Athena's Web Weekly Column

Week of June 2nd - June 8th, 2006

Sirius Rising

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  The calendar that we use today was first introduced to western civilization by Julius Caesar, who assigned an Egyptian, Sosigenes of Alexandria, the job of determining the calculations for this yearly event.
Sirius, the Dog Star in Canis Major

Sirius, in the constellation Canis Major

Many of the ancient civilizations used a lunar calendar rather than a solar calendar to help calibrate time. Today we know that the Chinese and the Hebrews use a lunar calendar, but so did the Greeks and many other Mediterranean cultures. In the early days of Egyptian culture, during what has come to be called the protodynastic phase, a lunar calendar was also used by the Egyptians before they switched to a solar calendar. One of the principal uses of the calendar was to determine when festivals should be celebrated.

Egyptian Calendar

Egyptian Calendar

  This use of a lunar calendar demonstrates the importance of the Moon, together with its association with agriculture, to the peoples of the past. But a lunar calendar requires much more observational maintenance than does a solar calendar. Lunar calendars must be continually watched to make sure they keep pace with the seasons, a solar function. Therefore a line of priests was employed to keep track of these two separate cycles and made sure that they stayed in step. The Egyptians measured their month from the morning when the old crescent of the waning Moon became invisible in the predawn eastern sky.

  In order to keep the Sun and Moon working together, an extra month would have to be inserted, about every three years, in order to keep the two cycles synchronized. Rather than having four seasons of three months each, the Egyptians had a year of three seasons of four months each, which were named Inundation, Planting and Growth, and Harvest or Low Water.
Circles of time

Circles of Time

These names were related to the annual flooding of the Nile. It had long been observed by the Egyptians that the twelve lunar months fell short of a tropical year by about eleven days. The last month of the year, which for them meant the fourth month of the third season, was named 'Sirius Rising,' and was the time of the year when the star Sirius (the brightest star in the sky) could indeed be seen rising out of the rays of the early morning Sun. Immediately prior to this period, Sirius would be too close to the Sun, lost in the Sun's glare and daylight. Whenever Sirius was observed to rise during the last eleven days of the month of Sirius Rising, an extra month (an intercalary month) was added.

  This use of a star to help keep a fix on their calendar was important to the Egyptians. Rather than start each month with the appearance of the New Moon in the western sky just before sunset, the Egyptians began each of their ten day weeks (you think making to the weekend is hard now!) with the nocturnal rising of either a star or asterism (a cluster of stars which form a recognizable pattern, like the Pleiades). This ten day week was known as a decan. The ten day week was also involved in the use of a star clock, which was not a mechanical device, but a diagram. A knowledgeable observer could use a star clock to determine, as long as he knew the day of the year, what time of night it was. We know of these star clocks because a dozen of them were included in tombs, and were obviously thought important enough to carry into the afterlife.

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