EG2007 Cultural Heritage Papers
=================================

Supplement movies for

Georg Zotti: Tangible Heritage: Production of Astrolabes on a Laser Engraver
----------------------------------------------------------------------------

These 2 movies demonstrate the flexibility of the procedural modelling
approach for the astrolabe parts. Please bear in mind that the target
application of the graphics is print, and the MPG compression blurs
some fine lines.

1) Rete-over-centuries.mpg

The star pointers change due to the change of the star positions
caused by precession.  The curves are PostScript spline curves where
the control points are subject to the half value of precession. At
some points in time, some star pointers would collide with the carrier
struts and must be hidden. Elsewhere, the control points must be
changed to avoid bad curves. 


2) Tympan-fromPoleToPole.mpg

The horizon plate, or Tympan, of a Northern astrolabe can be produced
for all latitudes between the poles. The light area is the sky above
the horizon, below are the 3 twilight zones as variant of the
classical crepuscular line. (civil/nautical/astronomical; when the sun
is below the astronomical twilight zone, we have dark night.)

The zones of the sky partitioned by thin dotted lines and labeled by
Roman numbers are the classical astrological houses.  The green dotted
lines at lower left are the Islamic afternoon prayer lines (both not
described in the paper).

For latitudes near the poles, where the sun is above/below the horizon
near the solstices, the lines for hours from sunrise/hours from sunset
(arcs that are rotated copies of the horizon, labeled red/blue)
degenerate to radial lines, when the hours may be counted from
midnight. (Else, they should be omitted as meaningless. Classical
instruments may show the 90-degree tympans which have no hour lines,
but typically have no tympans for latitudes near the poles.)

The Unequal Hour lines (arcs labeled 1..12 on the outermost border)
are shown in the classical approximation (red) and correct computation
(blue). Only for latitudes greater 50 degrees the difference is
notable, but for latitudes polewards of the polar circles, the
classical 3-point construction is not applicable.