An 819-day cycle that features in Maya calendars has been a question mark for researchers ever since it was first deciphered in the 1940s, as the timespan didn’t seem to match anything. But new research has found that the 819 days may refer to the synodic periods of the visible planets. A synodic period is the amount of time it takes a celestial object to return to the same point in the sky. According to the study, “Although prior research has sought to show planetary connections for the 819-day count, its four-part, color-directional scheme is too short to fit well with the synodic periods of the visible planets. By increasing the calendar length to 20 periods of 819-days a pattern emerges in which the synodic periods of all the visible planets commensurate with station points in the larger 819-day calendar.” In other words, each planet has a different synodic period, but it turns out that each one has a synodic period that fits nicely into the number 819 when you look at 20 periods. For example, Mercury has a synodic period of 117, which goes into 819 exactly 7 times. And there’s even a link to the 260-day Tzolk’in calendar. Here’s the paper again, “Rather than limit their focus to any one planet, the Maya astronomers who created the 819-day count envisioned it as a larger calendar system that could be used for predictions of all the visible planet’s synodic periods, as well as commensuration points with their cycles in the Tzolk’in and Calendar Round.”