[time-nuts] Solstice question, about 5000 years ago

[Alan Biocca]

The sun is illuminating the floor 4 minutes after sunset?

-- Alan

On Sun, Dec 21, 2008 at 8:09 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz
> wrote:

> Bruce Griffiths wrote:
> > Steve
> > Steve Rooke wrote:
> >
> >
> >> Michael,
> >>
> >> 2008/12/22 Michael Sokolov <msokolov at ivan.harhan.org>:
> >>
> >>
> >>> Bill Hawkins <bill at iaxs.net> wrote:
> >>>
> >>>
> >>>
> >>>> The passage grave at New Grange, Ireland, is one of those astronomical
> >>>> wonders where the rising sun at winter solstice shines down a
> relatively
> >>>> long tunnel to shine on carved stone at the far wall of a chamber.
> >>>>
> >>>> We know that solstice has the shortest day and the longest night.
> >>>>
> >>>> How'd they know that?
> >>>>
> >>>>
> >>> I'll abstain from answering the last question, but I'm more interested
> >>> in a different question: from what I understand, the exact shape of the
> >>> analemma depends on the misalignment between the line of apses
> (aphelion
> >>> and perihelion of Earth's slightly eccentric orbit) and the solstices
> >>> and equinoxes defined by Earth's obliquity.  These things do change
> very
> >>> slowly over the course of millennia, don't they?  Isn't that change
> >>> significant enough that the correct stone alignment would be different
> >>> between today and 5000 y ago?  If they got it right 5000 y ago for
> their
> >>> epoch, why does it still work now?  Hasn't the analemma shifted far
> >>> enough to break the alignment?
> >>>
> >>>
> >> This is a very interesting point, of course, so this is my 2c worth:-
> >>
> >> 1) We talk about nano/pico/femto second differences here but I wonder
> >> just how far the analema has shifted in the 5000 years and just how
> >> much difference it would make physically to this location in Ireland.
> >> I assume the slit of light that shines through the passage is not
> >> microns wide and the target, similar. It may be that the change only
> >> makes a small physical difference and is within the accuracy
> >> percentage of the measuring instrument, being The Passage Grave.
> >>
> >>
> > The sun has an angular diameter of about 1/2 degree so it would be
> > difficult to achieve an alignment accuracy much better (<10% of suns
> > diameter) than that without optical aids.
> > To get some idea of the potential accuracy and resolution of the setup
> > some dimensions would be useful.
> >
> >> 2) Is it possible that any shift in the analema may be circular over
> >> that time and has reverted to it's position of 5000 years ago.
> >>
> >> 3) Bill said that this alignment is noted at rising sun on the winter
> >> solstice. Is it possible that originally the alignment was at a
> >> different time of the day, say, mid-day which would seem to be a
> >> better target to aim for.
> >>
> >>
> >>
> > When the sun is at its highest point in the sky the vertical alignment
> > will be quite different than when it is near the horizon.
> > If the sun is near the horizon refraction is quite significant.
> > However regular refraction will have no effect on its apparent azimuth.
> >
> >> 4) Stonehenge dates from about the same time and there is a suggestion
> >> that this has an astronomical connection. If that is the case, it
> >> would also be affected by any shift in the analema but I wonder how
> >> much physical difference that would make considering, what appears to
> >> me, to be an instrument with quite a degree of accuracy latitude.
> >>
> >> 73, Steve - JAKDTTNW (yes, I got it right this time, said I was a
> troglodyte :-)
> >>
> >>
> >
> > Bruce
> >
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> >
> According to the Wikipedia entry on Newgrange the duration of the
> illumination of the floor at the end of the 18m long passage is 17
> minutes and the time at which the floor is initially illuminated on the
> Winter Solstice occurs at 4 minutes after sunset in our time. Precession
> over the intervening 5000 years means that when constructed the initial
> illumination would have occured exactly at sunrise. The geometry
> precludes using any other time of day for alignment.
>
>
> Bruce
>
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