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The philosopher Sir Anthony Kenny, in his 2004 book A New History of Western Philosophy v. 1, Ancient Philosophy claims that Thales of Miletus, an ancient Greek astronomer, was

the first to show that the year contained 365 days.

I was very surprised by the above claim. Is it true?

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    "Ours is not to reason why .." We don't deal with issues of personal motivation here as they're untestable and speculative, the rest is fine though. Welcome to skeptics. – Bitter dreggs. May 29 at 7:42
  • @Bitterdreggs.: I don't know what you are talking about. – user135187 May 29 at 9:11
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    You ask: " If not, why might Kenny have made this claim?" - That's off topic is what I'm saying. The other question appears to be fine though. – Bitter dreggs. May 29 at 9:56
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    Welcome to Skeptics! – Oddthinking May 29 at 11:51
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    But a year doesn't contain 365 days. It contains 365.2425 days. – Daniel R Hicks May 31 at 21:17
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This claim originally comes from Diogenes Laërtius, a biographer of ancient Greek philosophers, who wrote (Lives of the Philosophers (Philosophoi Biol), book I # 27):

[Thales] is said to have discovered the seasons of the year and divided it into 365 days

(You can read the entire bio here.)

A number of people besides Kenny repeat this assertion uncritically.

Thales of Miletus lived circa 620 BCE to circa 546 BCE. Diogenes lived circa 180 CE - circa 240 CE, quite a bit later.

However I find it hard to believe that the Egyptiians were able to predict when the Nile would flood thousands of years before Thales without realizing that there are approximately 365 days in a year.

According to his page in the Internet Encyclopedia of Philosophy

Thales’s Discovery of the Seasons

From Diogenes Laërtius we have the report: ‘[Thales] is said to have discovered the seasons of the year and divided it into 365 days’ (D.L. I.27). Because Thales had determined the solstices, he would have known of the number of days between say, summer solstices, and therefore have known the length of a solar year. It is consistent with his determination of the solstices that he should be credited with discovering that 365 days comprise a year. It is also a fact that had long been known to the Egyptians who set their year by the more reliable indicator of the annual rising of the star Sirius in July. Thales may have first gained the knowledge of the length of the year from the Egyptians, and perhaps have attempted to clarify the matter by using a different procedure. Thales certainly did not ‘discover’ the seasons, but he may have identified the relationship between the solstices, the changing position during the year of the sun in the sky, and associated this with seasonal climatic changes.

According to the book Thales of Miletus: The Beginnings of Western Science and Philosophy by Patricia F. O'Grady,

If [Thales] advocated the development of a calendar based on the 365 days of the year, there is nothing to testify to the fact apart from the report of Digenes.

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  • Yes I saw that too. But it seems to me that to "divide the year into 365 days" is somewhat different from being "the first to show that the year contained 365 days". Perhaps this is simply Kenny's poor paraphrasing of Laërtius. – user135187 May 29 at 4:02
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    en.wikipedia.org/wiki/Egyptian_calendar#Civil_calendar suggests a 365 day year by the mid-25th century BC or earlier – Henry May 29 at 8:34
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    @user135187 - What distinction do you see between "dividing the year into 365 days" and "showing that the year is made up of 365 days"? Because they seem to me like two ways of saying the exact same thing. – Dave Sherohman May 29 at 8:50
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    "the Egyptians were able to predict when the Nile would flood": When, as a teenager, I expressed surprise that the Ancient Egyptians accurately knew the length of a year, my aunt with an avid interest in Egyptology explained they had 12 months of 30 days, followed by a holiday/festival that just kept going until the Nile flooded. (This is by far the worst reference I have ever provided on Skeptics.SE!) – Oddthinking May 29 at 11:19
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    @user135187: "Showing the year contained 365 days" was done by megalithic structures in "barbaric" lands a couple thousand years before Thales of Miletus... (Warren Field, Durrington Walls, just to name two in Britain). And that is very different from "showing that the Earth went around the sun", which Thales did not. You're mixing things up quite badly here; please clarify what you are actually interested in here. – DevSolar May 31 at 21:46

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