Inex

The inex is an eclipse cycle of about 29 years. The cycle was first described by Crommelin in 1901, but was named by George van den Bergh who studied it half a century later.

An inex is 358 lunations (synodic months) long: this is almost exactly equal to 388.5 draconitic months or 30.5 eclipse years. This means that if there is a solar eclipse (or lunar eclipse), then after one inex a New Moon (resp. Full Moon) will take place at the opposite node of the orbit of the Moon, and under these circumstances another eclipse can occur.

An inex also is close (within 70 min.) to an integer number of days (10,571.95) so solar eclipses tend to take place at about the same geographical longitude at successive events, although at opposite geographical latitudes because the eclipses occur at opposite nodes. This is in contrast to the better known saros cycle, which has a period of about 6585 + ⅓ days, so successive solar eclipses tend to take place about 120° in longitude apart on the globe (although at the same node and hence at about the same geographical latitude).

Unlike the saros, the inex is not close to an integer number of anomalistic months so successive eclipses are not very similar in their appearance and characteristics. Indeed, unlike the saros, an inex series is not unbroken: at the beginning and end of a series, eclipses may fail to occur. However once settled down, inex series are very stable and run for a long time.

The significance of the inex cycle is not in the prediction, but in the organization of eclipses: any eclipse cycle, and indeed the interval between any two eclipses, can be expressed as a combination of saros and inex intervals. Also when a saros series has terminated, then often one inex after the last eclipse of that saros series, the first eclipse of a new saros series occurs. This in-coming and ex-iting of saros series separated by an interval of 29 years suggested the name for this cycle.

References

* A.C.D. Crommelin (1901): The 29-year eclipse cycle. Observatory xxiv nr.310,379, Oct-1901
* G. van den Bergh (1954): Eclipses in the second millennium B.C. Tjeenk Willink & Zn NV, Haarlem 1954
* G. van den Bergh (1955): Periodicity and Variation of Solar (and Lunar) Eclipses, 2 vols. Tjeenk Willink & Zn NV, Haarlem 1955