Geology Reference
In-Depth Information
henge III consisted of a horseshoe of trilithons on the midsummer sun-
rise/midwinter sunset axis.
Tropics
The band of latitudes on Earth within which the Sun passes
overhead in the course of the year. In each epoch its extent, north and
south, equals the value of Earth's axial tilt (the obliquity of Ecliptic) and
the radius of the Arctic and Antarctic Circles. The Sun is overhead at
the Tropic of Cancer on midsummer's day in the northern hemisphere,
around June 21, and at the Tropic of Capricorn on northern midwinter's
day (midsummer's day in the southern hemisphere), around December 21.
Vernal Equinox
The date at which the Sun crosses the celestial equator
moving northward, in the direction of increasing right ascension, around
March 21 each year; by extension, the position of the Sun on that date,
currently located in the constellation Pisces but confusingly called 'the
First Point of Aries' for historical reasons dating back to ancient Greece.
Whinstone
A variety of basalt quarried at Kilsyth and at Hillhouse
Quarry, Troon.
Wiltshire Lozenge
A bronze plaque, thought to have been worn as a
breastplate, found at Bush Barrow immediately south of Stonehenge in
1808; interpreted by Archie Thom as an astronomer's template, but al-
legedly flattened and polished in a 'restoration' that removed the crucial
markings.
Zenith
The point on the celestial sphere directly above the observer, at
an altitude of 90°.
Zodiac
The band of constellations, centered on the ecliptic, within
which the Moon and the planets normally move; the only ones that can
pass outside it are Mercury and Pluto (which is no longer officially a
planet).
Zodiacal Light
A diffuse band of light from the Sun scattered around
the plane of the ecliptic by interplanetary dust, linking up with the Ge-
genschein opposite to the Sun, from the observer's viewpoint. Zodiacal
light is often depicted by artists as twin cones to either side of the Sun,
but actually it is only visible before sunrise and after sunset, particu-
larly in the tropics. Until the mid-1970s it wasn't known whether the ef-
fect was local to the Earth-Moon system, but the
Pioneer 10
space probe
found that it persisted out to the orbit of Jupiter, showing the dust comes
from the Asteroid Belt and from comets passing through the inner Solar
System.