[LEAPSECS] Reliability

[Rob Seaman]

Hi Richard,

Yes, it's certainly true that sundials show apparent solar time.  I  
looked into buying or building a state of the art sundial when we  
moved into a new house a few years back.  The cost can be staggering,  
so this was hard to justify, but the state of the art is pretty spiffy  
these days.

Apparent is not more real than mean, however.  Like I said, it depends  
on point of view.  By combining multiple measurements of apparent  
apparitions, one attempts to recover an intrinsic parameter of the  
planet - mean solar time.

Another point of view is gain vs sensitivity (as used in Janesick's  
CCD bible, for instance).  Gain relates successive steps in a process  
in a forward direction.  Sensitivity recovers them in a backward  
direction.  Photons from astrophysical sources are more fundamental  
than electrons from solid-state detectors, and electrons more  
fundamental than the DNs from A/D converters.  Image processing  
measures DNs to characterize photons, however.

We measure apparent positions of solar system objects, comets as well  
as the Sun, in order to characterize the more fundamental parameters  
of orbital elements and the spinning bodies following those orbits (as  
well as weird and wonderful higher order wobbles).

The Earth is spinning like a top.  Its rate of spin is what we're  
interested in for civil time.  Far from denying this fact, both the  
current UTC standard and the ITU proposal rely on the high regularity  
of mean solar time for them to be even conceptually possible.

If we focus on ways to improve the logistics of the approximation  
scheme (without abandoning the underlying requirement), we'll reach  
consensus.

Rob
---

On Jan 2, 2009, at 6:18 AM, Richard B. Langley wrote:


> Rob:

> Just sending this offlist. Forward if you like. A conventional  

> sundial directly shows

> apparent solar time.

> -- Richard

>

> Quoting Rob Seaman <seaman at noao.edu>:

>

>> Tony Finch wrote:

>>

>>> On Thu, 1 Jan 2009, Rob Seaman wrote:

>>>>

>>>

>>>> Mean solar time will outlast artificial clocks and the species that

>>>> built them by a factor of something like 5,000,000,000 to 50,000.

>>>

>>> Not really, because mean solar time is also artificial and can't  

>>> exist

>>> without mechanical clocks and telescopes.

>>

>> And I suppose the refrigerator light goes out when the door is

>> closed :-)

>>

>> Once more from the top, mean solar time is just sidereal time offset

>> by a little bit to make up for the Earth lapping the Sun once a year.

>> Nowhere does humanity appear in the equation, just the Earth and Sun

>> and Stars.

>>

>> Apparent solar time is derived from mean solar time, not the other  

>> way

>> around.

>>

>> Rob