[LEAPSECS] Coming of age in the solar system

[Rob Seaman]

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