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 as viewed from the Earth reaches its farthest point north or the equator). To measure years accurately, some cultures built circular megaliths like that at Stonehenge in England, in which the alignment of the columns with the rising Sun or a star defines a specific day of the year.

The development of a calendar, a systematic arrangement of days in the year, proved to be a challenge, because we can’t divide the sidereal year (365.256 days). Different societies devised different solutions to the problem. For example, ancient Egyptians delineated 12 months of 30 days each and added 5 days to approximate a tropical year. Greeks and Romans adopted the Egyptian calendar, but changed the names of the months. Contemporary English names for the months follow the Latin names, which came from Roman gods, heroes, festivals, or numbers. A Roman emperor who lived around 700 B.C. E. arbitrarily established January 1 as New Year’s Day. Then, during the reign of Julius Caesar (100-44 B.C. E.), a Roman astronomer created the Julian calendar, consisting of 12 months of unequal length and a leap year every fourth year (to accommodate the extra approximately 0.25 days in the tropical year).

Different cultures have also chosen different points at which to begin recording time. For example, Chinese calendars begin with the dynasties of early emperors, while the Hebrew calendar traditionally dates from the biblical Genesis. Until 526 C.E., Romans counted years from the founding of Rome, but in that year, a Byzantine emperor reset the clock so that the year 1 coincided with the presumed birth of Christ.

The Julian calendar lost time by about 11 minutes per year because a tropical year is slightly less that 365.25 days, so by the sixteenth century the calendar had become misaligned with the seasons by about 11 days. As a result, a Jesuit priest and an Italian astronomer together designed a new calendar that, by order of Pope Gregory XIII, became the new standard at midnight on Thursday, October 4, 1582; to recalibrate the calendar, the following day was called October 15. Most people in the world today still use the resulting Gregorian calendar. But some cultures employ alternative calendars for determining dates of religious holidays. For example, the Mohammedan calendar counts months from July 15, 622 C.E., and does not take into account leap year. The Hebrew calendar uses lunar (synodic) months, and is adjusted to the solar year by adding an extra month every 19 years.

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