The Dog Days and Sothic Cycles of August

universetoday.com ^ | on August 4, 2015 | David Dickinson

[BenLurkin]

Egyptians livelihood rested on knowing when the annual flooding of the Nile was about to occur. To this end, they relied on the first seasonal spotting of Sirius at dawn. Sirius is the brightest star in the sky, and you can just pick out the flicker of Sirius in early August low to the southeast if you know exactly where to look for it.

Sirius lies at a declination of just under 17 degrees south of the celestial equator. It’s interesting to note that in modern times, the annual flooding of the Nile (prior to the completion of the Aswan Dam in 1970) is commemorated as occurring right around August 15th. Why the discrepancy? Part of it is due to the 26,000 year wobbling of the Earth’s axis known as the Precession of the Equinoxes; also, the Sothic calendar had no intercalculary or embolismic (think leap days) to keep a Sothic year in sync with the sidereal year. The Sothic cycle from one average first sighting of Sirius to another is 365.25 days, and just 9 minutes and 8 seconds short of a sidereal year.

But that does add up over time. German historian Eduard Meyer first described the Sothic Cycle in 1904, and tablets mention its use as a calendar back to 2781 BC. And just over 3 Sothic periods later (note that 1460= 365.25 x 4, which is the number of Julian years equal to 1461 Sothic years, as the two cycles ‘sync up’), and the flooding of the Nile now no longer quite coincides with the first sighting of Sirius.

Such a simultaneous sighting with the sunrise is known in astronomy as a heliacal rising.

[rjsimmon]

Sirius is the brightest star in the sky

Nuh, uh.

[BlueLancer]

Well, it’s a good thing that that cactus is holding the sun up, isn’t it ...

[JoeProBono]

HUGH & SERIES

[rjsimmon]

Somebody’s gotta do it. It ain’t like the sun is hanging there in a void.