http://antwrp.gsfc.nasa.gov/apod/ap081220.html
<<Explanation: Tomorrow's solstice marks the southernmost point of the Sun's annual motion through planet Earth's sky and the astronomical beginning of winter in the north. You can watch a live webcast of the the solstice sunrise from the megalithic tomb of Newgrange, in County Meath, Ireland. Newgrange dates to 5,000 years ago, much older than Stonehenge, but also with accurate alignments to the solstice Sun. In this view from within the burial mound's inner chamber, the first rays of the solstice sunrise are passing through a box constructed above the entrance and shine down an 18 meter long tunnel to illuminate the floor at the foot of a decorated stone.
The actual stone itself would have been directly illuminated by the solstice Sun 5,000 years ago.>>
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http://en.wikipedia.org/wiki/Newgrange
<<Once a year, at the winter solstice, the rising sun shines directly along the long passage into the chamber for about 17 minutes and illuminates the chamber floor. This alignment is too precise to have occurred by chance. Professor M. J. O'Kelly was the first person in modern times to observe this event on December 21, 1967.
The sun enters the passage through a specially contrived opening, known as a roofbox, directly above the main entrance. Although solar alignments are not uncommon among passage graves, Newgrange is one of few to contain the additional roofbox feature. The alignment is such that although the roofbox is above the passage entrance, the light hits the floor of the inner chamber.
Today the first light enters about four minutes after sunrise,
but calculations based on *
the precession of the Earth* show that 5000 years ago first light would have entered exactly at sunrise. The solar alignment at Newgrange is very precise compared to similar phenomena at other passage graves such as Dowth or Maes Howe in the Orkney islands, off the coast of Scotland.>>
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It is
NOT the [25,765 year cycle] precession of the equinoxes but
rather the [41,013 year cycle] variable obliquity of the axial tilt
that prevents the earth's winter solstice sunrise from occurring at
a point as far south (and as late in the morning) as it did 5000 years ago.
Hence:
Today the first light enters about four minutes after sunrise
; too late for:
the actual stone itself to be directly illuminated.
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http://en.wikipedia.org/wiki/Axial_tilt
<<The Earth currently has an axial tilt of about 23.44°.
The Earth's axial tilt varies between 22.63° to 24.35°, with a 41,013-year period, and at present, the tilt is decreasing.
The last maximum was reached in 8700 BC, the mean value occurred around 1550 and the next minimum will be in 11800.
Other planets may have a variable obliquity too, for example on Mars the range is believed to be between 15° and 35°, as a result of gravitational perturbations from other planets. The relatively small range for the Earth is due to the stabilizing influence of the Moon, but it will not remain so. According to Ward, the orbit of the Moon (which is continuously increasing due to tidal effects) will have gone from the current 60 to approximately 66.5 Earth radii in about 1.5 billion years. Once this occurs, a resonance from planetary effects will follow, causing swings of the obliquity between 22° and 38°. Further, in approximately 2 billion years, when the Moon reaches a distance of 68 Earth radii, another resonance will cause even greater oscillations, between 27° and 60°. This would have extreme effects on climate.>>
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