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When exactly in history did the Chachamim adopt the 24 hour day?

Our calendar goes by chalakim (which are sections of the amount of daylight), but when it comes to waiting a certain amount of hours it goes by hours based on the 24 hour cycle. Offhand, the only halacha we could think of that requires hours based on the 24 hour cycle is waiting from meat to milk. In that case you need to wait a certain amount of hours based on the 24 hours, rather than halachic shaos/chalakim.

Was it always like this? Wasn't the 24 hour day a relatively recent invention introduced with the Gregorian calendar? I've seen that it might go back to Egyptian times but I'm looking for good sources. Could it have been that waiting x number of hours between meals was at one point based on chalakim rather than hours from the 24 hour cycle?

Additionally, can anyone think of other halachic cases that use the hours from the 24 hour cycle to wait for something?

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    I see no evidence Chakhamim ever adopted a 24 hour day.
    – Double AA ♦
    Jul 27, 2017 at 15:48
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    "waiting from meat to milk. In that case you need to wait a certain amount of hours based on the 24 hours, rather than halachic shaos/chalakim." The Peri Chadash does not hold that way, but rather that in the winter you wait less time and in the summer you wait more time. This makes some sense as the Gemara in Pesachim appears to give meal times for breakfast in terms of daylight-hours, and indeed people's eating patterns likely revolved around their sleep patterns which depended on daylight.
    – Double AA ♦
    Jul 27, 2017 at 15:50
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    The fact that someone waits a fixed amount of time between two things doesn't mean they have adopted a 24 hour day, so I don't understand what the point of your previous comment is. Some things follow the time of day and some things need an absolute interval. There is no contradiction there.
    – Double AA ♦
    Jul 27, 2017 at 15:53
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    Adding to the last comment - I would think that marking times for the end of Shema, start of Mincha, etc. based on adjusted "hours" is easy for people to remember these days as one can publish a fixed schedule for each day of the year based on location, and this is an area that applies to the public. The waiting time between meat & dairy is a private matter. I could see many errors occurring if each day you had to recalculate how long 6 "hours" were. I know that I certainly don't track how long the halachic hour is each day. If I went on a distant trip even 100 miles, my "hour" changes.
    – DanF
    Jul 27, 2017 at 17:44
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    related judaism.stackexchange.com/q/4792/759
    – Double AA ♦
    Jul 27, 2017 at 20:55

2 Answers 2

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Sanhedrin 38b - Rabbi Yochanan ben Chanina discusses the day Adam was created and says that the day consisted of 12 hours.

The first hour Adam's dust was gathered. The second hour the dust was kneaded into a shapeless mass. The third hour his limbs were shaped. The fourth hour the soul was infused into him. The fifth hour he arose and stood on his feet. The sixth hour he gave the animals their names. The seventh hour Chava became his wife. The eighth hour they ascended to bed as two and descended as four. The ninth hour Adam was commanded not to eat of the tree. The tenth hour he sinned. The eleventh hour he was tried. The twelfth he was expelled from Gan Eden and departed.

Based on this the day was divided into 12 hours from the beginning of time.

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  • how do you know that this wasn't "sha'ah zmaniot"?
    – Menachem
    Jul 27, 2017 at 21:43
  • @Menachem Seemingly it was on the equinox (or within a couple days of it) so there's no real difference.
    – Double AA ♦
    Jul 28, 2017 at 15:16
  • @DoubleAA but no proof either
    – Menachem
    Jul 28, 2017 at 15:42
  • @Menachem - The way I see it is that it does not matter whether it was full 60 minute hours or not, we see here a division of 12 hours to the day.
    – Gershon Gold
    Jul 28, 2017 at 15:44
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    @GershonGold good point. I misunderstood the question. I thought it was when did they split the day into 24 equal hours.
    – Menachem
    Jul 28, 2017 at 16:00
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Partial answer (to your last question)

The interval of the molad, is based on a fixed clock. It predates the Gregorian calendar by numerous centuries! The minutes and chalakim are calculated according to a fixed 60 minute hour.

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