On August 21st, people will gather along a thin strip stretching kitty-corner across the United States. Their lawn chairs will be set up in the path the Moon’s shadow will travel on its way to totally blocking the Sun. They will be in the Moon’s umbra— the inner, narrowing cone shadow. The source of light is completely blocked anywhere inside the umbra.

For those who can’t take the day off—ahem!—to travel to a spot along the path of the Moon’s umbra (known as the “path of totality”), there’s good news. Most of us will still have the opportunity to see a partial eclipse. That’s because the Moon’s penumbra will also track across the nation. The penumbra is the Moon’s outer, widening shadow. It becomes lighter and fuzzier in a spreading cone.

Shadow Play

Turn on a single lamp at one side of a room. Make a shadow puppet on the opposite wall. Imagine the lamp as the Sun, your hands as the Moon, and the wall as the Earth. As you move closer to the wall and the shadow puppet becomes dark and focused, you are seeing the umbra shadow. As you move away from the wall, your shadow becomes lighter and fuzzier. It is not reaching the wall. The umbra is giving way to the penumbra.

Eclipse Viewer

One safe way to look at the solar eclipse is with a pinhole eclipse viewer. Open one side of a long cardboard box. Cut a two-inch hole in one end. Cover the hole with tin foil. Poke a sewing pin through the foil. On the inside of the other end of the box, tape a sheet of white paper. Hold the box over your head so that you are looking at the paper inside the box. Point the pinhole side back behind you at the sun. The image of the sun should show on the white paper. Make a viewing tube for a larger image of the sun. (The longer the box or tube, the larger the image of the Sun will be.) The same idea applies. Put the foil with the pinhole at one end. Put white paper inside the other end. Cut a small viewing window in the side of the tube near the paper. Experiment with your box or tube. Try smaller or larger nails to make your pinhole. Test your viewer so that everything is ready for the big day.