The Rest of The Story: What Is A Year?
by Stephen Marshak In prehistory, nomadic people defined time based on the phases of the Moon. We now refer to the time from new moon to new moon as the synodic month. Since the Moon passes through four phases, people divided the month into 4 weeks, each consisting of 7 days. With the advent of agriculture, farmers needed a larger unit of time, the year, to specify seasons for planting. But years defined by multiples of synodic months soon get out of sync with the seasons. Fortunatelym, by 2000 B.C. E., observers realized that a given star sets at a different place along the horizon each night, but on any night, the star sets at about the same place it had 365 nights earlier. This 365-day-long interval, the sidereal year, represents the time it takes for the Earth to complete an orbit of the Sun and provides a convenient basis for defining seasons. It almost equals another unit, the tropical year, which is the time between successive summer or winter solstices (on a solstice, the Sun a
