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Penrhiwen 2

Megalithic equinoxes alignmentsLechwedd Penrhiwen, Rhayader, Powys.

The Stone Hand at the Head of the White Water.

This is a fine flat faced example of an orientated blade stone with it's two long faces precisely aligned on two diametrically opposed hill notches one to the west and the other to the east. The sunset alignment. This survey was made from a series of good photographs taken as the Sun settled into the notch to the west on the evening of 21st. September 1997.

The ideal declination of the centre of the Sun at the Megalithic Calendar equinoxes is +0 deg. 26.4 min. At the time these photographs were taken the Sun was still some four and a half hours short of making this position, however the careful construction of this alignment makes sure that there could be no ambiguity in finding the correct day, given clear weather at sunset. Here is a montage with the setting Sun's position shown for the three days surrounding an equinox of the Megalithic Calendar.

Rate of declination change at an equinox. At the equinoxes the Sun is making it's greatest declination change per day. The rising and setting points on the horizon are taking their largest daily steps. The difference per day is 24 arc minutes almost as great as the fulldiameter of the Sun which is 32 arc minutes, over half a degree. The Sun rarely sits conveniently on the horizon at the exact moment it is achieving a required declination. Errors of nearly plus or minus 12 hours are inherent, which means, at these times of year, there may be discrepancies of the position of the Sun on the horizon nearly as great as it's own semi-diameter. The range of error is + or - 12 arc minutes; the semi-diameter of the Sun is 15.5 arc minutes. Due to these large inherent errors it would be difficult to arrange single pin-point foresights indicating an upper or lower limb of the Sun as they would not be capable of sorting one day from another reliably.

These are the positions of the Sun with the maximum possible errors at an equinox.

The function of back-to-back alignments. When observations can be made to alignments which operate on the same day further reliability may be ascertained. If one observation is border-line then then the sun will sit accurately on the other alignment at the prior, or next, observation. If, for example, the sunrise to the east is ambiguous then either the previous day's sunset or the sunset of the same day will sit with good accuracy on the opposite alignment showing whether the precise moment of the interval fell on the previous evening or the morning following. This would render the astronomers the capacity to measure time to an accuracy of + or - 12 hours.

The sunrise alignment.

Sunrise 21st September 2001 The declination of the centre of the Sun's disc at the time of sunrise on September 21st 2001 was +0 degrees 40 minutes. The ideal declination for the Sun at the Megalithic Calendar intervals nos. 1&9 is +0 deg. 26.4 min.- some 13.6 minutes lower than in the photograph. See Penrhiwen 1 for the original photographs. Here is the digitally corrected position showing how carefully the alignment is arranged to bring the first flash of the Sun up from the bottom of the notch on the correct days.

Sunrise 22nd March 2003. A good set of images of the rising Sun were secured on the morning of the 22nd March 2003. At the time of this sunrise the Sun's declination was very close to the ideal for this calendar date. Sun's declination at 06.00 22nd.March 2003 = + 0 deg. 28.75 mins. Ideal declination for the megalithic equinox = +0 deg. 26.4 mins. The ideal declination had been reached less than 3 hours before the photographs were taken. Again we see how the diameter of the solar disc when it is half risen is clearly framed by a horizon feature, in this case a rocky ledge protruding from the side of the Penrhiwen gorge.

Bracketing techniques for equinoctial alignments. A variety of bracketing methods have been observed for alignments indicating dates around the equinoxes. These may use enclosing foresights which are natural features such as Lechwedd Penrhiwen, or artificial structures such as a linear bank at right angles to the alignment of just the length to delineate the Sun's diameter when hull down. A sample of this style has been found in Llananno.

See html page Brook Cottage.

Twin tumuli set apart may be arranged to mark the left and right curves of the Sun when it sits fully on the horizon as with the alignment S1, Llananno to Gorslydan, Llanbister.

See html page S1, Llananno/Gorslydan.

Another fine example of bracketing of the disc when 'hull down' can be examined in the S2, Llananno to Warren Hill alignment. This instance uses a tumulus for the left bracket and a hill notch for the right.

See html page S2, Llananno/Warren Hill, and Horizon Astronomy 2.

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