Geology Reference
In-Depth Information
positional horizon observations were all the Neolithic builders could
do. The plane of Earth's orbit around the Sun is the ecliptic, the cen-
ter line of the zodiac; as the Sun moves along it over the course of the
year, its horizon position varies from its most southerly midwinter
rise and set, when it's overhead at the Tropic of Capricorn, to its most
northerly midsummer rise and set, when it's overhead at the Tropic
of Cancer. It's universally agreed that the Stonehenge Avenue and the
later structure both mark the midsummer sunrise.
The plane of the Moon's orbit is inclined to the ecliptic by
approximately 5°. Under the pulls of the Sun and Earth's equatorial
bulge, the Moon's orbital plane swings around in the sky, against
the background of stars, with a period of 18.61 years (regression
of the lunar nodes). When the Moon reaches its highest possible
position above the ecliptic, it rises and sets at its extreme north-
erly positions, and at its extreme southerly positions 14 days later.
These events are termed the Major Standstill. As we've seen, few
archaeologists agree with Gerald Hawkins that the 'station stones'
of Stonehenge I mark the extreme northern and southern posi-
tions of the Moon's 18.61 year cycle; and still fewer with Alexan-
der Thom, that the megalith builders had a sophisticated program
of lunar observatories, spread over the British Isles.
Just about 9.3 years after the Major Standstill, the Moon comes
to an intermediate position called the Minor Standstill, flanking the
solstice risings on the other side. This was easiest to illustrate using
the planetarium at Glasgow Science Centre. A friend of the author's,
Chris O'Kane, stood at the midsummer sunrise position on the edge of
the dome, while we advanced the projector setting by 6 months, and I
then stood at midwinter sunrise position. Then the operator brought
the Moon to the Major Standstill northerly rise, on Chris's right; and
we advanced 14 days to the southerly rise, on my left. Then, advanc-
ing 9.3 years, we repeated the exercise for the Minor Standstill, with
the northerly moonrise now to Chris's left and my right. There was
no time to practice beforehand, and it was a great relief to see the
Moon rise and set at those positions.
When it comes to star alignments, the position is more com-
plex. Because Earth's equatorial plane doesn't coincide with the
ecliptic, nor with the orbit of the Moon, the combined pulls of
the Sun and Moon on Earth's equatorial bulge cause Earth's axis to
'wobble' around the ecliptic pole with a period of 26,000 years
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