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The Ancient Egyptians were meticulous astronomers and recorded the passage of the heavens in extraordinary detail. The goal was to mark the passage of time and  to understand the will of the Gods who kept the celestial machinery at work. 

Egyptian astronomers used what they learnt to make predictions about the future. They drew these up in the form of calendars showing lucky and unlucky days. 

The predictions were amazingly precise. Each day was divided into three or more segments, each of which was given a rating lying somewhere in the range from very favourable to highly adverse.

One of the best preserved of these papyrus documents is called the Cairo Calendar. Although the papyrus is badly damaged in places, scholars have been able to extract a complete list of ratings for days throughout an entire year somewhere around 1200 BC.

An interesting question is how the scribes arrived at their ratings. So various groups have studied the patterns that crop up in the predictions. Today, Lauri Jetsu and buddies at the University of Helsinki in Finland reveal the results of their detailed statistical analysis of the Cairo Calendar. Their conclusion is extraordinary.

These guys arranged the data as a time series and crunched it with various statistical tools designed to reveal cycles within it. They found two significant periodicities. The first is 29.6 days–that’s almost exactly the length of a lunar month, which modern astronomers put at 29.53059 days.  

The second cycle is 2.85 days and this is much harder to explain. However, Jetsu and co make a convincing argument that this corresponds to the variability of Algol, a star visible to the naked eye in the constellation of Perseus.

Algol is interesting because every 2.867 days, it dims visibly for a few hours and then brightens up. This was first discovered John Goodricke in 1783, who used naked eye observations to measure the variability.

Astronomers later explained this variability by assuming that Algol is a binary star system. It dims when the dimmer star passes in front of the brighter one. 

Nothing else in the visible night sky comes close to having a similar period so it’s reasonable to think that the 2.85 and the 2.867 day periods must refer to the same object. “Everything indicated that the two best periods in [the data] were the real periods of the Moon and Algol,” say Jetsu and co.

And yet that analysis leaves a nasty taste in the mouth. The ancients were extremely careful observers. If Goodricke measured a period of 2.867 days (68.75 hours), the Egyptians ought to have been able to as well. 

This is where the astronomy becomes a little more complex. The period of binary star systems ought to be easy to predict. But in recent years, astronomers have discovered that Algol’s period is changing in ways that they do not yet fully understand. 

One reason for this is that Algol turns out to be a triple system with a third star in a much larger orbit. And of course, the behaviour of triple systems is more complex. It is also hard to model based on real data since observations of Algol’s variability go back only 300 years.

Or so everyone had thought. Jetsu and co now think that the difference between the ancient and modern measurements is no accident and that the period was indeed shorter in those days. So the Egyptian data can be used as an additional data point to better constrain and understand Algol’s behaviour.

So not only did the ancients’ discover the variable stars 3000 years before western astronomers, the data is good enough to help understand the behaviour of this complex system. A truly remarkable conclusion. 

Ref: arxiv.org/abs/1204.6206: Did The Ancient Egyptians Record The Period Of The Eclipsing Binary Algol – The Raging One?  

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