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Horizon Astronomy 2

1.2.1   Foresights The upper and lower tips of the discs of Sun and Moon are termed 'limbs' by astronomers. Here are a selection of foresights designed to pin-point one or other limb in a high precision manner. Only alignments of this standard are of use in an operable observatory.

1.2.2

A selection of typical Megalithic foresights which indicate and optimise natural horizon features.

1.2.3      Bracketing foresights.      When the Sun is near the equinox it´s day-to-day movement is very large- almost it´s own diameter in 24 hours. The declination shift during the course of the required day will be large and could lead to confusion if a single point foresight were relied upon. To assist in identifying calendar dates at these times of year a broad foresight such as twin horizon markers, a flat hill notch or a linear bank might be used as foresights. These arrangements are intended to contain or 'bracket' the solar disc whilst standing on the horizon or 'hull down' at semi-diameter. For details of how these foresights might have worked see Calendar 2, 2.2.2 and Study Sites pages:-

Lechwedd Penrhiwen,

Gorslydan,

Warren Hill,

Brook Cottage

These images were secured on the correct day of a Megalithic Equinox or Megalithic Calendar date close to an equinox. Note how the earthworks or horizon feature frames the Sun.

1.2.4      Single point foresights. When the Sun is near the solstices it's daily movement becomes extremely small- finally 0.2 arc minutes in the 24 hours preceding and succeeding the solstice.

The refined use of hill notches allowed the Megalithic astronomers to create high resolution observatories.

1.2.5  The use of hill slopes parallel to a declination track.

The very best foresights employ hill slopes which match closely the rise or set angle of Sun or Moon. Very high resolution foresights were created using the natural intersection of hill flanks They may be indicated by an adjacent cairn which would appear on the disc before it settles into the notch. 1.2.6   These are two accurately extrapolated images derived from a good photograph of moonset at a lunar extreme on the S1, Llananno/Drygarn Fawr lunar alignment.

1.2.7   Drygarn Fawr twin cairns. Here is a superb pair of foresights in Mid Wales giving the upper limb of the Moon at the Southern Major Standstill about 1660 B.C.   The cairn on the left indicates, on the face of the descending Moon, where the hill slope and final notch is. In this notch the last gleam of the setting Moon will disappear when it is at the southerly extreme of it´s orbit. To the right the cairn for the limits of Cyclic Perturbation, a small wobble in the Moon´s path, again indicates an appropriate hill flank and notch.

1.2.8    Prolonging the moonset. By taking particular care in their choice of foresights the ancient astronomers could prolong the setting of the Moon by several minutes. These critical extra minutes allowed the observers the extra time to shift their ground and so ´move´ the Moon precisely into the bottom of the notch. This specific place of observation was then marked by driving a stake into the ground at the observer's feet. This observing practice would continue night after night with a fresh stake driven at each observation until a curve had been described on the ground. Subsequent calculations would be made based upon the curves described by the daily progression of these stake positions.

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