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

[Bruce Griffiths]

Alan Biocca wrote:
> The sun is illuminating the floor 4 minutes after sunset?
>
>   
Oops! Thanks, I thought sunrise and typed sunset.

Actually the sun is visible after geometric sunset due to atmospheric
refraction.

corrected version:

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 sunrise in our time. Precession
over the intervening 5000 years means that when constructed the initial
illumination would have occurred exactly at sunrise. The geometry
precludes using any other time of day for alignment.



Bruce
> -- 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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>> and follow the instructions there.
>>
>>     
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