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Astronomy/Lunar eclipse Saros series numbering

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Expert: - 6/21/2009


Question
As far as I know, Saros series are numbered according to when the eclipse that has the least gamma in absolute value occurs. However when I view NASA's lunar eclipse Saros series page, I find that Saros 138 peaks on 2369/03/24 and Saros 139 on 2235/11/25 which is earlier than 2369. I would like to ask why this is the case and what is the rule and reason behind this (a bit peculiar) arrangement? Thanks a lot!

Answer
The answer is simpler than I thought, so I was able to figure it out within a few spare minutes. Even Saros refer to one node, and odd to the other, so they are keeping track of different things. An even Saros will always peak before the next even Saros, and an odd Saros before the next odd Saros, but the two series can progress at different rates. Thus, Saros 138 must peak earlier than 140, and 137 must peak earlier than 139; but the odd series might peak before the even series, or vice versa, as you noticed.

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Courtney Seligman

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I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.

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I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy.

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Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

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I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

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(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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