I meant to credit Timothy Ferris for the borrowed subject line:
http://www.harpercollins.com/books/Coming-Age-Milky-Way-Timothy-Ferris/?isbn=9780060535957
Apologies!
On Sep 3, 2010, at 5:55 PM, Rob Seaman wrote:
> Q: How long is one Earth day?
> A: The sidereal rotation period of the Earth is 86164 SI seconds (23h56m4s).
>
> Q: Why was the SI second not specified to be 86164/86400 as long to make this
> come out an even 24 hours?
> A: Because only those who study the stars need to keep time by them.
>
> Q: Why was the SI second chosen such that 24x60x60 of them fit into one solar
> day?
> A: Because everybody else keeps time by the Sun - sexagesimally since the
> Sumerians.
>
> One "day" doesn't mean one day by the stars. One day is one day by the Sun -
> that is, "one mean solar day". A day is indeed the sidereal period of the
> Earth, simply adjusted to subtract off one rotation/year due to our lapping
> the Sun.
>
> Q: How many days are in one Earth year?
> A: Having circled the Sun, the Earth will have rotated 366.25 times:
>
> 24(60)(60)/86164 x 365.25 = 365.25 + 1
>
> All this talk of the political vagaries of DST offsets or of the apparent
> wanderings of the Sun in the sky (the Earth is tilted and its orbit is
> elliptical) are red herrings. They confuse modest periodic effects with the
> unbounded secular (monotonic and accelerating) effect that ceasing leap
> seconds would cause. By seeking to redefine UTC, the ITU is mucking with the
> definition of the word "day". I don't see how this is within the purview of
> the International Telecommunications Union.
>
> Warner asks:
> Q: "And why does MEAN solar time matter more than ACTUAL solar time?"
> Q: "And what flavor of MEAN solar time is best?"
> Q: "What does solar time mean on mars, the moon, etc?"
>
> A: Mean solar time (ignoring legal tap dancing on the head of a pin about the
> definition of "GMT") matters because it *is* "actual" solar time. It has not
> previously mattered what flavor - the ITU is "breaking the symmetry" of UTC
> and GMT. What humans have already done during trips to Mars and to the Moon
> is to structure mission operations in concert with local sunrise and sunset.
> Presumably the equation of time on other solar system bodies will be taken
> into account just as on Earth. It is not as if the ITU has a coherent plan
> for timekeeping needs on the other planets any more so than they do on Earth.
>
>
> ITU = flat-earthers
>
> Civil timekeeping is based on mean solar time because our clocks are
> fractional representations of the calendar. The only reason the ITU is able
> to contemplate this cheat of redefining UTC (originally defined as the
> "general equivalent" of GMT) is because the rate of atomic clocks was chosen
> to be very similar to the canonical rate of solar clocks. It was chosen to
> be similar to our ubiquitous solar clocks because civil timekeeping was
> recognized to be equivalent to mean solar time.
>
> Clocks go round and round - and so do we.
>
> Rob Seaman
> National Optical Astronomy Observatory
> --
>
> "It cannot be that axioms established by argumentation can suffice for the
> discovery of new works, since the subtlety of nature is greater many times
> than the subtlety of argument."
> - Sir Francis Bacon
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