Posted: 11/21/2006 7:43:58 PM EDT
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Please bear with me for these question come from my tortured brain. 1st, we all know how long one second is. What I would like to know is how was it decided and by who was it decided the amount of time one second would be? For example why wasn't the time one second represents equal to what we now consider 10 seconds? Does it have something to do with the rotation of the earth? I would believe that has something to do with it. How did all the cultures around the world agree to the same measurement of time? Were there many cultures that came to the same conclusion on the measurement of time and with the mixing of these cultures time was a well established principal and there was little dispute over the measurement of time? I know it's a dumb question but it has kept me up many nights thinking about it. I just find it fascinating since most of the world is rarely able to agree on anything. |
Ancient civilizations used 24 because it is twice 12, and 12 was very important to them. It is the number of months in a year for example, so why not divide a day (sans night) the same way? That was the logic of the ancient world anyway. A minute was simply the division of an hour, and a second the "second division" of an hour, or smaller division. Babylonians used a base-60 counting system for astrological/astronomical purposes, so that's where the 60 comes from. So 1 second is 1/60th of 1/60th of 1/24th of a day. Those are the origins anyway, the terms have been redefined somewhat to make them more uniform and scientific, but they all have their basis in these two ancient cultures. |
well not exactly a full rotational period....otherwise it wouldn't take into account for leap seconds. |
That's because it was devoloped in Europe many hundreds of years ago. |
A rotational period is 23 hours, 56 minutes and 4.1 seconds, IIRC. But that's taking into account the Earth's revolution around the Sun. It takes that long for the Sun to move from one position, all the way around, and back to exactly that position. If you add the 4+- leap minutes/day (1 degree around the sun per day), it's 24 hours. Correct me if I'm wrong, but I'm pretty sure that's right. |
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From Wikipedia: Originally, the second was known as a "second minute", meaning the second minute (i.e. small) division of an hour. The first division was known as a "prime minute" and is equivalent to the minute we know today. The factor of 60 comes from the Babylonians who used factors of 60 in their counting system. However, the Babylonians did not subdivide their time units sexagesimally (except for the day). The hour had been defined by the ancient Egyptians as either 1/12 of daytime or 1/12 of nighttime, hence both varied with the seasons. Hellenistic astronomers, including Hipparchus and Ptolemy, defined the hour as 1/24 of a mean solar day. Sexagesimally subdividing this mean solar hour made the second 1/86,400 of a mean solar day. Hellenistic time periods like the mean synodic month were usually specified quite precisely because they were calculated from carefully selected eclipses separated by hundreds of years—individual mean synodic months and similar time periods cannot be measured. Nevertheless, with the development of pendulum clocks keeping mean time (as opposed to the apparent time displayed by sundials), the second became measurable. The seconds pendulum was proposed as a unit of length as early as 1660 by the Royal Society of London. The duration of a beat or half period (one swing, not back and forth) of a pendulum one metre in length is approximately one second.[1] In 1956 the second was defined in terms of the period of revolution of the Earth around the Sun for a particular epoch, because by then it had become recognized that the Earth's rotation on its own axis was not sufficiently uniform as a standard of time. The Earth's motion was described in Newcomb's Tables of the Sun, which provides a formula for the motion of the Sun at the epoch 1900 based on astronomical observations made between 1750 and 1892. The second thus defined is the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time. This definition was ratified by the Eleventh General Conference on Weights and Measures in 1960. The tropical year in the definition was not measured, but calculated from a formula describing a tropical year which decreased linearly over time, hence the curious reference to a specific instantaneous tropical year. Because this second was the independent variable of time used in ephemerides of the Sun and Moon during most of the twentieth century (Newcomb's Tables of the Sun were used from 1900 through 1983, and Brown's Tables of the Moon were used from 1920 through 1983), it was called the ephemeris second. With the development of the atomic clock, it was decided to use atomic clocks as the basis of the definition of the second, rather than the revolution of the Earth around the Sun. Following several years of work, two astronomers at the United States Naval Observatory (USNO) and two astronomers at the National Physical Laboratory (Teddington, England) determined the relationship between the hyperfine transition frequency of the caesium atom and the ephemeris second. Using a common-view measurement method based on the received signals from radio station WWV, they determined the orbital motion of the Moon about the Earth, from which the apparent motion of the Sun could be inferred, in terms of time as measured by an atomic clock. As a result, in 1967 the Thirteenth General Conference on Weights and Measures defined the second of atomic time in the International System of Units (SI) as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. The ground state is defined at zero magnetic field. The second thus defined is equivalent to the ephemeris second. The definition of the second was later refined at the 1997 meeting of the BIPM to include the statement This definition refers to a caesium atom at rest at a temperature of 0 K. In practice, this means that high-precision realizations of the second should compensate for the effects of the ambient temperature (black-body radiation) within which atomic clocks operate to extrapolate to the value of the second as defined above. Furthermore, it indicates that the ultimate atomic clock would contain a single caesium atom at rest emitting a single frequency. |