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.
Related links
No links available

S1, Llananno to Two Tumps, Kerry Hill, Dolfor.

High precision alignment indicating the upper limb of the Sun on two megalithic calendar dates-
CIs 4 & 6- one interval before and one interval after the summer solstice CI 5.
Rock cut seat S1 at the centre of Brondre Fawr stone circle, Llananno, North Radnorshire.

S1 to two tumps notch

These two close- set earthen tumuli are still prominent enough to capture the first flash of the rising Sun when at, or near, declination +22° 3.6'

Many observations of sunrise on this alignment have been made at, or near, the appropriate dates for two intervals in the 16 part British Megalithic Calendar CIs 4 & 6. The summer solstice in this ancient calendar would be marked as interval 5. Intervals 4 and 6 mark 22 days before and after the solstice.
Today, in the Gregorian calendar, CI 4 falls on the 1st or 2nd of June and CI 6 on the 11th or 12th of July.

Several sets of good quality photographs and video of the rising Sun on this alignment have been secured which allow high resolution digital surveys to be generated.

Sunrise 12th July 1990.

Here, at sunrise on the 12th July 1990 the declination of the Sun is 22° 0.84' - less than 3 arc minutes beyond the ideal position for these two CIs = 22° 3.6'.
The notch between the tumuli is perfectly arranged to collect the first gleam of the rising Sun at declination +22° 3.6'.
This second image was taken 2 minutes after the first.

02 June 2007 sunrise on Kerry Hill .

The two tumuli are distinct on the upper limb as the first flash had appeared to the left of the tumuli. On the previous day, 1st June, it would have risen to the right of the tumuli.
The required declination, +22° 3.6', for CI 4 had been attained 13 hours prior to this sunrise with the declination +22° 7.9'.
Properly, the sunrise before, at 11 hours distant from the ideal would have been the closest.
From the images secured on 02 June 2007 we may make another detailed survey of the declination of the notch between the tumuli.
Extrapolating backwards to the ideal required declination for CI 4 we see that the track of the upper limb of the rising Sun must first flash in the notch between tumuli at the required declination.
When seperate still images are layered accurate extrapolations of the Sun's position to other declinations may be performed.
The two tumuli lie close to a major ancient trackway leading from the West Midlands into Mid Wales

Sunrise 1st June 2002.

Haze obscured the first flash between the tumuli but subsequent image layering may still establish the true path of the Sun.
At the moment of sunrise the Sun at +22° 01.3' declination is close to the required +22° 3.6' declination for CI 4..

The Sun is 2.4 arc minutes short of required declination.

A high resolution prehistoric solar alignment

Prior to 35mm film photographic studies of this alignment several naked eye observations had been made. It had been noted that the first flash of the Sun did not always sit in the notch but may rise on the skirts of the tumuli. When precise declinations were read from USNO ephemera it was seen that these movements were directly related to the difference in time between the moment of sunrise and the instant when the Sun passed through the required declination. When conditions were open enough to resolve the solar disc distinctly then reliable establishment of the correct dates of the calendar intervals, either CI 4 or 6, could be made without modern equipment.

Astronomical refraction

Across the range of observations little variation in the declination of the notch due to astronomical refraction was found. The successful modern observations were made in clear open weather and at the same times of day, seasons and year as the original establishment of the alignment and should therefore offer close matches with conditions of air temperature and humidity pertaining at the ancient observations.

Terrestrial refraction

After several years of controlled theodolite observations Thom concluded that most errors due to refraction variations could be introduced to observations when an alignment ran close to high ground between foresight and backsight. Here with S1 to two tumps we have the ideal situation with the intermediate ground dropping entirely to the River Ithon head water avoiding any incidence of terrestrial refraction.