Algol, aka the Demon Star, is actually a binary star in the Perseus constellation, and has been discovered by a group of Finnish researchers to be first noted by the Egyptians some 3200 years ago. It is one of the best known eclipsing binaries, the first such star to be discovered, and also one of the first (non-nova) variable stars to be discovered.
The eclipses in binary stars give precise information of orbital period changes. Alogl brightens and dims every 2.867 days, a phenomenon first described in semi-modern western astronomy by John Goodricke, who wrote about what he saw with the naked eye back in 1783.
The irregular orbital period changes of this longest known eclipsing binary continues to puzzle astronomers. The mass transfer between the two members of this binary should cause a long-term increase of the orbital period, but observations over two centuries have not confirmed this effect.
The Finnish reserach team presents evidence indicating that the period of Algol was 2.850 days three millenia ago. For religious reasons, the ancient Egyptians recorded this period into the Cairo Calendar, which describes the repetitive changes of "The Raging one." Cairo Calendar may be the oldest preserved historical document of the discovery of a variable star.
The data from the Cairo Calendar however describes the binary period as 2.85 days --slightly less than the 2.867 days observed today. The Helsinki team doesn’t believe this difference is due to error, they think it’s because the period has changed over time, and that they say, gives credence to the theory that the Algol binary is actually a three star system as new research suggests: Beta Persei A, B, and C.
Image credit: With thanks to http://www.mpl3d.com/solar.html
Source: The Daily Galaxy - arxiv.org