on 3/31/2002 11:00 AM, kilopascal at [EMAIL PROTECTED] wrote: > I thought the day was actually shorter than 24 h. The day is 23 h 56 min > long. Something like that. So, the true day is 4 min shorter than the 24 h > day.
You are thinking of the sidereal day. That is equal to one complete rotation of the Earth relative to the stars. It is 23 h 56 min (and some seconds) long. Basically, it is the time from the passage of a star (any star) past any particular meridian in the sky until that same star again passes that same meridian. But the day which naturally governs our human activities is the time for one complete rotation of the Earth relative to the sun. This is the length of time that is set to be "exactly" 24 hours (or at least very close to 24 hours). It is called the solar day. The Earth moves around the sun and this motion makes the time for one rotation relative to the sun different from the time for one rotation relative to the stars. The solar day is the time from the passage of the sun past any meridian until the next time it passes that meridian. Neither of these "days" is exactly constant during the year because of minor complexities arising from the tilt of the Earth's axis, the variability of the speed of the Earth in its orbit about the sun and some actual fluctuations of the Earth's rotational speed. For purposes of making clocks and other timing mechanisms, it is usually the average length of time that is used for either one of these "days". The solar day was at one time defined as the average for one year, and not any year but a particular year (1900) because the average was not even exactly the same every year. The important point in all this is that the definition of all of our units (Including time units) MUST result in units which are constant in size. The day and the year (as well as the month) are not constant in length, therefore (1) they can't EVER be made to be a fixed number of (constant) seconds long; and conversly, (2) it is impossible to define the units of time (like the second) in terms of these natural times because that would make the fundamental time units variable. To try to force our time periods like the day and the year to be an exact and fixed number of seconds long is futile. Let's stop trying. If we don't like hours and minutes, we don't have to use them. We can use the SI unit "second" or more conveniently one of its multiple (like the kilosecond). A day is 86.4 kiloseconds long, plus or minus an extra leap second every so often. A kilosecond is between 15 and 20 minutes long (16.667 more precisely). It would not be hard to structure our day around time periods of 1 kilosecond IF WE WANTED TO. But nobody wants to. Regards, Bill Hooper physics professor (retired), Florida, USA ======================== Keep It Simple - Make It Metric! ========================
