# Is the 24.hour day really based on earth's revolution? **URL:** \ **Category:** Factual Questions\ **Created:** [May 16, 2015, 2:36am UTC](https://boards.straightdope.com/t/is-the-24-hour-day-really-based-on-earths-revolution/720155 "2015-05-16T02:36:05Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![GusNSpot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gusnspot/32/436_2.png) [@GusNSpot](https://boards.straightdope.com/u/GusNSpot)\ **Post date:** [May 16, 2015, 7:15pm UTC](https://boards.straightdope.com/t/is-the-24-hour-day-really-based-on-earths-revolution/720155/21 "2015-05-16T19:15:01Z")
> [@Learjeff](#): > > Right! And RPM stands for … oh, never mind. We could also add the fact that navigation used 360° (degrees) … '60 (minutes) & "60 (seconds) until the electronic age more or less. :eek: ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [May 16, 2015, 9:21pm UTC](https://boards.straightdope.com/t/is-the-24-hour-day-really-based-on-earths-revolution/720155/22 "2015-05-16T21:21:31Z")
> [@Learjeff](#): > > Twelve is simply a nice small number that’s evenly divided by a number of other smaller numbers (2, 3, 4, 6). I suspect that’s the reason for its choice, just as for 60 minutes and 60 seconds. There are also twelve finger knuckles on each hand, and at least some duodecimal cultures counted this way, moving your thumb to each knuckle (or, more accurately, the part of the finger following the knuckle). Though it’s quite possible the math came first. ---
**Author:** ![Don\_t\_Panic](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/don_t_panic/32/398_2.png) [@Don\_t\_Panic](https://boards.straightdope.com/u/Don_t_Panic)\ **Post date:** [May 16, 2015, 9:37pm UTC](https://boards.straightdope.com/t/is-the-24-hour-day-really-based-on-earths-revolution/720155/23 "2015-05-16T21:37:44Z")
> [@Flyer](#): > > Tell that to the Romans. Heh. Pointless personal anecdote: Once upon a time, when I first had Greek and Roman time keeping explained to me, the person doing the explaining made it sound terribly complex. "The day was based around the available daylight, and unlike our hours, which are all the same length, the _Roman hours were variable,_ and the length of an hour would c_hange with the seasons…"_ Wow, I thought to myself, in wonderment. That sounds _super complicated!_ \*How \*did they do that?! And _why?_! Then a second later the light bulb went off over my head: Oh, right. Because sundials. Never mind. :smack: (Although, come to think of it, I’m still not sure exactly how they went about things for the night hours. Well, beyond the fact that they had [water clocks](http://en.wikipedia.org/wiki/Water_clock).) ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 16, 2015, 9:39pm UTC](https://boards.straightdope.com/t/is-the-24-hour-day-really-based-on-earths-revolution/720155/24 "2015-05-16T21:39:57Z")
> [@Chronos](#): > > They’re not “very, very different”. The length of time from noon to noon varies over the course of the year, since the Earth’s orbit isn’t a perfect circle and its speed therefore isn’t perfectly constant. But the variation is quite small, and the average duration of that interval is what was used to define the hour (before the modern atomic standard, which was defined to match that as closely as it could be measured). All told, the difference between the two definitions of “day” is so small that I can’t imagine any context in which the distinction would matter: Anything that’s sensitive enough to matter would have its duration measured in seconds anyway. Poor writing on my part. Sorry. I agree 100% that the \*values \*are similar to many more decimal places than matter for ordinary use. And were carefully chosen by experts to be that way. But as a matter of logic, ontology, philosophy, etc., the two kinds of day are based on completely different foundations. With different consequences for how we ought best to think about them and how they will vary (or not) over time. And what other truths we can derive from them, etc. Two people engaged in a semi-technical conversation about days (imagine two half-drunk physics undergrads late on some random Saturday), where one is thinking in terms of cycles of solar altitude and the other is counting caesium transitions will end up talking past each other pretty soon. The OP’s question somehow reminded me of a question along the lines of “My friend and I have a bet about the day: (tedious details omitted). Who’s right?” and it turns out they both are due to their mutual ignorance of competing definitions of their terms. I was trying to call attention to that distinction. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [May 16, 2015, 9:49pm UTC](https://boards.straightdope.com/t/is-the-24-hour-day-really-based-on-earths-revolution/720155/25 "2015-05-16T21:49:46Z")
> [@Leaffan](#): > > Hey, hang on a centon! 🙂 ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [May 16, 2015, 9:56pm UTC](https://boards.straightdope.com/t/is-the-24-hour-day-really-based-on-earths-revolution/720155/26 "2015-05-16T21:56:28Z")
> [@Ranger\_Jeff](#): > > …and if there aren’t enough people, well, you’ll notice your speedometers measures miles or kilometers per hour and not [smoots](http://en.wikipedia.org/wiki/Smoot)per hour. Smoots was honcho in chief of ANSI _and_ ISO. I wonder what his frat brothers would have thought of that then. ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [May 17, 2015, 1:30am UTC](https://boards.straightdope.com/t/is-the-24-hour-day-really-based-on-earths-revolution/720155/27 "2015-05-17T01:30:57Z")
> [@Hermitian](#): > > A full rotation actually only takes 23 hours, 56 minutes and 4.1 seconds. > > However, it does take 24 hours for the sun to be in the same place (a solar day). This is called a sidereal day. ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [May 17, 2015, 1:41am UTC](https://boards.straightdope.com/t/is-the-24-hour-day-really-based-on-earths-revolution/720155/28 "2015-05-17T01:41:59Z")
> [@Martian Bigfoot](#): > > (Although, come to think of it, I’m still not sure exactly how they went about things for the night hours. Well, beyond the fact that they had [water clocks](http://en.wikipedia.org/wiki/Water_clock).) The stars, planets, and constellations move over the course of a night. Those motions could be used to keep track of time both per night and over the course of seasons and years. ---
**Author:** ![Ranger\_Jeff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranger_jeff/32/2065_2.png) [@Ranger\_Jeff](https://boards.straightdope.com/u/Ranger_Jeff)\ **Post date:** [May 17, 2015, 5:46am UTC](https://boards.straightdope.com/t/is-the-24-hour-day-really-based-on-earths-revolution/720155/29 "2015-05-17T05:46:18Z")
> [@Leo\_Bloom](#): > > Smoots was honcho in chief of ANSI _and_ ISO. > > I wonder what his frat brothers would have thought of that then. Yeah. I love that irony. 😃 [Previous page](https://boards.straightdope.com/t/is-the-24-hour-day-really-based-on-earths-revolution/720155.md?page=1)