Credit: Matthew Zimmerman @ English Wikipedia
The Saros cycle can be used to predict eclipses of the Sun and Moon, and is usually defined as 223 lunar synodic months, or about 11 days over 18 years.
But there are a few other lunar-related periods which can used to arrive at 223.
One Saros cycle can be said to be the difference between the number of:
— anomalistic months and full moon cycles (239 – 16)
— draconic months and draconic years (242 – 19)
— tropical months and tropical years (241 – 18)
That may be fairly well known, but then there are the lunar evections.
One Saros cycle can also be the sum of the number of:
— draconic years and lunar evections in latitude (204 + 19)
— full moon cycles and lunar evections in longitude (207 + 16)
In each case (except tropical months/years) both numbers are marginally lower than the whole number, but the difference is still exactly 223.
With tropical months/years the evections are marginally higher than the whole number, but the sum is still exactly 223.
To convert the ‘marginally’ inexact numbers to whole ones we need to multiply by 766, but that’s another story.
via Tallbloke’s Talkshop
May 7, 2019 at 05:03AM
Iowa Climate Science Education 