West entrance gnomon shadow at Knowth
interesting feature of the passage entrances at Knowth
is that they both had standing stones positioned just outside
the entrance kerbstones. When the site was excavated, both stones
were found in a fallen position, but the western standing stone
was later re-erected.
the evening of maximum penetration of the setting sun into Knowth
West, March 3rd, the standing stone casts a shadow on the
entrance kerbstone. It does this for a number of evenings at certain
times of the year, but it is on the day of maximum penetration
that the shadow appears to line up against the central vertical
line of the entrance kerbstone just as the sun sets.
photographic sequence below shows the actual event in progression.
These photographs were taken on March 5th, two days after maximum
penetration, but they aptly demonstrate how the event happens.
Regrettably, the shadow becomes very faded and obscure by the
time it reaches the vertical line, but naked eye observations
confirmed the shadow was still visible at this critical point.
additional difficulty is the sunlight, which becomes very diffused.
This may be partly due to a large tree which sits in a direct
line with the setting sun - it seemed that the foliage of the
tree was scattering the sun's light, causing the definition of
the shadow to fade.