Tuesday, March 02, 2004

Sothic Period and Sothic Year

Although people all tend to talk about the "Sothic Year", including yours truly, there is a subtle difference between the "Sothic Period" and the "Sothic Year".

Here is what Duncan Steel writes in his book "Marking Time: The Epic Quest to Invent the Perfect Calendar", John Wiley & Sons, 2000, p. 40:

"...the Egyptian civil calendar followed a grand cycle with duration equal to what is know as the Sothic period or cycle, this being 1,461 Egyptian years (of 365 days) long, or 1,460 Julian years (of 365.25 days)."

That is usually what is meant when people talk about the long Sothic year, the essential point being that 1461 "solar" years of 365 days = 1460 "star" years of 365.25 days, so that a gigantic "leap" year is required to conform the two calendric systems.

But as Steel writes, p. 41:

"... the period between heliacal rising of Sirius (the Sothic year, as opposed to Sothic period) is not equal to the Julian year of 365.25 days... A handful of millennia back the sidereal year was about 365.2564 days long, but this is determined for a star on the ecliptic. For a star some distance from the ecliptic, as is Sirius, the annual round will be slightly different. The center of Egyptian astronomy was at Memphis... and at that latitude ... in that epoch the average interval between heliacal rising of Sirius was 365.2507 days."

This made for a difference of 2 days in 3000 years.

Plus, you have the fact that the tropical year is 365.2424 days, which adds another variable.

I recommend Steel's book for in-depth reading on stars and calendric matters.