# Why are there 24 hours in a day and...
**URL:** https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101
**Category:** Factual Questions
**Created:** [March 15, 2001, 4:12pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101 "2001-03-15T16:12:40Z")
**Posts on this page:** 12
**Page:** 1
### Author:  [@Mahaloth](https://boards.straightdope.com/u/Mahaloth)
#### Post date: [March 15, 2001, 4:12pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101/1 "2001-03-15T16:12:40Z")
60 minutes in an hour and 60 seconds in a minute? This is such a basic question, I’ve never gotten around to asking it. So, why?
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### Author:  [@capybara](https://boards.straightdope.com/u/capybara)
#### Post date: [March 15, 2001, 4:32pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101/2 "2001-03-15T16:32:18Z")
Quote:
“Why the twenty-four? Historians do not help us much. The Egyptians did divide their day into twenty-four “hours” --“temporary,” of course. Apparently they chose this number because they used the sexagesimal system of numbers, based on multiples of six, which has been developed by the Babylonians. This pushes the mystery back into earlier centuries, for we have no clear explanation of why the Babylonians built their arithmetic as they did. . . . Our “minute,” from the medieval Latin pars minuta prima (fist minute or small part). originally described the one-sixtieth of a unit in the babylonian system of sexegisimal fractions. . .”
and so on. Daniel Boorstin, The Discoverers, (New York: Random House, 1983), 41-42.
So it goes back to Babylonian/ Egyptian sexegisimal math, via Ptolemy, and was picked up with the arrival of the mechanical clock in Europe around 1330. Perhaps.
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### Author:  [@Junchbailey](https://boards.straightdope.com/u/Junchbailey)
#### Post date: [March 15, 2001, 4:36pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101/3 "2001-03-15T16:36:19Z")
try this link:
> **[How Time Works](https://science.howstuffworks.com/science-vs-myth/everyday-myths/time.htm)**
>
> Ever wonder why we start our year on the first of January? Or why we have January and the other 11 months in the first place? Find out all about time.
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### Author:  [@Phobos](https://boards.straightdope.com/u/Phobos)
#### Post date: [March 15, 2001, 5:51pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101/4 "2001-03-15T17:51:06Z")
12 was one of the sacred numbers (Babylonians) so the day was divided into 12 daytime hours and 12 nighttime hours.
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### Author:  [@Evnglion](https://boards.straightdope.com/u/Evnglion)
#### Post date: [March 15, 2001, 5:59pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101/5 "2001-03-15T17:59:15Z")
I have heard that there is actually only 23 hours and 56 minutes in the day. That is why we have leap year every once in a while… 🙂
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### Author:  [@Great\_Dave](https://boards.straightdope.com/u/Great_Dave)
#### Post date: [March 15, 2001, 6:09pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101/6 "2001-03-15T18:09:59Z")
I think, instead of starting with one day and dividing it down into 24 hours, etc., the originators started with the second and multiplied. I’m sure Cecil has answered this question.
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### Author:  [@Phobos](https://boards.straightdope.com/u/Phobos)
#### Post date: [March 15, 2001, 6:26pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101/7 "2001-03-15T18:26:57Z")
> [@](#):
>
> \*Originally posted by Evnglion \*
> \*\*I have heard that there is actually only 23 hours and 56 minutes in the day. That is why we have leap year every once in a while… 🙂 \*\*
Close, but not quite right.
from [http://www.treasure-troves.com/astro/LeapYear.html](http://www.treasure-troves.com/astro/LeapYear.html)
> [@](#):
>
> A leap year is a year in which an extra day in added to the calendar in order to synchronize it with the seasons. Since the tropical year is 365.242190 days long, a leap year must be added roughly once every four years (four times the fractional day gives ). In a leap year, the extra day (known as a leap day) is added at the end of February, giving it 29 instead of the usual 28 days.
In other words, it takes the Earth a little longer to make it once around the sun than our calendars estimate. So, an extra day is added every now and then in order to account for that extra time. Otherwise, the seasons would start shifting into new months on our calendars.
You were talking about the sidereal day (with respect to the stars)
[http://www.treasure-troves.com/astro/SiderealDay.html](http://www.treasure-troves.com/astro/SiderealDay.html)
> [@](#):
>
> The length of time which passes between a given “fixed” star in the sky crossing a given projected meridian (line of longitude). The sidereal day is 23 h 56 m 4.1 s, slightly shorter than the solar day because the Earth 's orbital motion about the Sun means the Earth has to rotate slightly more than one turn with respect to the “fixed” stars in order to reach the same Earth-Sun orientation.
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### Author:  [@Great\_Dave](https://boards.straightdope.com/u/Great_Dave)
#### Post date: [March 15, 2001, 6:38pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101/8 "2001-03-15T18:38:45Z")
[http://www.straightdope.com/classics/a1\_125.html](http://www.straightdope.com/classics/a1_125.html)
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### Author:  [@Irishman](https://boards.straightdope.com/u/Irishman)
#### Post date: [March 15, 2001, 6:45pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101/9 "2001-03-15T18:45:57Z")
Cecil did, indeed, address this question.
[Who decided the day should be divided into 24 hours?](http://www.straightdope.com/classics/a1_125.html)
BigDaddyD, you are incorrect. They specifically started with the hour, and then made divisions off the hour. Thus you have minutes and “second minutes”, or seconds. All modern definitions of the second (like the SI system) are based on matching this original 1/3600 of a day.
Think about it - how would they start with seconds and mysteriously end up with an exactly even number in a day?
Evnglion, you are not quite correct. Leap years are not corrections for the number of hours in a day, but the number of days in a year. The Earth does not quite make an even number of rotations (days) in a single revolution about the sun (year). The extra quarter day is what is factored in to make the leap year (extra day every four years). Of course the numbers aren’t that neat either, and the leap year has to be finagled to match reality.
[http://aa.usno.navy.mil/faq/docs/leap\_years.html](http://aa.usno.navy.mil/faq/docs/leap_years.html)
You’re thinking of the **sidereal day** vs. the **solar day**.
> [@](#):
>
> A sidereal (pronounced sigh-dear’-real) day refers to the rotation of the Earth measured relative to the stars. It is the time it takes the Earth to rotate 360 degrees and is equal to 23 hours, 56 minutes and 4 seconds.
>
> A solar day is 24 hours long. This is the time it takes from one noon (sun overhead) to the next noon.
>
> The difference in the two “days” arises from the fact that during a day the Earth also travels nearly a degree further on its yearly trek around the Sun.
[http://liftoff.msfc.nasa.gov/academy/rocket\_sci/satellites/sidereal.html](http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/sidereal.html)
Sidereal vs. solar is the difference in the amount of rotation it takes to reach the exact previous orientation vs. the extra distance to rotate to align the same orientation to the sun, because of the motion of the Earth around the sun.
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### Author:  [@Doctor\_Jackson](https://boards.straightdope.com/u/Doctor_Jackson)
#### Post date: [March 15, 2001, 6:48pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101/10 "2001-03-15T18:48:36Z")
> [@](#):
>
> I have heard that there is actually only 23 hours and 56 minutes in the day. That is why we have leap year every once in a while…
A “true” day is roughly 23 hours and 59.17806 minutes. We add a day to the calendar every 4 years (leap year) because it takes roughly 4 years to “lose” 1440 minutes, the equivalent of one day. 1 minute per day times 365 days per year times 4 years equals 1460 minutes. Because that does not work out to exactly 1440 minutes we have the “no leap year if divisible by 400” rule, etc.
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### Author:  [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)
#### Post date: [March 15, 2001, 9:21pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101/11 "2001-03-15T21:21:12Z")
> [@](#):
>
> \*Originally posted by Irishman \*
> \*\*Cecil did, indeed, address this question.
>
> [Who decided the day should be divided into 24 hours?](http://www.straightdope.com/classics/a1_125.html)\*\*
I’m not sure I agree with Cecil. [I think](http://mentock.home.mindspring.com/ifaq.htm#IFAQ009) that the Sumerians preceded the Babylonians in dividing the day into 12 parts, but the parts were 2 hours long. Later, the Babylonians or the Egyptians divided both daylight and nighttime into 12 parts (because their sundial solar batterys failed at night).
Of course, 12 is a nice, divisible number, but I’m more inclined to believe that they attempted to divide the lunar cycle into the same number of parts as the solar cycle. That is, two hours is about the same part of a lunar month as one day is of the solar year. That it worked out so closely to 12 parts in a day probably convinced them of its worthiness.
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### Author:  [@Irishman](https://boards.straightdope.com/u/Irishman)
#### Post date: [March 15, 2001, 10:30pm UTC](https://boards.straightdope.com/t/why-are-there-24-hours-in-a-day-and/58101/12 "2001-03-15T22:30:11Z")
No, Doctor Jackson, simulposts got you (and also made my post redundant, doubly so). Again, the leap year has nothing to do with the sidereal day, i.e. an extra day every four years is _not_ because the “day” is slightly less than 24 hrs.
See above.
RM, I make no claim as to who decided the length of our hours. For what I know, it was Caveman Cliff sitting in the woods and he had an extra set of fingers (one per hand), so 12 seemed natural to him. If you have a problem with Cecil’s column, take it up with him. 😉
I do like your suggestion that 12 was important because of the lunar cycle, and they attempted to match the solar cycle to it.
I do maintain, however, that minutes and seconds were derived from hours, and not vice versa. I don’t think you dispute that.