Sitting about 93 light-years away from us in the constellation of Perseus is the pair of bright stars named Algol, also known collectively as the "Demon Star."
The stars orbit and eclipse each other, causing variations in brightness and dimming as regular as clockwork -- so much so that this cycle, which can be seen with the naked eye, may have been used to regulate ancient Egyptians' Cairo Calendar, or CC.
A new study offers a possible explanation of how the "demon star" would have been used to keep track of days in the calendar, providing evidence that ancient Egyptians were the first to describe this elusive star.
"First of all, they discovered Algol 3,000 years before modern astronomers," Dr. Lauri Jetsu, a researcher at the University of Helsinki in Finland and lead author of the study, told The Huffington Post. "Secondly, they used this information in constructing the prognoses of CC. The moon and Algol had religious meanings to them. Of these, the role of Algol is something completely new."
The researchers analyzed the text of the ancient Cairo Calendar found on a papyrus (above) that dates to sometime between 1244 and 1163 B.C.
The CC, like other ancient Egyptian calendars, aimed to predict which days of the year would be "lucky" and which would be "unlucky." The researchers' analysis found that the 2.85-day period of the CC's lucky days strongly correlates with the Algol's bright-to-dim cycle during that same time in history. The researchers also found correlations between the CC's pattern of days and the cycle of the Earth's moon.
All of this suggests that ancient Egyptians not only noticed the star, but also observed that it had a regular dimming pattern.
"They probably noticed that their constellation containing Algol changed," Jetsu said. "It is the star where it is easiest to discover periodic variability with naked eyes."
Sebastian Porceddu, a Ph.D. student at the University of Helsinki and a co-author of the study, told HuffPost that Algol is either the only star, or one of just two or three stars, that the ancient Egyptians monitored for their variability in brightness.
"This variability was considered strange and threatening behavior from a star they considered a divine being," Porceddu said. "We were rather surprised to find the period of Algol in the Cairo Calendar because it wasn't known beforehand that this star would have had mythological properties to the ancient Egyptians."
It was previously thought that Italian astronomer Geminiano Montanari discovered Algol in the late 1660s, but the researchers concluded that the "demon star" was discovered much earlier -- and that the ancient Egyptian papyrus is the oldest preserved historical document that references a variable star like Algol.
The study, which was published in the journal PLOS One on Dec. 17, further supports the researchers' previous studies suggesting that ancient Egyptians tracked the eclipsing binary star.