# Why we measure a day 24h?

Asked by Dine (233 ) April 6th, 2008

And then we have a leap year,could we measure time with equation of time and make a watch that will erase a 29th February ?

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as far as i have learned, nobody really knows for certain where the 24 hour day, 60 minute hour, and 60 second minute come from.

we do know that the babylonian and sumerian civilizations used a base 60 counting system, and so it seems reasonable to assume that they would develop a base 60 method of keeping time. that would explain the minutes and seconds… but hours? 12 is 1/5 of 60, and so perhaps these civilizations had 5 as some kind of important number (easy to imagine… five fingers on the hand). then you could imagine the decision to mark off 12 hours of day and 12 hours of night, which puts us at 24 hours.

what i find totally unbelievable is the apparent coincidence of the 24 hour day versus the 24,000 mile equator. seriously!? there is one hour in the day per 1000 feet of earth circumference, and it’s coincidence!? yet i have found no evidence that suggests there is any link.

of course, the circumference of the earth is actually around 24,900. but still, that’s pretty close. i guess truth really is stranger than fiction.

samkusnetz (1177 )

It takes roughly 24 hours for the earth to rotate once around on it’s axis, which is where the circumference/time co-ordination comes into play. As to the history, I couldn’t say, but I’d guess it just grew out of the light/dark cycles that accompany rotating out of and then back into direct sunlight.

@intransigence… you misunderstand. what i’m saying is that there’s no historical link between the measurement of one hour and the measurement of one mile, yet since there is the coincidence of 24 hour days and the 24,000 mile circumference, i would think that there is some reason for it. but no, nobody can find any evidence to say “the measurement of the mile grew out of the measurement of the hour” or the other way around.

samkusnetz (1177 )

We have only 4 days in a year that are 24h:

minimum −14:15 11 February
zero 00:00 15 April
maximum +03:41 14 May
zero 00:00 13 June
minimum −06:30 26 July
zero 00:00 1 September
maximum +16:25 3 November
zero 00:00 25 December

Dine (233 )

true, but at the time that folks first settled on a definition of “mile” and of “hour”, our methods of measurement were not advanced enough to make such distinctions. for centuries, every day was 24 hours.

samkusnetz (1177 )

And that’s why we have Daylight Saving Time, to catch up on those days?

klaas4 (2164 )

No, daylight savings was a system thought up so that farmers would have more time to plant/harvest their crops during the warmer parts of the year. At least that’s my understanding. We “catch up” with leap years and then there’s some adjustments that happen every century or millennium or whatnot. I recall one of these in 2000 on leap day that only happens every 100 or 1000 years or something.

gorillapaws (14333 )

@gorillapaws: Sorry, not the farmers… why would a farmer care what time it is? (although they apparently do, or at least like to mess with the journalists who come asking*) (See the graph on this page: http://photo.benpeoples.com/2006/11/24/sunrisesunset/ it will make DST make a little more sense)

The main reason for not switching to some other system is that we’ve gotten pretty close to a worldwide standard for time and date with the current time and calendar system. There are a few cultures that still keep other calendars, but everyone (to my knowledge) uses 24/60/60, and yes, leap days to keep up.

* Apparently farmers have problems with the cow’s milking schedules when DST kicks in. What I have never been able to understand is why they don’t just ignore DST and keep milking them on the same schedule. The only thing in a farmer’s life that really has to do with clock time (vs. solar time) is the TV schedule. Oh, and you (gorillapaws) are right about 2000 being an exception—years that end in 00 (1900, 1800) aren’t leap years unless they’re also divisible by 400 (1600, 2000, 2400).

bpeoples (2516 )

Thanks for the correction bpeoples. I always thought it had to do with harvesting and public school schedules back in the 19th century or something like that. I stand corrected.

gorillapaws (14333 )

@gorillapaws: no worries! I just hate DST with a passion, so try to make sure people are educated about exactly why DST happens. (Mainly it’s about flattening the curve of when the sun rises, to gain us some extra evening hours—I expect that if you surveyed people who didn’t like DST, they’d be morning people like me more often than night owls)

bpeoples (2516 )

365 rotations of the Earth don’t exactly equal a full rotation around the Sun. So once every four years, we have to allow the Earth’s orbit to catch up during an extra day, and then we’re back to four full cycles around the Sun. If we didn’t have a leap year, after some 4*180 years winter would fall in summer and vice versa.

Day light savings: as I’ve once understood it, it’s to save energy (lighting mostly); once lamps (gas; electrical) light were invented, people’s schedule shifted to later times. And apparently with DST we’re trying to play catch up with that behaviour (not sure why this doesn’t happen during winter. Probably dark anyway in the evening). But DST does indeed have nothing to do with leap years.

The 24 hours and 24000 miles are purely coincidence, although it is interesting to compare nautical miles and the clock: a nautical mile is one minute of arc at the equator, hence 1/60 of a degree. With 360 degrees in a circle, the equator spans 60*360=21600 nautical miles. Any “closeness” to 24 hours then comes from the fact that 360/24*60 = 900, close to 1000. Doesn’t explain the landmiles, so you’d have to look into how land miles and nautical miles compare, and which came first.

Unfortunately, I cannot explain the original question: 24 hours per day. Having a guess: 12 months (~ moons) in a year, 12 divisions in a day. But hey, there’s night as well (said the Sumerian, or someone long past), so let’s double that. Hence 24 divisions per full day.
Another one would be: morning, noon, evening, night. 4 parts of the day, each with a subdivision into 6 (following the aforementioned Babylonian/Sumerian tradition): 24 hours per day.

Evert (167 )

I vaguely remember there being a 12-hours-in-a-day division, with another 12 at night. The length of these divisions varied throughout the year.

12 months in a year
12 signs of the zodiac
12 hours in a day
12 hours in a night

bpeoples (2516 )

@evert: That was my question.Now we know that Earth goes round the Sun in 365.4 days (24h days).Why can’t we measure 1st January (23:56:48) and 2nd January (23:56:20) and…?
Is it very difficult to do that with our computers,satellites,....?

Dine (233 )

Dine, you can do that, no problem. Only, that would be horribly confusing: imaging starting your day at 20:00 hours somewhere in June, because you’re shifting each day a little (and the Sun would rise, say, at 16:00 hours). We live, as practically any organism on this planet, by the day-night cyclus, so that’s what we’ll have to do with. Since we also live by the seasons, there’s also a year involved; and those two just don’t add up in whole numbers.
Actually, Nature would tend to fix this, that is, making the yearly orbit a whole number of days. Mercury has this behaviour, where 3 rotations around its axis equal 2 rotations of the planet around the Sun. The reason this doesn’t happen on Earth are several; most importantly 1/ the Moon is disturbing this resonance effect, and 2/ the Earth is quite a bit further from the main centre of mass, the Sun, that contributes to this.

Now that I’m on this, there is another thing to point out, just to confuse things further: one day on Earth (from Sun in the South to Sun in the South; 24 hours exactly) is not the same as a rotation around the Earth’s axis. Since the Earth moves a bit in its orbit in the meantime, it’s need to rotate a bit extra to align with the Sun again: 4 minutes extra. So a day (Sun to Sun) is 24*60=1440 minutes, but a revolution of the Earth 1436.
If you think this carefully through, you’ll see it has nothing to do with the leap year though. It’s just another inconvenience, mostly for astronomers: to them, the night sky appears to drift every day with 4 minutes (as you will notice yourselves, since different constallations appear at different points in the year, but at the same position on the same time). This 1436 minute day is called a siderial (“star”) day.
When I mentioned the orbit/axis rotation resonance for Mercury, I was mentioning the actual rotation (siderial day). So you wouldn’t really notice it when you’d be standing on Mercury.

@bpeoples: that was my last point. 12 months & 12 zodiac signs equal 12 (well, roughly, but hey) Moons in a year.

Evert (167 )

We are human and therefore resist change.

gooch (5674 )

Check out wikipedia’s explanation of the solar day vs the sidereal day. That’ll getcha thinkin’! ;)

outerspace (21 )

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