
The Ancient Egyptian Discovery of Binary Star Algol’s Period Confirmed - diodorus
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0144140
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Florin_Andrei
BTW, observing the magnitude variations of Algol with the naked eye is not
hard. Spend enough time looking at Perseus from a dark place, and the shifting
brightness of that star will eventually become apparent.

The article's claim is that this might be the oldest documented observation of
the variability of Algol. Previously, the oldest confirmed observation was in
the 1600s.

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codezero
Yep, the trick, is to look at the brightness relative to a non-variable, or
less variable star.

Also, note that the observations of Algol from the 1600s were just the first
documentation of the star – observations of variability didn't happen until
the 1700s – I'm not smart, I just checked Wikipedia as I was reading up around
this topic :)

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teraflop
You might want to read that Wikipedia article more closely; it mentions that
Algol was written about by Ptolemy in the 2nd century.

~~~
codezero
You're totally right, and the variability was in the 1600s – foot in mouth!
Thanks for pointing this out to me.

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codezero
I was curious whether or not the fact that P_moon being 29.6 (in their data)
and P_algol being 2.85 would lead to the chance that they were measuring some
other fraction of the periodicity of the Moon, more specifically, 2.96 – which
is P_moon divided by the length of an Ancient Egyptian week of 10 days.

The linked article doesn't discuss this (that I can find), but an article the
same authors posted to Arxiv in 2012 (and updated thereafter) does discuss
this and they found that the two periodicities are completely out of phase, so
that's pretty neat:
[http://arxiv.org/abs/1204.6206](http://arxiv.org/abs/1204.6206)

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dang
Also
[https://news.ycombinator.com/item?id=10800225](https://news.ycombinator.com/item?id=10800225).

