Glossary X
At a lunar backsight where permanent or temporary ancillary extrapolation gear has been set up there should be a length preserved between major stones which is the 4G for this site. This 4G represents, on the ground, the lateral distance an observer must move in order to 'shift' the Moon through the declination range K. K is the change in the Moon´s position during the 24 hours preceding a standstill. At a Major Standstill it is 46.5 arc minutes and at a Minor Standstill 30 arc minutes. See also- K, ANGULAR DISPLACEMENT, EXTRAPOLATION PROCEDURE, LUNAR STANDSTILL.

S1, Llananno to Gors Lydan tumuli,
Llananno

High precision alignment indicating the fully risen disc of the Sun on two megalithic calendar intermediate dates- CIs 10 16 which occur in the Gregorian calendar @ 27th February @ 15th October.
When the exact declination of Sun is -8° 27.6' the lower limb will sit on the mid point between the two tumuli and the disc will be framed.
Gorslydan_Survey
This foresight has been engineered to frame the left and right limbs of the rising Sun when close to the divisions for megalithic calendar CIs 10 & 16.
This image has been digitally extrapolated from a series of photographs taken at or near CIs 10 16. See below.
S1 to Gors Lydan is one of the most sensitive observatories in Mid Wales Ideal declinations are rarely reached at the moment of observation - the rise or set. At the times of year when this alignment would be employed the daily change in declination is almost equivalent to the solar disc itself. The arrangement here can offer optical callibration of the slight differences from ideal ensuring the correct day has been established.
The Sun's rate of daily change at these times of year is 22 arc minutes hence an observatory must have a working resolution of better than + and - 10 minutes. The apparent diameter of the solar disc is 31.6 minutes. Here on Gors Lydan with the careful framing by the tumuli, reliable optical estimates of 1/3rd and 1/2 of the disc may be made.
Yet more refined observations might be carried out by bringing one or other limb in line with a tumulus by stepping aside at the observation and setting stakes.

S1 stone circle Brondre Fawr to Gors Lydan tumuli, Crown Estates, Llananno.

Gorslydan

S1 to Gors Lydan OS.

Gors_Lydan_OS
As with all the tumuli groups in Llananno the mounds are arranged on the exact summit of the hill establishing the relationship of the megalithic calendar dates to this hill top.


Sunrise from S1 with declination = -8° 31.9'

S1_to_Gorslydan_15th_Oct_1989
The Sun here is within 4.3' of the ideal declination for CIs 10 & 16.

Sunrise from S2 with declination = -8° 32.7'

S2Gorslydan27thFeb08
The Sun here is within 5.1' of the ideal declination for CIs 10 & 16.
These are stills from a video shot from S2 at sunrise of the 27th February 2008. Cloud obscured reliable observation.

Sunrise from S1 with declination = -8° 51.6'

S1_to_Gorslydan_26th_Feb_95
The Sun here is within 24.0' of the ideal declination for CIs 10 & 16.
The Sun is 24 hours short of the required declination. This deficiency is readibly observed. The following sunrise the northern, left, tumulus will sit at the centre of the disc and the lower limb will stand on the mid-point between tumuli.

Sunrise from S1 with declination = -8° 33.0'

S1_to_Gorslydan_15th_Oct_1997
The Sun here is within 5.4' of the ideal declination for CIs 10 & 16.