|The constellation Perseus and Algol, the Bright Star in the Gorgon's headJohannes Hevelius, Uranographia, 1690|
An eclipsing binary system is made up to two stars that rotate around each other. On earth, it looks like any other star, but it twinkles in a very different, and periodic way. One star in the system is brighter than the other and when the dimmer star is the one in front we see the star as dark. But when the system turns and the brighter star is in front, we see it as brighter. Its possible to measure the period of rotation by watching how the system twinkles. The ancient Egyptians thought this dimming and brightening could predict if the day would be good or bad so they very meticulously recorded its period. Research by Jetsu et al suggests that the Egyptians not only measured the period of one of the most famous eclipsing binary systems, Algol, also called the Demon Star, but found it to be ever so slightly different than the system's period today. This ancient data point is extremely valuable to astronomers who are rarely able to take data on such a time scale.
The ancient Egyptians were amazing astronomers. They charted the movement of a vast number of stars, planets and moons and recorded them in intricate calendars. The best preserved of these is the Cairo Calendar dated to 1271-1163 B.C. Archeologist working with astronomers were able to determine which heavenly bodies' movements were being cataloged. One period was giving them all a bit of trouble. It was clear that something bright and visible to the naked eye was changing with a period of 2.85 days but there was nothing in modern day astronomy that had that as its period. However, Algol came close with a period of 2.867 days. That may not seem like a large difference, but if astronomers today and those of 3000 years ago were indeed measuring the same thing, the period shouldn't differ by even this small amount. The Egyptians had proven themselves to be much better observers than that.
Algol, better known as the Demon Star, appears in the constellation Perseus as the eye of the Medusa's head. Though on earth it looks like a star that gets brighter and dimmer with a period of 2.867 days, it is actually two stars rotating around each other with their orbit affected by a distant third star. The more massive Algol A is about 4.5 times more massive than Algol B and 2 times more massive than the very distant Algol C.
In these type of systems it has been theorized that "mass transfer" may occur. The mass of one star is transferred to the other and consequently the period of rotation is changed. But the mass isn't transferred quickly. Since this system was first measured in 1783 there hasn't been enough mass transfer to measurably change the period. But, over 3000 years the stars' masses may have changed enough to notice a difference. The discrepancy between the Egyptian's data and modern data is just the right amount to be explained by mass transfer. Without this ancient data, it wouldn't be possible to measure this. Though only a difference of 25 min over 3000 years this new period data point, found by archeologists and explained by astronomers adds a new level of understanding to eclipsing binary star systems.