Geology Reference
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F IG . 2.4 The changing position of Stonehenge midsummer sunrise (After
Peter Lancaster Brown) [ 6 ]
But even precessional change varies slightly with time, because
the tilt of Earth's axis relative to its orbital plane (the obliquity of
the ecliptic) changes gradually. When Stonehenge I was built the
Obliquity was very nearly 24°, but it has now declined to 23° 27 ¢ .
One effect of this is that the Sun now rises over the Heelstone on
Stonehenge Avenue, but in Neolithic times it rose to the left, on
the center line of the avenue (Fig. 2.4 ) .
The Moon's standstill rising and setting positions have altered
similarly. But in addition to that, other forces, such as the pull of
the planets, bring about multiple, small, superimposed changes in
the tilt of Earth's inclination and of the Moon's orbit, all altering
its rising and setting positions on the horizon; and Thom main-
tained that all these had been detected by the Neolithic observers,
and recorded in multiple alignments at complex lunar observato-
ries such as Temple Wood at Kilmartin in Argyllshire.
Astronomers can partly get around the problem of coordinate
change by giving star positions in ecliptic latitude, which remains
nearly constant, and ecliptic longitude, which changes smoothly
 
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