RISE OF THE SIDEREAL YEAR. Hipparchus of Nicea, who lived in the second century B.C.E., was one of the first to realize the importance of using a sidereal year—a measuring system that calculates the time it takes the sun to return to the same position with respect to fixed stars. Julius Caesar (whose bust is pictured here) first introduced a calendar compatible with Hipparchus’s observations. Beginning with the Julian calendar, an accurate form of time measurement was available to the ancient world.