On August 21 this summer, the long shadow of the Moon will race across the United States, offering millions of people a look at the Sun’s magnificent atmosphere, its corona. But for the millions more who do not make it to the narrow path of total eclipse, the Sun’s light will still be partially cut off by the Moon. Everyone in the United States, and almost all of Canada, will see a partial eclipse that day. How much of the Sun will be covered by the Moon depends on where you live. In Tucson, Arizona, to cite an example, will see a maximum eclipse of 58%.
A partial eclipse lacks the drama of a total eclipse, but it is well worth watching nevertheless. The beginning is subtle. The Moon’s first bite is barely noticeable, appearing at first as a tiny flat area cutting into the edge of the Sun. Within a few minutes that “line” becomes curved as the Moon cuts into ever-larger regions of the Sun. As the eclipse progresses, more and more of the Sun gets cut off. Within an hour half the Sun will be covered, and our star will take on the appearance of a crescent.
By this time you need to pay close attention to the warnings about possible eye damage during a solar eclipse. Never look at the Sun without protection for your eyes. It is dangerous even when there is no eclipse. During an eclipse, the Sun’s light is reduced, and you do want to gaze at it, but the dangerous UV is still there. (During the moment of total eclipse, and only then, is it perfectly safe to gaze at the Sun without protection for your eyes.) Most people have access to eclipse glasses fitted with strips of Mylar plastic that offer excellent protection from the Sun’s ultraviolet rays. If you do not have access to a pair, you can project the Sun’s image through a pinhole onto a second sheet of paper or cardboard. But do not look through the pinhole. If you have a telescope, you can project the sun’s image onto a piece of cardboard, a wall, or even the ground.
The closer you are to the path of totality, the thinner the crescent will get. If there is a tree nearby, try looking at the spaces between its leaves. You should be able to see dozens of crescent Suns, each one projected through a space between the leaves. Their appearance is really quite wonderful. If the eclipse gets deeper than 80%, the sky will begin to darken slightly and a general sense of quiet will start to descend across the land.
After maximum eclipse, the story reverses. The sky lightens up, the crescent gets much wider, and after another hour the Moon leaves the Sun and the eclipse is over.
I know all these things because I have witnessed 90 eclipses since 1959, about 30 of which were either partial or the partial phases of total eclipses. An eclipse offers absolute proof that the Earth is moving through space around the Sun, that the Moon circles the Earth, and that because of a cosmic coincidence, the Moon and the Sun get in each other’s way and that occasionally there is an eclipse. We are a part of this graceful movement. Eclipses teach us that we are a part of the solar system, and on August 21, throughout most of North America, we will get a first-hand demonstration.