moon, the Moon is approximately 400 times closer to the Earth than the Sun. As a result, the Sun and Moon appear to be the same size. So, when the Moon falls directly in the line of view between the Earth and the Sun, a solar eclipse occurs, and the Moon casts a shadow, known as the umbra, on the Earth.

When the Moon appears slightly larger than the Sun, it can block out the everyday Sun. Thus, when the Moon goes centrally across the Sun, we have a total solar eclipse (TSE). This event is the most dramatic form of an eclipse. At the moment the Moon first seems to touch, or “kiss,” the Sun, we have first contact. For the next hour or more, the lunar silhouette passes over the solar disk. The second contact occurs when the Moon appears entirely within the solar disk.

Between the first and second contacts, part of the Sun is visible, but it is too bright to look at safely with your eyes, so you must use a special filter or a pinhole camera (see page 27). As the second contact occurs, a few beads of everyday sunlight peek through valleys aligned on the edge of the Moon, and we have “Baily’s beads.” The last Baily’s bead is so bright with respect to everything else in that part of the sky that it looks like a diamond on a ring (see page 6), with the band of the ring becoming a whitish circle—the solar corona. After this “diamond-ring effect,” we have totality (a total eclipse) that lasts from seconds to about seven minutes until third contact—between two minutes and two minutes forty seconds on the centerline of the 2017 United States eclipse. At this point, we see a second diamond-ring effect and another set of Baily’s beads.