Algol, a variable star in the constellation Perseus. (A variable star is one that changes in brightness.) Algol, which was observed by the ancient Greeks and named by the Arabs, is now known to be a complex system of at least four stars revolving around one another. A pair of the stars forms an eclipsing binarytwo stars that pass in front of each other in turn, causing a periodic (cyclical) dimming of their combined light. Algol dims regularly every 21 days and 21 hours.
Pluto is relatively round and orbits the Sun, so why doesn't it qualify as a planet?
The changing phases of the moon have given us an enduring curiosity about the dark side of the moon. But is there really a dark side of the moon? What would we see there?