Order in Eclipses

Solar eclipses were, at one time, considered omens of bad times to come. After living through a few eclipses without issue, that is unless someone turned one into a self-fulfilling prophecy, it seems solar eclipses would be considered interesting but benign.

Interested cultures dug into everything about solar eclipses. Assyrian and Babylonian mathematicians studied the occurrence of total solar eclipses over centuries and compiled vast databases. The cool thing about well organized databases is that when you step back from the details, patterns often emerge. The amazing pattern that emerged from this ancient data was that total solar eclipses occurred in cycles, albeit cycles that seemed to drift over time. I guess that keeps things interesting.

The Moon, of course, plays a role in this mechanism but I’m staying away from that complexity. Suffice it to say, the Moon’s orbit contributes to the drift in these cycles.

The Babylonians continued to study and document eclipses, developing catalogs but no one knows for sure how they were applied. In later centuries this data was known by Hipparchus, Pliny, and Ptolemy.

Fast forward to the 17th century, Edmund Halley, culling data from the Suda, a 10th century Byzantine encyclopedia, identified the cycles and used the name saros cycles. In the 1950s Dutch Astronomer George van den Bergh devised the saros cycles numbering system, calculating from ancient data that solar saros cycle 0 began on May 23, 2956, BC. Each full cycle lasts between 12 and 15 centuries! We are currently in cycle 139 which began on May 17, 1501 and ends on July 3, 2763.

The drift I mentioned earlier has to do with the Moon’s influence on Earth’s rotation. Between each total solar eclipse in the series the Moon’s orbit causes Earth’s rotation to lag by 120 degrees relative to the Moon. This means that the follow up eclipse, about 18.6 years later, will be parallel but 120 degrees from the previous one in the series. This oversimplification does not take into account other variables that affect which solar eclipses are part of the series. Saros cycles are highly complex mathematical models and made for specific eclipse types. For example, the annular solar eclipse of 2023 is in a different saros cycle (138) than the total solar eclipse coming on April 8, 2024 (139).

The Moon also has saros cycles, but I’m not touching that. I’m already confused.

What’s in the Sky?

January 25; sunset; east: Full “Wolf” Moon