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Astronomical Calendar

Astronomical Calendar

A human invention, the was created to measure and record the passage of . For our early ancestors, survival depended on observing changes in the sky. The moon’s changing shape and position were easy to see, which is why most ancient calendars used the synodic period or luna­tion, the interval between successive full moons, as an intermediate measure between the solar day and the solar . Herein lies one of the primary difficulties with calendar design.

Lunar months vary between 29 and 30 days, and the number of lunations fluctuates between 12 and 13, resulting in a calendar year of varying lengths. When to begin a new year was another concern. The oldest calendar, first used around 4236 BCE, is that of predynastic Egypt. The begin­ning of this 365-day calendar, divided into 12 months of 30 days and 5 extra days, was marked by the rising of the brightest star. Because the , the interval between successive equi­noxes, is slightly longer, at approximately 365.25 days, this calendar gradually drifted into error.

The earliest , 735 BCE, began in March. Composed of 6 months of 30 days and 4 of 31, it resulted in a 304-day year. The remaining days, which fell in winter, were uncounted until Numa Pompilius, the second king of Rome, hoping to collect more taxes, added 2 months (Januarius and Febrarius), thereby lengthening the year to 354 days. With their names based on Latin numerals, the seventh, eighth, ninth, and tenth months became our September, October, November, and December.

Following the advice of the astronomer Sosigenes, Julius Caesar abandoned the republican calendar and began reforming the old Egyptian one, length­ening the year to 365.25 days, and arranging the months according to the solar year. Their lengths were either 30 or 31 days; February had 28, with an additional day inserted every 4 years. July, named after Julius Caesar, had 31 days. Augustus Caesar, unhappy that August had only 30, ordered that it be changed to 31, which would have meant three consecutive 31-day months. To avoid this, the rest of the months were rearranged, hence the rhyme, “Thirty days hath September. . . .” In order to realign the calendar with the seasons, Julius Caesar ruled that 46 BCE, known at the time as “the year of confusion,” would have 445 days.

Although used widely for more than 1,500 years, the was about 11 minutes and 14 seconds too long, the tropical year being 365.24219879 days long, not 365.25. By 1580, the resurrection of Christ, tied as it is to the spring equinox, fell on March 11, which was 10 days too early. Pope Gregory XIII ordered that October 5, 1582, became October 15, 1582. He further decreed that February would have one extra day in years divisible by 400, resulting in an average year of 365.2425 days. By modern day calculations the , in use today, is about 26 seconds too long. Alternate calendaring systems continue to be proposed and their merits examined and debated. However accurate, none are really viable in the long run. As noted in 1878 by the American astronomer Simon Newcomb, the years are slowly decreasing by about a half a second each century. From time to time, future generations will find adjustments are necessary.

See also Egyptian Calendar; Roman Calendar; Comets; Meteors and Meteorites; Observatories; Planetariums; Galactic Time; Measurements of Time; Sidereal Time

Further Readings

Holford-Strevens, L. (2005). The history of time: A very short introduction. New York: Oxford University Press.

Richards, E. G. (1998). Mapping time: The calendar and its history. New York: Oxford University Press.

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