What is the X in X-mas?
in astronomy, interval of 18 years 1113 days (1013 days when five leap years are included) after which the Earth, Sun, and Moon return to nearly the same relative positions and the cycle of lunar and solar eclipses begins to repeat itself; e.g., the solar eclipse of June 30, 1973, was followed by one of roughly the same latitude and duration on July 11, 1991. As the relative positions of the bodies are slightly changed after each saros, an eclipse cycle ends after a number of saroses. A saros series lasts between 1,226 and 1,550 years and comprises 69 to 87 eclipses. As one series ends, another is born. On average, 42 series are running in parallel at a given time.