  The Picture Rocks Sun Dagger is a spiral petroglyph on a hillside northwest
of Tucson that shows sun dagger events at both the summer solstice and the
equinoxes. On each of these dates, a wedge-shaped sunbeam with opening angles
20{\deg}-30{\deg} touches the center of the spiral, with both of these being
confidently intentionally constructed by peoples of the Hohokam culture c.
800--1300 AD. More generally for claimed sun daggers throughout the American
Southwest, the critical question is whether the ancient indigenous peoples
intentionally placed the petroglyph so as to create a solar marker. The
confident starting point for proving the intentionality of sun daggers in
general is a histogram measured by the Prestons showing highly significant
peaks for indicated declinations within 2{\deg} of -23.4{\deg}, 0.0{\deg}, and
+23.4{\deg}, with this being not by chance. In a review of solsticial and
equinoctal sun daggers, we find that they all have beams of light shaped like a
long-thin triangle with an apex opening angle of <40{\deg} that touches the
center of the petroglyph symbol. The majority of the sun daggers use a spiral
petroglyph, with circles and other symbols being used. Where a spiral or
circular petroglyph has a coincidental light/shadow display, the false alarm
rate is measured to be 20% to 33%. Sun daggers that have indicated declinations
other than +-23.4{\deg} or 0.0{\deg} are false alarms, including claims for
alignments to cross-quarter days and lunar standstills, which are certainly
wrong. Intentional sun daggers are not related to any form of calendric
regulation, astronomical tools, or public ceremony. Rather, abundant
ethnographic evidence shows that sun daggers are a part of sites, called Sun
Shrines, where a local Sun watcher would have lone vigils, with offerings and
prayers to the gods on the solstices and equinoxes.
