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According to Wikipedia, concepts superficially similar to the modern system of hours have been around since at least ancient Egypt c. 2800 BC. But their system divided "daytime" (after dawn and before sunset) and "nighttime" (after sunset and before dawn) into 12 hours each, whose length would of course vary at different times of the year.

At some point in the intervening ~5000 years, we moved to a system of a day being composed of 24 equal hours that daytime or nighttime could take up a varying quantity of at different times during the year. But I'm having trouble determining at what point this concept first took hold.

There's a related question on here that was closed for being confusing and unclear what exactly was being asked, so to avoid the same fate, here's exactly what I'm asking: when (and by who) was the system established in which each day has 24 hours, each of which are supposed to be of identical length rather than being dependent on varying values such as the length of time that the sun is in the sky?

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    @Alex You mean the one I specifically mentioned in my question and explained why this is different because, unlike that confused question, I have a very clear, articulable understanding of exactly what it is I want to know the answer to? Possible duplicate of that question? – Mason Wheeler Feb 27 at 22:14
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    As the other question was closed for being unclear (which it is, though as far as I can work out it's not asking exactly the same thing as this one), it seems reasonable to leave this one open as it is clear. – Lars Bosteen Feb 27 at 23:04
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    By „established“, do you mean „known and used among the educated people“ or rather „used by the majority of ordinary people“? This is probably a big difference. – not2savvy Feb 28 at 10:35
  • This is most likely very pragmatic - you need to have a use for it. Unless you need to agree with others (or making devices that do not observe the sun) there is no reason for this. – Thorbjørn Ravn Andersen Feb 28 at 13:56
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From Scientific American:

Hipparchus, whose work primarily took place between 147 and 127 B.C., proposed dividing the day into 24 equinoctial hours, based on the 12 hours of daylight and 12 hours of darkness observed on equinox days. Despite this suggestion, laypeople continued to use seasonally varying hours for many centuries.

In terms of when the 24 hour day began to be relevant to ordinary people in their daily lives, my impression is that this would have come with advancements in mechanical clocks over the course of the Middle Ages and early modern period. See the history of timekeeping devices on Wikipedia, which states:

The appearance of clocks in writings of the 11th century implies that they were well known in Europe in that period. In the early 14th-century, the Florentine poet Dante Alighieri referred to a clock in his Paradiso; the first known literary reference to a clock that struck the hours.

However that article also points out with specific reference to sundials: "The idea of using hours of equal length throughout the year was the innovation of Abu'l-Hasan Ibn al-Shatir in 1371."

  • Hipparchus was most likely borrowing from the Babylonians and Egyptians. – Matt Balent Feb 27 at 18:41
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    @MattBalent Did they have equal-hour days? – Mason Wheeler Feb 27 at 18:42
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    I'm thinking that Brian Z has a good point here in that the concept of uniform hours would have been largely irrelevant to any culture that didn't have a reliable mechanism for measuring them. Perhaps the concept became more functional with the spread of clock towers and/or hourly bell ringing? – jeffronicus Feb 27 at 23:59
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    @jeffronicus The ancient Egyptians used water clocks to measure time. – Matt Balent Feb 28 at 12:51
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    @MattBalent Yes, which obviously don't vary with the seasons, but were they ever actually used in conjunction with a system of 24-hour equinoctial hours? And even if they were, how widely? – Brian Z Feb 28 at 13:17
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Agreed with the 1st above comment.

The 24-hour time system has its origins in the Egyptian astronomical system of decans, which itself dates back to 10th Dynasty (2100 BCE).

There were 36 decans (36 X 10 = 360 days) and an additional 5 days to compose the 365 days of a solar year.

Eventually this system led to a system of 12 daytime hours and 12 nighttime hours, varying in length according to the season. Later, a system of 24 "equinoctial" hours was used.

References: - Neugebauer, Otto (1983) [1955]. "The Egyptian "Decans"". Astronomy and History: Selected Essays. New York: Springer. pp. 205–209. - Neugebauer, Otto (1969) [1957]. The Exact Sciences in Antiquity (2 ed.). Dover Publications. pp. 81–88.

Historical adoption:

1886: the Canadian Pacific Railway train at Port Arthur began using the 24-hour clock.

1893: Italy became the first to adopt the 24-hour clock nationally.

1909: The French Army began using the 24-hour clock while the rest of France did not start using this time system until 1912.

1915: The British Royal Navy began using this time system during the First World War and the Allied forces would follow suit as well.

1916: Denmark adopted the system.

1917: Greece adopted the system, along with the Canadian armed forces

1918: the British Army adopted the 24-hour clock.

1920: The United States Navy was the very first organization in the U.S.A. to utilize the 24-hour clock.

1920: Spain, Belgium, Portugal, and Switzerland finalized the switch, as most of the countries in Latin America.

1925: Turkey.

1927: Germany.

1942: US Army.

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    AFAIU, the whole question is about precising the adverb Later in your last sentence. – Evargalo Feb 28 at 10:00
  • I have added when the system was officially adopted in various countries, but I think the origins are the important part. Unlike the monthly system, the hourly system maintained its logic. – Overmind Feb 28 at 11:22
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    I fail to see how this answers the question in any way. How is it relevant when the US Army started using the 24 hour clock? – pipe Feb 28 at 12:03
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    I think there's a confusion here between two different things: the practice of dividing the daily cycle into 24 equal parts (regardless of how those parts were labelled), and the practice of numbering those parts 0 to 23 (rather than 1 to 12, then 1 to 12 again). The question is asking about the first, but the references to "using the 24-hour clock" in this answer are talking about the second. – IMSoP Feb 28 at 15:01
  • The beginning of this answer is also very confusing: "the Egyptians divided the year in this way; this led to dividing the day this way". – IMSoP Feb 28 at 15:04

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