Researchers from the University of Helsinki have proposed that ancient Egyptians 3,000 years ago were the first to record the variability of a distant star – and their records could provide useful information for astronomers today.
A new paper published in PLOS ONE explains how the Egyptian Cairo Calendar from 1244 to 1163 B.C. describes the variability of a binary star system called Algol. In the calendar, there are two significant periods of time for two gods – 29.6 and 2.85 days. The former relates to the period of the Moon, while the latter almost perfectly matches the variability of Algol – which today is 2.867 days, or two days, 20 hours, and 49 minutes.
This theory had been proposed in 2013 but, understandably, had been met with some skepticism. However, the researchers now say they are more confident in their claims, and say that Algol relates to the deity Horus.
“I would have serious doubts, if someone claimed, for example, that the Bible contains information about water in Mars,” said lead author Lauri Jetsu in a statement. “We claimed that Ancient Egyptian religious texts contain astrophysical information about Algol. It was no surprise to us that there were, and there still are, sceptics.”
Shown is an extract of the Cairo Calendar papyrus, used courtesy of Lauri Jetsu
An eclipsing binary is a pair of stars that, as viewed from Earth, rotate around each other and block each other's light. Thus, this particular star dims regularly in brightness as it orbits its companion. Algol is found in the constellation Perseus about 92.8 light-years from us; the larger star is about 3.5 times the radius of the Sun, and the smaller about 2.7. They are separated by about 0.062 astronomical units (AU, one AU is the Earth-Sun distance).
The variability of Algol, which can be seen with the naked eye, was thought to have been first recorded by Italian astronomer Geminiano Montanari in 1667, although it was not until 1783 that British astronomer John Goodricke suggested another object may be the cause of the dimming. Based on this latest assumption, however, the record for discovery of this star's variability may have to be re-awarded.
Perhaps most interestingly, the discovery reveals that the variability of the star has decreased very slightly over three millennia, by about 0.017 days. Rather than being an error, the researchers postulate that this could be due to the transfer of mass between the two stars affecting their orbits.
“In fact, this would be the first observation that confirms the period increase of Algol and it also gives an estimate of the mass transfer rate,” added Jetsu, possibly providing an important tool for astronomers today to learn more about eclipsing binaries.