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Newgrange roof box

It is well known that Newgrange has a roof box, through which the winter solstice sun light penetrates up to the chamber of Newgrange.
This window, through which the sun shines on the chamber floor, is at present times trapezium formed because the light is blocked by the orthostats R21 and L20, which are leaning towards each other. These orthostats were not repositioned due to uncertainty on structural issues.

Former sky window

Looking at the information on hand, the azimuth and apparent altitude of this former sky window is determined here:

O'Kelly states that the sun is visible on the chamber floor from 08:58 until 09:15 BST on Dec. 21^st, 1969 (page 123 until 124 of Newgrange).
Patrick J. states that the azimuth of the sun can vary between 133° 42' and 138° 24' and the apparent altitude between 51' and 2°40' (Midwinter sunrise at Newgrange, Nature, 249, 1974, pp 517-519).
Ray T.P. states that the azimuth of the sun can vary between 133° 49' and 137° 29' and the apparent altitude between 55' and 2°11' (The winter solstice phenomenon at Newgrange, Ireland: accident or design?, Nature, 337, 1989, pp 343 - 345).

Remark: The max. apparent altitude of 2°11' is too small (also compared with the report of O'Kelly [1982] and Frank Prendergast and Tom Ray [pers. comm., 2001]).

Using the sky window determined by Ray [1989] and using the max. apparent altitude of 2°40' from Patrick [1974], the sun would come into the chamber appr. some 48 days (24 days before and after winter solstice day):

Sun location on Febr. 6^th, 3101 BCE (Julian calendar)

Present day sky window

Information on the present day sky window can be deducted from the above ideas of the former sky window and the following observation:

Martin Brennan states that the first sunlight is on the chamber floor 11 days before and after winter solstice day (page 80 of The stars and the stones).

The following trapezium (or triangular?) formed sky window (determined by roof box and the leaning in orthostats R21 and L20) is popping up:

the right down corner of the trapezium window
the right upper corner of the trapezium window
the left upper corner of the trapezium window
the left down corner of the trapezium window

More study has to be done to determine the left side of the trapezium/triangle in more detail.

Full moons

Full moons can be used to determine the roof box window in more detail and it also provides the opportunity to simulate the sun light 5000 years ago. Remember that the below mentioned dates will only be valid for the region around Newgrange and rise time. Other conditions (other place or set time) need to be calculated with a planetarium program (like Skymap). This is due to fast moving moon on its path.

Sun at winter solstice 3101 BCE, Gregorian calendar

Sun at winter solstice 2001 CE

Full moon risings in the coming years (2001 until 2020) that will cross the sky window are determined with JPL ephemeris.

July 5^th, 2001, 22:18 - 22:36 BST (sun at civil twilight). The moon has simulated the sun's path of winter solstice day 5000 years ago*.

Moon on July 5^th, 2001 CE at 22:27 BST

The weather had improved much during the day, but clouds were still in the way near the horizon:-(. But between 22:24 and 22:32 BST there was a spell: the moon was visible in the sky (video made beside standing stone GC1 [O 00783 72692] is available).

Moon seen from GC1 at 22:28 BST

During this spell the moon light (as was expected at civil twilight conditions and such a low apparent altitude of 1°38') was faintly experienced inside the chamber. The in-chamber pictures made by Dúchas don't show the moon in the roofbox (they were made at the wrong position).
From 22:42 BST onwards the moon became clearly visible in the sky (not in the chamber, because it was outside the sky window of the roofbox).

Moon seen from GC1 at 22:44 BST

To be able to witness this from within Newgrange was a great experience and I am very glad I and others were given this opportunity. I wish to thank Dúchas (Kathleen O'Keeffe, Clare Tuffy, Claire Mullen and Sharon Roche), Frank Prendergast and Michael Ryan for providing this opportunity!
I hope we will be able to repeat this unique sighting on July 14^th, 2003. That sighting will be better, because it will then be nautical twilight at moon rise.
A radio interview by Áine Ni Chairbre on this event has been broadcasted by Louth Meath FM Radio (95.8 MHz) on July 8^th, 2001 19:48 - 19:59 BST.

July 6^th, 2001, 23:11 - 23:25 BST (sun at nautical twilight)
July 14^th, 2003, 23:12 - 23:37 BST (sun at nautical twilight). The moon will simulate the sun's path of winter solstice day 5000 years ago*. The duration of the expected shadow inside Newgrange is prognosed in this section.

Moon on July 14^th, 2003 CE at 23:21 BST

On July 14^th, 2003 no moon rise was visible due to clouds. The first moment part of the the moon was visible was around 23:43 BST. A picture of the full moon form at 23:47 BST is below (still a little obscured by clouds and haze):

July 31^st, 2004 (sun at civil twilight)
May 23^rd, 2005 (sun at civil twilight)?
May 13^th, 2006 (sun at nautical twilight)?
May 19^th, 2008 (sun at civil twilight)?
May 9^th, 2009 (sun at nautical twilight)?
July 7^th, 2009 (sun at civil twilight)
May 17^th, 2011 (sun at civil twilight)?
June 6^th, 2020 (sun at nautical twilight)
July 5^th, 2020 (sun at civil twilight)

The * full moons in 2001 and 2003 are at the same position as the winter solstice sun was in 3100 BCE (the possible building date of Newgrange)!!!
The ? mean that it is not known if the moon will shine on the floor of the chamber (depends on the true dimensions of the sky window)

Twilight conditions at moon rises

The following definitions exists with regard to twilight conditions:

full daylight
civil twilight
nautical twilight
astronomical twilight
full darkness

One of the drawbacks of moon shadow sightings within Newgrange could be that the sun is still close to the horizon, and thus no full darkness. So tests have been done with a mock-up of Newgrange. This mock-up was made with a appr. 1m long tube and 5 cm in diameter (so creating a sky window comparable. the size of the roof box). One end is open and the other end is covered with opaque paper (to be able to see shadows). The side of the opaque paper also had a black vale, so that while watching the shadows on this paper, no other light was on it, thus creating a kind of dark chamber;-).

There are two differences between this experiment and the actual location: in the experiment the moon light is perpendicular to the opaque paper, while in Newgrange the moon light is only a few degrees over the gravel on the ground.

Sightings were done on two full moon rise dates:

July 16^th, 2000 (civil twilight)

22:08 BST middle of moon at 1° 27'

chamber

22:10 BST middle of moon at 1° 39'

chamber

22:15 BST middle of moon at 2° 10'

July 17^th, 2000 (nautical twilight)

22:41 BST middle of moon at 1° 24'

chamber

22:46 BST middle of moon at 1° 55'
22:52 BST middle of moon at 2° 30'

Certainly at nautical twilight, shadows can be seen . At civil twilight perhaps not the start can be seen, but at least the end of the phenomena is visible.
Some proof that full moon (98%) can be photographed when sun is at nautical-astronomical twilight; look in Brennan [1983, page 99]. This provides a picture of the moon light on stone 6 of Cairn T on April 1^st, 1980, around 20:10 GMT.

The visibility of shadows can also be determined with a model.

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Major content related changes: July 6, 2001