Glossary - The Stones and the Stars: Building Scotland's Newest Megalith - page 307

of the rising or setting Moon, relative to the bearing measured from the
center of Earth.
Planisphere A representation of slightly more than a hemisphere of the
sky on a flat plate, often with a mask that can be rotated to reveal only
the stars visible at particular dates and times. A planisphere shows the
relationships of the stars accurately in declination, but increasingly dis-
torted in right ascension towards the edges, like the polar regions on a
Mercator projection map of the world.
Plough, the A.k.a. the Big Dipper or the Drinking Gourd, in the Unit-
ed States, but depicted as a plough on Mesopotamian cylinder seals; the
seven brightest stars in Ursa Major, and part of an open cluster through
which our Sun is currently passing.
Polaris The brightest star in Ursa Minor, the northern pole star of mod-
ern times, now at its closest to the celestial pole - see precession of the
equinoxes.
Posthole A prepared socket for an upright standing stone or wooden pil-
lar; or simply the hole left behind by an upright post.
Precession of the Equinoxes A 26,000-year wobble of Earth's axis, caused
by the pull of the Sun and Moon upon Earth's equatorial bulge. Although
the current north pole star is Polaris, in 3000 to 2500 b. c. it was Thuban
in Draco, and 13,000 years ago it was Vega in Lyra.
Proper Motion The movement of a star across the celestial sphere, seen
from here, as a component of its true motion in space. The stars in open
clusters such as the Pleiades, Hyades and the Plough, q.v., have shared
proper motion with individual components added.
Radiocarbon Dating A method of determining the age of organic mate-
rial by the decay of the radioactive isotope carbon-14, formed in the upper
atmosphere by cosmic rays and solar radiation. Discrepancies between
C 14 dates and known historic dates were largely removed in the late 1970s
when it was realized that the ratio of carbon-14 to other isotopes in the
atmosphere varies considerably in proportion to solar activity. Correla-
tions with tree rings, ice cores and other indicators have pushed back the
dates for Neolithic Britain to a range starting well before the unification
of Egypt around 3100 b. c., much older than the Minoans and Mycenaens,
who were once thought to have built or inspired the British megaliths.
Recumbent Circles Stone circles found particularly in northeastern
Scotland and in Ireland, where the stones are graded in size away from