Sunday lived up to its name this past weekend, as countless skywatchers in the western U.S. took in a rare annular solar eclipse. An annular eclipse occurs when the moon passes directly in front of the sun but its apparent size, from Earth's perspective, is too small to completely cover the solar disk. During the May 20 eclipse, the moon covered as much as 94 percent of the sun, leaving a narrow "ring of fire" in the sky. (The apparent diameters of the sun and moon change over time; this is especially true for the moon, because its distance to Earth varies significantly over the course of its monthly elliptical orbit.)
