Download Henges, Heel Stones, and Analemmas

Published by the Applied Probability Trust
© Applied Probability Trust 2008
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Henges, Heel Stones,
and Analemmas
JOHN D. MAHONY
Introduction
There are various henges in existence around the world, some old – for example Stonehenge,
UK – and some new – for example Stonehenge Aotearoa, New Zealand. An understanding of
heel stones and their positions around a henge may be appreciated from figure 1. In figure 1,
six heel stones are shown distributed around a henge centre to mark the apparent positions of
sunrise/sunset (as seen from the centre of the henge) at the equinoxes and the solstices. The
solstice positions mark the extremes of the Sun’s apparent motion during a year and they are
usually established by measurement with reference to distant star sets. The heel stone axis
thus appears to swing through an angle 2max during the course of one year. It is a simple
matter to measure the magnitude of the swing but, since the phenomenon involves primarily
only the Sun and the Earth, it should be possible to obtain a simple formula that will determine
an approximate value for at any point of the Earth’s orbit, not just at the solstices where it
is a maximum.
Results from the formula should be in reasonable agreement with known facts.
1. At Stonehenge (UK, Northerly latitude circa 51.2◦ ) the apparent magnitude of the swing
at the solstices is about 80◦ (see reference 1, pp. 109–110).
N
Winter solstice sunset − SH
Summer solstice sunset − NH
Equinox sunset
Winter solstice sunrise − SH
Summer solstice sunrise − NH
max
W
Summer solstice sunset − SH
Winter solstice sunset − NH
E
Equinox sunrise
Summer solstice sunrise − SH
Winter solstice sunrise − NH
S
Figure 1 Henge showing six heel stones on Earth that mark the apparent position of sunrise/sunset at the
solstices and equinoxes with reference to an east/west axis. The stars denote heel stones; ‘NH’ and ‘SH’
denote ‘northern hemisphere’ and ‘southern hemisphere’ respectively.