The sun is illuminating the floor 4 minutes after sunset? -- Alan
On Sun, Dec 21, 2008 at 8:09 PM, Bruce Griffiths <[email protected] > wrote: > Bruce Griffiths wrote: > > Steve > > Steve Rooke wrote: > > > > > >> Michael, > >> > >> 2008/12/22 Michael Sokolov <[email protected]>: > >> > >> > >>> Bill Hawkins <[email protected]> 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 > > > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > > > > 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 > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
