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
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