On May 20, a solar eclipse was visible from China, across Japan and the north Pacific, to the western USA. During this eclipse the moon was not close enough to the earth to fully obstruct the sun, which would have been a total eclipse of the sun. Instead the moon obstructed most of the sun, an annular eclipse, where the sun forms a ring or crescent around the moon.
In the San Francisco Bay Area, the moon blocked only part of the sun, forming a crescent. During the eclipse I took a series of pictures of the sun shining through a tree onto a west-facing wall. The first photo was taken at 6:24 pm. In the shadow of the tree, note that the light coming through the tree forms many crescent shapes, with every crescent open to the right, the south.
In the second photo, taken two minutes later, the crescents are still open to the right.
At 6:31, the crescents have rotated and are open to the right and down.
During the peak of the eclipse, at 6:33 pm, the moon blocked 84% of the sun. See the distinct spots of light on the wall and walkway. Every crescent is open mostly downward and a bit to the right.
Five minutes later, at 6:38, the opening of the crescent has rotated and is open to the bottom-left.
Finally, here’s a picture of the Chinese pistachio tree that helped produce the shadows. Taken at 6:31, near the peak of the eclipse, it’s hard to tell there’s an eclipse from the photo. The exposure, 1/25 second at f/8, shows that the light was dim so the camera lengthened the exposure in order to produce this pleasing exposure. This photo was taken 1 3/4 hour before sunset, but almost 84% of the sunlight was blocked by the moon.
The pictures raised several questions. Here are my thoughts.
Why the crescent spots of light? During this eclipse, the moon covered most of the sun in the Bay Area, forming a crescent.
How did we see the crescent of the sun and moon on the wall? The branches and leaves of the Chinese pistachio tree are fine and dense, blocking almost all direct sunlight. Some light penetrates the dense tree, forming a number of small beams of light. Shining on the west-facing wall and walkway, each small beam of light produced a pinhole camera, forming an image of the sun and the moon.
Why do all the crescents point in the same direction? Each spot of light is an image of the sun, as blocked by the moon. The images point in the same direction because the sun and moon form a single crescent, and each beam of light forms an image of the sun and moon. That said, every pinhole camera inverts the image, so each image still has the same orientation as the other images.
Why do the crescents rotate as the eclipse progressed? I believe the crescent changes as the earth rotates under the moon shadow, changing the relative location of the sun and moon and therefore the portion of the sun covered by the moon. We see this change as the orientation of the crescent changing.