Astronomers have long recognized that the concept of a "day" cannot be uniquely defined. The so-called solar day is the one we set our clocks by; it lasts 24 hours, and defines the length of time it takes the Sun to return to the same point in the sky each day.

One can also define a sidereal day, however, which is the length of time it takes a star to return to the same point in the sky each day. Ancient astronomers knew that the sidereal day was approximately four minutes shorter than the solar day, meaning that a given star rose about four minutes earlier every day. This leads to a continual change in the appearance of the sky over the course of a year. In early evening in the winter, Orion dominates the southeastern sky in the northern hemisphere, while in the summer, Scorpius dominates...