| From: MichaelT | 4/07/99
10:39:28 |
| Subject: Why 24hrs in a day? | post id:
21653 |
| Why was the number 24 chosen as
the number of hours in a day? Was it because of it's highly divisible nature? I can probably guess the reason why there are 60 seconds in a minute and 60 minutes in an hour, due to there being 360 degrees in the rotation of the minute hand - does this have any relevence or just a coincidence? Regards, MichaelT | |
| From: Michael Gunter | 4/07/99
11:24:36 |
| Subject: re: Why 24hrs in a day? | post id:
21654 |
Friday, January 12, 1996DB: This is Earth & Sky. A listener asked, "Why are there 24 hours in a day? . . . Is 24 some historically arbitrary number, or is there some scientific basis for it? Does it stem from Arabic culture, like our numbers, or Christian prayer cycles, or the number of breaths per day of some long-forgotten monarch, or what?"
JB: Our system of 24 hours comes from Egyptian culture. The stages of its development are documented in monuments and tombs. In about the 30th century B.C., the Egyptians made a calendar of 365 days. It had 12 months of 30 days --plus five holidays at the end of the year. Most importantly, each month was divided into three periods of 10 days each.
DB: Now consider that all stars move in a yearly cycle, due to Earth's motion around the sun. The first appearance of a star before sunrise -- at the start of its new season of visibility -- is known as its heliacal rising.
JB: The Egyptians came to associate the heliacal rising of a particular bright star with each of their 10-day periods over the course of a year. There were 36 of these stars in all -- but night came to be associated with 12 stars -- about how many would rise each night during the summer months. Meanwhile, the Egyptians divided the period from sunrise to sunset into 10 parts. And they added an hour before sunrise and after sunset for twilight periods -- for a total of 24 hours in a day/night period.
DB: The length of these original "hours" varied with the seasons -- they were called "seasonal" hours. Hours of equal length were introduced by Greek astronomers. But as late as the Middle Ages, when mechanical clocks were coming into use, seasonal hours were still used in everyday life. With thanks to the National Science Foundation -- we're Block and Byrd for Earth and Sky.
Author(s): LeRoy Doggett, Tara Ross, Deborah Byrd I would have just put a link to this info, but I had to cust and paste it from a web page that only displayed the background in Netscape navigator 4.0 The offending page is http://www.earthsky.com/1996/es960112.html (does it work OK in Internet Explorer?) Cheers, Michael Gunter | |
| From: Tyler Henderson | 4/07/99
15:55:14 |
| Subject: re: Why 24hrs in a day? | post id:
21684 |
| There is actually 23 hours 56
minutes and i think 47 seconds in a day. 24 is just rounded up to make it
easier for everyone. This equals 1 day over 4 years and is why we end up
with a leap year. | |
| From: Daryn Voss | 4/07/99
16:19:36 |
| Subject: re: Why 24hrs in a day? | post id:
21691 |
| The length of the solar day is 24
hours (pretty much exactly). The solar day is the average amount of time
it takes for the sun to go from being at its highest point in the sky,
until the next time it is at the highest point in the sky. The length of the sidereal day is about 23 hrs 57 minutes. This is the actual period it takes for the earth to spin on its axis (with respect to the stars as it were). The solar day is slightly longer than the sidereal day because from one day to the next, the earth moves in its orbit around the sun. We have to turn just that little bit further to bring the sun back into the same position. The reason for leap years is that the solar day does not go evenly into the year. There are about 365 and 1/4 solar days in a year. So three out of every four years has 365 days and the fourth (the leap year) has 366. This brings the calendar into line. (Actually, the year is slightly less than 365 1/4 days, so 1700, 1800, 1900, 2100etc are not leap years.) Furthermore, the "year" that we use is generally the seasonal year, which is not the same as the length of time that it takes the earth to go around the sun, because of the precession of the earth's axis. 8^) | |
| From: MichaelT | 5/07/99
8:39:28 |
| Subject: re: Why 24hrs in a day? | post id:
21786 |
| Thanks Michael, I didn't even
think of an astrological reason for it, it seems logical now. That page works fine under the browser Opera, and the link from that page http://www.earthsky.com/1996/esmi960112.html is even more informative. (explains the minutes/seconds bit as well) Regards, MichaelT | |
| From: greg mcquaid | 12/08/99
11:09:43 |
| Subject: time? | post id:
30166 |
| why do we have the measurement
units we have in our perception of time it's easy to work out planet
rotation etc.but why 24hrs in the day,60min hours,60sec minutes is it
related to the roman measurement in 1st,2nd,3rd & 4th watches (ie on
guard).&why not a more accurate metric system.
| |
| From: Stephen Bosi | 12/08/99
13:21:21 |
| Subject: re: time? | post id:
30266 |
| The ancient Babylonians had a
number system based on the number 60. Modern science has some curious
vestiges of this, 60 seconds/minute and 60 minutes/hour or degree and 360
degrees per circle. This may seem odd, but actually at a time where
calculations had to be done in the head, base 10 is arguably not the best
base since it only has two (non-trivial) factors, namely 2 and 5. A base
with more factors means there are more shortcuts in calculations. Before
the availability of calculators, some engineers used to learn base 12 and
did their calculations in that base because 12 has 4 non-trivial factors
2,3,4 and 6 making multiplication and division in the head
easier. | |