Photos of the Sun Dagger at various times can be seen here:
http://www.cpluhna.nau.edu/People/sw_archaeoastronomy.htm
The following page gives a concise description of the Sun Dagger in relation to Sun and Moon.
http://paganastronomy.net/nahist.htm
At Chaco Canyon, we find the most famous sun clock known. Discovered in 1977, the site includes two petroglyph carvings of spirals. The larger spiral has 18.6 grooves, representing the 18.6 year lunar standstill. At the summer solstice, sunlight causes a shaft or "sun dagger" of light to fall onto the large petroglyph spiral on an inner rock wall. At the winter solstice, two daggers appear on the spiral [46].
Summer solstice: Single dagger appears through center of large spiral
Winter solstice: Two daggers bracket the large spiral
Equinoxes: Single dagger appears through center of small spiral
Lunar northern extreme: Moonlight shadow falls on first groove on large spiral
Lunar southern extreme: Moonlight shadow falls on center groove on large spiral
I haven't understood this yet. Can somebody help me?
1) How deep are the openings through which the light shines? Are they more like a narrow window or more like a separation between two slabs? Does light pass through for just a few minutes a day or for many hours.
2) Are the observations reported made at sun/moon-rise or at "noon-day"? (http://www.colorado.edu/Conferences/chaco/tour/fajada.htm refers to "noonday", "midday", and "moonrise".) Does the light enter through the same slit for all the observations?
These are my main questions. Depending on the answers, I have other questions about how this is supposed to work. In addition, I have a couple secondary questions:
3) What process caused a "settling" between 1977 and now that spoiled the alignment, but which was apparently not active the 7 centuries before that?
4) How can the endpoints (especially the inner endpoint) of a spiral in rough stone be determined to be 18.6 revolutions, as opposed to, say, "about 18 and a half"?
Thanks,
Art Carlson
