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:
  1. O'Kelly states that the sun is visible on the chamber floor from 08:58 until 09:15 BST on Dec. 21st, 1969 (page 123 until 124 of Newgrange).
  2. 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).
  3. 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).
  4. 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. 6th, 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: The following trapezium (or triangular?) formed sky window (determined by roof box and the leaning in orthostats R21 and L20) is popping up: 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.

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: 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:

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 1st, 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