[LEAPSECS] Reliability

[Rob Seaman]

Our humble and long suffering moderator informs me that this message  
bounced a few days back since the attachment was too big.  My  
apologies, since my more recent messages were predicated on folks  
having seen this plot.

I've put the attachment online as I should have in the first place:

	http://iraf.noao.edu/~seaman/images/HowLongIsADay.pdf

Rob
--

On Jan 2, 2009, at 2:29 AM, Rob Seaman wrote:


> Let's see if an attachment will help.  Here's a slide from a  

> conference session a few years back.  I think this was in San  

> Lorenzo de El Escorial, Spain, which I mention since the highlight  

> of that conference was fittingly a solar eclipse.  (Well, my  

> personal highlight was a day trip to see Guernica at the Reina Sofía.)

>

> If the attachment makes it through, imagine zooming out by a factor  

> of 200X so the y-axis reaches one full day from 0 to 86400 seconds.   

> The wiggle due to the eccentricity of the Earth's orbit and the tilt  

> of the poles will vanish in the anti-aliasing.  There will be a just  

> barely visible offset between the sidereal ("relative to the stars")  

> day length and the mean solar day length - just under 4 minutes out  

> of 1440 minutes per day.  Add up 4 minutes per day times 365 days  

> and you end up with an extra day relative to the stars.

>

> Except that this is looking at it backwards.  We really spin with  

> respect to the stars.  All the solar system action is foreground  

> folderol.  There is the simple offset to the mean solar day from  

> lapping the sun once per year.  And the wiggle on top of that of the  

> apparent solar day from the elliptical orbit and the tilt.  And the  

> equation of time / analemma (not shown) from integrating the slight  

> wiggle throughout the year.  (Well, maybe it makes more sense to put  

> the tilt into the analemma.)

>

> My thesis is that a lot of the thrashing on this list over the years  

> has come from allowing the apparent solar issues to cloud the more  

> fundamental mean solar day.  (Yes, the pun was intended, get over it.)

>

> Rob

> ---


http://iraf.noao.edu/~seaman/images/HowLongIsADay.pdf