The Eclipse Year

In Chapter 2 we met a length of time termed the eclipse year. It was noted to last for about 346 days. Now we will see how it comes about.

A few pages back we saw that the time required for the lunar nodes to revolve once is 18.61 years. If the lunar orbit were stationary, in that the nodes were fixed, then the Sun would pass through those nodes once per solar year and we would get eclipses only on certain calendar dates. This is not the case, though. Because the nodes are regressing the Sun gets to them earlier, producing a type of year that is somewhat shorter than the solar or calendar year. Just how short may be calculated as follows.

Adding unity onto 18.61 to account for that revolution of the nodes, one derives a period equal to 18.61/19.61 times the solar year of 365.24 days (not worrying too much about the last decimal places), or 346.6 days, and this is the eclipse year. Pairs of solar eclipses are easily identified in tables, separated by about half an eclipse year, such as June 10 and December 4, 2002.

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