Algol, the second brightest star in the northern constellation Perseus, is the finest example of an eclipsing variable star in the entire sky. In this compact stellar system, two stars revolve around their common center of mass in a rhythmic and precise gravitational dance, and the resulting eclipse causes the brightness of the star to vary like clockwork to a degree that’s easily perceptible to the human eye. And you can watch it from your backyard. No telescope required.