Archaeoastronomy in the British Isles - The Stones and the Stars: Building Scotland's Newest Megalith - page 62

time with its phases, rising north of east for half of the month and
south of east for the remainder. But, whereas the Sun comes back
to the same most northerly and most southerly points year after
year, only next month the Moon either passes or fails to reach the
north and south markers set out for it last time.
To the speeded-up observer at the center of the transparent
Earth it is obvious that the Moon's orbital plane is slipping back-
ward, pulled by the Sun and also, although not obviously, by Earth's
equatorial bulge. The outside observer would find it much harder
to work out what was happening, but in due course he would have
a full record of the results. Once every 18.61 years the Moon rises
and sets at the most northerly point on the horizon it can reach,
and, 14 days later, at its furthest south. It rides so high, and then
so low, because at the extremes its position north or south of the
equator is the sum of the tilt of the Moon's orbit and the tilt of
Earth's axis, both measured from Earth's orbital plane. For the
next 4.65 years, as month follows month, the most northerly and
southerly risings draw together until they meet at due east.
Then they move out again, but only to a lesser maximum
which is defined by the difference between the Moon's orbital
plane and Earth's axial tilt. Then it's back to due east again, before
drawing back out to the most extreme northerly and southerly
positions to complete the 18.61-year cycle.
For a while, when the Moon reaches its maximum and sec-
ondary maximum positions, the change from month to month will
be too slight to be noticeable to the naked eye. For that reason,
referring to the appearance of the phenomena to ancient observ-
ers, Alexander Thom designated these events as the 'major and
minor standstills,' and those terms will be used for the rest of this
book. The major standstill, when the Moon is at furthest north
and furthest south a fortnight later, recurs every 18.61 years, with
the minor standstill 9.3 years later at the midpoint of the cycle.
The observer on the ground finds that the markers for solar ris-
ing and setting - furthest north at midsummer, furthest south at
midwinter - are each flanked by the lunar markers for major and
minor standstills. Just to find that out, confirm it, and place the
markers with recognizable accuracy, is a considerable triumph for
perseverance in an age when the average life expectancy was little
more than 30 years. And that brings us to the first point in reply to