Ancient Egyptian astronomers may have discovered variable stars, and calculated the period of a well-known one called Algol, thousands of years before Europeans. But they buried those observations in a calendar designed to predict lucky and unlucky days, wrapped in religious narratives, so it's taken some work for modern scholars to tease out the hidden discovery.

Not all of the stars in the night sky shine steadily. Some, called variable stars, appear to fade and brighten at regular intervals. These stars are actually part of binary systems, and when the dimmer member of the pair passes between us and its brighter sibling, it causes an eclipse, so the bright star seems to fade from the night sky for a few minutes or hours. European astronomers first described a variable star called Mira in 1596, and another called Algol in 1669. John Goodricke calculated the orbital period of Algol's two stars over a century later, in 1783 -- but it turns out the ancient Egyptians had worked that out over a millennium and a half earlier.

It's worked into the Calendar of Lucky and Unlucky Days, an ancient Egyptian text that recorded which days were likely to be lucky or unlucky, with specific advice for things like travel, feasts, religious offerings, and the likely outcomes of childbirth or illness. The timing of good days and bad followed astronomical events, the timing of the Nile flood, and seasonal shifts in weather.

Certain words -- especially the name of the god Horus -- pop up at regular intervals that seem to line up with a 2.85 day period. That's awfully close to Algol's current 2.867 day period, and in a recent paper in Open Astronomy, astrophysicist Sebastian Porceddu and his colleagues say it makes perfect sense for Algol's period to have stetched out a bit over the last 3,000 years, because Algol A is slowly losing mass to Algol B, and that transfer bleeds energy and gradually slows the system's rotation.

But if Algol's rotation really is behind the pattern of lucky portents written into the Calendar of Lucky and Unlucky Days, it's a bafflingly obtuse reference, especially for a culture with thousands of years of impressively accurate astronomical observations to its name. Porceddu and others say the Egyptians must have discovered Algol and its variability, because of all the variable stars, it's the easiest to see and track with the unaided eye.