Hold an index card with a hole in it near a viewing surface, and you’ll probably see the spot of light in the shape of the hole. As you pull the card away from the surface, though, that spot of light should gradually look circular, no matter what shape the hole is. You’ve revealed the shape of the light source, the spherical Sun!
When the Sun shines around us, everything looks blanketed in light. This light is the reflection of overlapping images of the Sun. When you block the light with your index card, you reveal only those images of the Sun that pass through the hole. With a small hole held at the right distance, you will see the circular image of the Sun.
Rather than letting an image of the Sun through the hole, the mirror reflects the image of the Sun. If the mirror is very large, many images will overlap and look like a bright spot. A small mirror held far enough away from the viewing surface will reflect only a small area of sunlight, and you’ll see the circular image of the Sun.
Have you ever noticed bright sparkles on the surface of moving water in the sunlight? Little waves in the water act like small mirrors, reflecting images of the Sun toward your eye. If you take a photo, you can capture a moment in time and notice that each sparkle is a circular image of the Sun, as in the photo below (click to enlarge).
During a solar eclipse, the moon blocks part or all of our view of the Sun. This means that, during an eclipse, the images of the Sun that shine onto the earth form partial circles, which we might not notice if they all overlapped.
Viewing sunlight through a pinhole or reflecting it off a mirror can isolate the image to reveal the true shape of the light source hitting the earth. In a total solar eclipse, the Sun’s image will be a partial circle until it is completely blocked by the moon, or has reached totality. In an annular eclipse, when the moon is too far away to block the full Sun, the image will be a ring of light, called an annulus.