From ancient times, lunar eclipses and solar eclipses have been regarded both as signs of awe and fear or of beauty and amazement. It is therefore understandable that astronomers have continuously searched for methods of predicting their occurrence and circumstances.
Long before the theories of the relative motions of the Sun and the Moon had reached the stage of development so that the circumstances of a lunar or a solar eclipse could be solved from first principles, astronomers noted that eclipses occurred at semi-regular intervals. The earliest of such periods to be employed would have been the Semester (5 or 6 lunar months) and the lunar year, after which would probably come the Hexon (35 lunar months), the Hepton (41 lunar months) and the Octon (47 lunar months).
Although numerous eclipse cycles of varying lengths can be constructed by combining basic cycles such as the Saros and the Inex in different ways (see below), there is no evidence that ancient astronomers were aware of any eclipse cycles longer than the Saros (18.0 years), the Metonic Cycle (19.0 years), the Exeligmos (54.1 years), the Hipparchus Cycle (345.0 years) and the Babylonian Period (441.3 years). New World (Maya) astronomers appear to have been familiar with the Hepton, the Octon, the Tritos (10.9 years), the Thix (25.6 years) and the Triple Tritos (32.7 years). The Tritos and the Triple Tritos were also employed by ancient Chinese astronomers for predicting eclipses.
Many of the longer eclipse cycles listed in the online catalogue were first studied in the early 1950’s by George van den Bergh (1890-1966), a Dutch amateur astronomer and professor of law at the University of Amsterdam. With these cycles he devised simple recipes, to be used together with Theodor von Oppolzer’s eclipse tables (Canon der Finsternisse), to predict the approximate circumstances of lunar and solar eclipses occurring before or after the period spanned by Von Oppolzer’s eclipse tables. As the character of a lunar or a solar eclipse also strongly depends on the solar anomaly, many of Van den Bergh’s cycles were chosen to approximate a whole number of solar years in length.
Van den Bergh also discovered that the lunar and solar eclipses calculated by Von Oppolzer could be grouped in a large Saros-Inex Panorama from which numerous interrelations between eclipse families can be derived.