Is the 24.hour day really based on earth's revolution?

Hello Everyone,

Is the concept of a 24 hour day a really based on the time it takes the earth to compete a rotation or was it just a random system devised just because? Does the earth make a precise rotation every 24 hours or is it just close enough?

The day is an obvious unit of measurement. The hour is just a result of dividing the day into 24 even length units, which was done in ancient times. The ancient Egyptians, Babylonians and other early civilizations settled on the 24 hour day for a variety of reasons.

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).

You’re presupposing that it was 24 hours that the day was fit into. Instead it was a day that 24 hours was made to fit.

Note also that most days are not exactly 24 hours, but vary by as much as 30 seconds throughout the year.

Actually, if I recall my history correctly, it was not so much the day as we understand it was divided into 24 hours, but that the daylight was divided into twelve hours. In the days of sundials that wasn’t too difficult, but it did mean that the higher your latitude the more variable the length of the hour, depending on the season. Since the average person was probably more likely to speak of “dawn”, “noon”, “mid-afternoon”, “sunset” and so forth rather than hours this wasn’t a big deal.

Later on, the night was likewise divided into 12 hours.

Then the period from noon-to-noon (or whatever point in the Earth’s rotation was used) was divided into 24 hours and suddenly the hour was a constant, but the number of hours in daylight or night wasn’t anymore. Which is our current system.

Tell that to the Romans.

Just to be the first to mention, 12 is a handy number. It has twice as many integer factors (2, 3, 4, 6) as base 10, but you do lose some convenience having everything except time in base 10.

Hey, hang on a centon!

Fixed hour length was invented in the days of sundials.

Since “the days of sundials” encompassed a few centuries we could both be right on that, you know? I tried to keep my post short, which necessitated keeping it simplified and missing a few details.

The modern definition of the hour is just 3600 seconds, and the modern definition of the second is based on the properties of a particular atom, which can be easily replicated in the laboratory. So no, it is no longer inherent fact that the day is 24 hours long.

However, the new definition was devised to be as close as possible to the old definition, which really was 1/24 of the average synodic period of rotation of the Earth, so the day is still exactly 24 hours long to an incredibly high degree of precision, and this is no accident.

Yup, I could have included that, but it slipped my mind as I cut the rest of my post for being wrong and/or unsupported by immediately available evidence.

Bolding mine.

I agree 100%; well said. But IMO it’s worth emphasizing to the non-technical OP that at that point “day” becomes a unit of time measurement that is no longer tied to the cycles of sunrise and sunset. It’s merely more-or-less similar to that interval.

So we now really have (at least) two competing concepts for “day”: one based on Sun-and-Earth motions and one based on counting the ticks of various clocks. By design they’re mostly interchangeable in casual i.e. sloppy, conversation. But in reality they’re very, very different things.

Well, I imagine the first and most accurate measure of a day was the time elapsed from one local noon to the next day’s local noon. Just stick a STRAIGHT rod in the ground, use a plumb bob to make sure it’s perpendicular, and start timing from the shortest shadow it casts today to the shortest one it casts tomorrow. So the duration is pretty much there.

As for dividing that period into 24 equal periods, I believe the bottom line is that decision was pretty much arbitrary. They could have just as easily decided to divide the day into 42 Cecils or 20 CK Dexter Havens of equal length. It just takes enough people to use and push a standard unit until it becomes the accepted convention. Sure, after the fact you can come up with all sorts of justifications for your choice, but I still think it’s still fairly arbitrary. And if there aren’t enough people, well, you’ll notice your speedometers measures miles or kilometers per hour and not smootsper hour.

If you can’t trust a scientist named Chronos on this, who can you trust?

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.

There’s an error in the title-the earth rotates around its axis, but revolves around the sun.

Timing with what? It actually worked the other way around: when we first devised mechanical timekeeping devices, we calibrated them or measured their accuracy against existing cyclic phenomena, the most obvious (but hardly simple) were celestial.

The history of measurement of time is interesting and complicated. I remember reading an excellent short but thorough treatment of the subject by David Mills, author of the Network Time Protocol that our computers use to know what time it is. Sadly, the last time I looked for it not long ago, it wasn’t listed on his UDEL site.

But I believe that Broomstick has it right: the day was at first divided into 12 equal-ish parts from sunrise to sunset, and only later was that concept extended into the dark hours. Later still was time codified well enough so that a time could be discussed without also specifying a specific location. (Heck, even dates used to be ambiguous, during certain periods, due to using different calendars, say between England and France. As it turns out, the Pope was more correct in that case!)

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.

Right! And RPM stands for … oh, never mind.