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Right now the clock just struck 1PM in England. If I went exactly 1000 hours back in time, I don't know exactly what time it would be in England, but it would be the top of the hour regardless. But what if I went 10,000 hours back in time, or 100,000 hours, or a million hours?

My question is, when did our current 24-hour day cycle begin? That is to say, what is the largest number x such that if I started at the top of the hour in England and then traveled exactly x hours back in time, it would still be the top of the hour in England, give or take (say) 5 minutes?

Note that I don't want to know when it was first proposed to divide the day into 24 hours, or even when the duration of the hour first became around 3600 seconds, but rather when the 24-hour day cycle was first at least roughly in sync with how it is right now.

Keshav Srinivasan
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  • I'm not sure I understand the question. The 24-hour clock has been used since we decided there were 60 seconds in a minute, 60 minutes in an hour, etc. It's not like back in time it took less or more than 24 hours for the earth to make one rotation... The question should be when the 24-hour clock was first used. – ezra Oct 30 '17 at 13:29
  • What has your preliminary research shown? Probably the answer involves the last leap year, or perhaps the adoption of BST. What research have you done on time zones and the measurement of time? – MCW Oct 30 '17 at 13:30
  • @ezra I'm not asking when the 24-hour clock was first used, but rather when the 24-hour clock used in such a way that it was at least roughly in sync with our 24-hour clock. If I started at the top of the hour in England and went back 300 years back in time, going exactly a whole number of hours back in time, would it still be the top of the hour in England, or would it be (say) 37 minutes past the top of the hour? – Keshav Srinivasan Oct 30 '17 at 13:35
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    Its really tough to grok what you want out of this question, but my best guess is never. The earth's rotation isn't as regular as we'd all like to imagine, and we often have to add leap seconds to keep our clocks aligned with the earth. Its never been more than one a year that I can tell (except in 1972 when there were two), but some years we don't need them. – T.E.D. Oct 30 '17 at 13:39
  • @T.E.D. That's why I said give or take 5 minutes. – Keshav Srinivasan Oct 30 '17 at 13:44
  • did you mean "24 hour" rather than "24 day" If you're going to permit a 5 minute variation, then the answer is probably the establishment of railroad time (before that, there was a greater than 5 minute variation in people's watches). How is this question meaningful? What are you really asking and why? – MCW Oct 30 '17 at 13:56
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    This is the first question in my memory where I've VtC as "Unclear what you're asking" really truly fits. As you can see, we're all just a bit confused as to what you are really asking. – CGCampbell Oct 30 '17 at 14:32
  • Don't forget leap seconds added because of the inconsistent rotation of the Earth that has already been mentioned. You can look up when leap seconds have been added.. You can go back as far as the previous New Year's day when a leap second was added, then you'll be at least one second out of sync with "the top of the hour" from then on. – Cody Oct 30 '17 at 19:14
  • @Cody That is why I said "give or take 5 minutes". – Keshav Srinivasan Oct 30 '17 at 21:06
  • Keshav, but what Cody says is what, exactly, it appears to me you are asking. "when the 24-hour day cycle was first at least roughly in sync with how it is right now" ... what does that MEAN? Do you mean, when were accurate clocks developed? Inaccurate? Do you mean to ask when humans decided there were 24 hours of 60 minutes each versus 12 hours of 120 minutes each? What do you MEAN? Until you can ask your question in an unambiguous fashion, in a way that a majority of readers understand, it will remain closed as unclear. – CGCampbell Oct 31 '17 at 12:14
  • I have now down-voted this question, in addition to VtC. The OP has continually added what he doesn't want to know, instead making it clear to everyone what he does want. I am also deleting my answer, despite up-votes, as it obviously does not answer his request. – CGCampbell Oct 31 '17 at 12:17
  • @CGCampbell What I want to know is best summarized in this sentence from my question: "That is to say, what is the largest number x such that if I started at the top of the hour in England and then traveled exactly x hours back in time, it would still be the top of the hour in England, give or take (say) 5 minutes?" Tell me if there is something unclear in that sentence. – Keshav Srinivasan Oct 31 '17 at 15:09
  • @CGCampbell Now that I've told you what I do want, let me also tell you what I don't want and why. I don't want to know when the day was first divided into 24 hours, I want to know about something that happened later than that. And I don't want to know when clocks were perfectly accurate to the second or nanosecond, I want to know about something that happened earlier than that. What I'd like to know when the top of the hour was at least roughly in sync with how it is now, as opposed to starting at a time (say) 37 minutes different from when it starts now. – Keshav Srinivasan Oct 31 '17 at 15:14
  • I think the OP wants to know how long it took the slowing down rotation of the Earth to accumulate to a difference of 5 minutes (300s) w.r.t. today. This is an astronomy question, not history. The variable is called Delta T and is obtained from astronomical observations. The current value is ~70s, it was 370s between 1300 and 1400 (~1370) CE (see deltat.dat on this page). – AstroFloyd May 10 '19 at 07:19
  • The question might be clearer when expressed in years rather than hours - if the Sun is transiting in the south at 12 'o clock noon today in Greenwich, it was too (nearly, but not quite!) exactly 1 year ago. How long ago was the Sun transiting outside the 11:55 - 12:05 range? The answer to this question is in my comment above. BTW, if your question had been 'give or take 1 minute' (solar transit outside 11:59-12:01 range), the answer would have been 1910, when Delta T was ~10s. – AstroFloyd May 10 '19 at 07:25

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Since the base unit of time is now defined as the second (minutes, hours and days, etc are all derived from it). The "current 24-hour day cycle" would have been defined as recently as 1997 when the S.I. definition of the second was last changed..

KillingTime
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