References:
L Jetsu, S Porceddu, J Lyytinen, P Kajatkari, J Lehtinen, T Markkanen, and J Toivari-Viitala (2013). ‘Did the Ancient Egyptians Record the Period of the Eclipsing Binary Algol – the Raging One?’ Astrophysical Journal 773:1-14. [LINK]
L Jetsu and S Porceddu (2015) ‘Shifting Milestones of Natural Sciences: The Ancient Egyptian Discovery of
Algol’s Period Confirmed.’ PLoS ONE 10 (12). [LINK]
In what follows, we refer to either or both of these as ‘Jestsu & Porceddu’, or ‘J&P’.
For our readers’ convenience, there are pdf copies of each on the shelves of our own reading room [LINK].
A pair of recent reports analyze a set of ancient Egyptian astrological records, and concludes that Egyptian astronomers recognized Algol as a variable star. This finding nicely aligns with our conjecture that the stars of the northern sky were shaped by the Greeks of the 14th century BCE.
Perseus, as depicted in the sky, carries a bag containing the head of Medusa, marked by four stars. The brightest of these is Algol (β Per); while typically depicted as one of her eyes, Ptolemy’s star list simply lists four stars as “in the Gorgon’s head”, a modest quadrilateral anchored by Algol. (The name derives from Arabic – al Gol, the Ghoul.)
Algol has an unusual property – it’s a variable star. Every 2.867 days, its magnitude dims from 2.1 to 3.4 magnitude, then returns to baseline, over a period of roughly 10 hours. If you know where and when to look, the dimming of Algol is readily observed with the naked eye, by comparing its brightness over the course of an evening with nearby stars of well-defined magnitudes.
Knowing when and where to look is key; this is a subtle phenomenon that even expert observers might easily miss unless they were actively searching for variable stars. But science is a community endeavor; it only calls for a single observer to recognize a particular phenomenon, and to teach others where and when to look for the blinking star. The evidence presented by Jetsu and Pordeccu suggests that a community of Egyptian astrologers and stellar adepts had learned to track the blinking of a star which they associated with Horus. Before turning to their reports, let’s consider: Who might that first observer of Algol have been?
Let’s try to imagine how the sky unfolded to ancient eyes.
In the nighttime sky there are a few really bright stars, and a lot of bright stars, a whole lot of not-quite-so-bright stars, and endless numbers of ever dimmer lights. Early astronomers described the brightness of individual stars with a system of stellar magnitude, still in use today. By comparing the relative brightness of stars within a field of view, ancient observers used an iterative process to assign stellar magnitudes, on a scale of 1 to 6, bright to dim.
It’s easy to envision how this might have developed. The very brightest of the bright stars are easily identified – those stars or clusters bright enough that one can simply point and say ‘that one, there’. Deneb and Vega and Altair, in the summer sky. Betelgeuse, Aldeberan and the Pleiades in the winter. Various bright asterisms are also easily described – the seven stars of the Dipper (part of the Bear), the great square of the Horse, or the curling tail of the Scorpion. By visual observation, even a novice observer can sort these easily-found stars into categories of brightest, next brightest, third brightest. Dedicated students of the sky used these brighter stars to map out dimmer stars in many patches of the sky (see String Theory LINK), then used these secondary stars to define a tertiary set, and so on to the catalogue of Ptolemy, encompassing stars as dim as the sixth magnitude.
The human eye and brain comprise a remarkable optical instrument, capable of distinguishing relative stellar brightness over a wide range. It wasn’t until the late 19th century that instruments and methods were developed for more precise determination of stellar magnitudes. Ptolemy’s list of stars visible from Mediterranean latitudes [LINK] includes all stars of magnitude 1, 2 and 3, most of 4th magnitude, some of 5th, and a few of 6th, along with a handful of ‘dim’ stars and ‘nebulous’ objects. Modern assessments of magnitude align well with Ptolemy’s accounting.
The existence of a catalogue like Ptolemy’s tells us that somebody (or more likely, somebodies), thousands of years ago, conducted a formal census of the brightness of the known stars.
Give some thought to just how such an accounting might proceed. This must have involved repetitive evenings of observation, comparison and cataloging of the whole of the visible sky, sector by sector by sector, ideally by several independent observers, a project spanning several years at a minimum.
In the course of such an endeavor we can easily imagine someone reviewing a particular corner of the sky for the tenth or twentieth time, only to note that the star we know as Algol – normally a star of second magnitude – was not as bright as it ought to be. Checking again, several hours later, Algol was Algol as Algol had been known to be – curiouser and curiouser. Might make one inclined to look at Algol a little bit closer in the future. Within a few weeks, several similar dimmings of the star were noted, prompting further vigilance, and communication with like-minded observers of the sky. Once two or more pairs of eyes were focused upon the phenomenon, the parameters of Algol’s dimming were roughly sketched out, and later refined.
This is information of a sort that most people would never observe. But to those who see, to those who know, this was a remarkable celestial omen.
Evidence presented by Jetsu and Porceddu (J&P) argues that ancient Egyptian astrologers knew of the variable brightness of Algol, and tracked its blinking.
In brief: These authors analyze a papyrus text, dated to the 12th or 13th century BCE, of Egyptian astrological omens, spanning a period of roughly a year. For each of 3 segments of a day, a span of time was deemed favorable or unfavorable (+ or -) on the basis of several astronomical omens.
Now, if there was only one factor contributing to the determination of (+ or -) for a given period, then the pattern of + / – scores over time should follow a simple pattern. For example, if + or – was determined by the timing of the full moon, we’d expect a simple pattern of + and – with a periodicity of roughly 29½ days. But other factors – perhaps the appearance of Venus as a morning or evening star, or the retrograde motion of Mars or Jupiter – also influenced any individual determination of + or – . The resulting table is a superimposition of several astronomical cycles, each with its own periodicity.
Starting with a dataset spanning roughly 280 periods of + or -, analysis of the pattern of +/ – scores over time reveals strong evidence for a periodicity of roughly 29½ days; no surprise there.
But J&P tested the dataset for evidence of other periodicities – a spectral analysis, in geek-speak. One result which stood out sharply against background noise was a signal with a periodicity of 2.85 days.
Modern observers cite the periodicity of Algol as 2.867 days.
That’s too close to be coincidence.
J&P conclude (and we concur) that astronomers of ancient Egypt knew Algol as a variable star, and tracked its regular darkening – when it was visible.
J&P further argue that a gradual increase in this period over several thousand years is consistent with the currently accepted model of Algol as a star system which includes a dark star orbiting a bright star in manner that results in a regular eclipsing, as viewed from the vicinity of Earth. We haven’t the maths to judge their judgment on this one. . .
For the periods which fit this pattern, the author(s) of the table cite the Eye of Horus as a prominent omen. According to ancient Egyptian theology, Horus was an important god, who engaged in a furious battle with his uncle Seth; Horus lost his eye, while Seth lost his life. The eye of Horus was recovered, and served as a talisman with important ritual uses. (Egyptian religious beliefs and practices were complex and nuanced; your author has only a passing acquaintance. Readers with more knowledge are encouraged to share in the comments.)
Whatever the significance of the Eye of Horus, Egyptian astrologers apparently discovered that the star we know as Algol was subject to periodic fluctuations in brightness, and came to associate this star with the Eye of Horus. (Perhaps the dimming of the star was viewed as the blinking of the Eye?) Whatever the particulars, the star we call Algol played a role in Egyptian astrology.
What has this to do with the Greek sky? We’ve argued that the Mycenaean Greeks reshaped the sky around 1400 BCE (+/- 50) [LINK]. If true, it’s likely they consulted with expert astrologers and astronomers of the day. In fitting the stars of this region to the figure of Perseus, we propose that Algol became the eye of Medusa not by happenstance, but by design.
We’ve argued [LINK] that The Rescue of Andromeda was not a matter of folk tradition, but a story and set of constellations shaped by royal fiat. Let’s ponder just how that might have played out.
Evidence of myth suggests that the Mycenaean Greeks adopted planetary astrology, as developed in Mesopotamia [LINK]. If true, we can safely infer that Mycenaean palaces kept royal astrologers.
And if, as we propose, Mycenae and its allies redefined the sky around 1400 BCE or so, royal astrologers would certainly have been involved. If so, it’s not a far stretch to suggest that some of these astrologers might have known of Egyptian traditions – such as the Eye of Horus.
We don’t claim this as iron-clad proof that Algol was purposefully made part of Medusa.
But we’re loathe to ascribe to chance something easily explained by choice.
An additional reference we chanced across in our parsing of Algol:
Wilk, Stephen R. (1996). Mythological Evidence for Ancient Observations of Variable Stars. JAAVSO 24:129-133. [LINK]
While we agree with Wilk that Algol’s location in the head of Medusa is significant, we differ with his assessment of when and why Perseus might have been constellated.
JAAVSO: Journal of the American Association of Variable Star Observers.
We love that there is an AAVSO, and that they have a J of their own.
Science is a community endeavor; but specialized interests define scientific neighborhoods.
Observational astronomy still has neighborhoods with room for amateur scientists.
We count that a good thing.