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Atlatl Rock Engraving Marks Equinoxes

Valley of Fire State Park, NV, USA

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Atlatl Rock Analysis 2008

Fortunately, I always have set the time of my camera to UT (± ~5 minutes) so that determining the sun position during the exposures was possible.
almanac 2008
Almanac for 2008 from
archeoastronomy.com

Location

Atlatl Rock coordinates from Google Earth:
      36°25' 25.39" N;     114°33' 03.23" W;       elevation: 686 m (2250 ft);
      decimal: latitude is 36.4275°,   longitude is 114.5509 ° W;

Atlatl Rock coordinates from Lat-Long.com:
      36°25' 24" N;     114°33' 02" W;       elevation: 681 m (2234 ft);
      decimal: latitude is 36.4233°,   longitude is 114.5506° W;

Alatl Rock in Google Maps
(Atlatl Rock is at the very center of the map.)

Pictures

Pictures taken at
    2008-09-12-18:30 UT (11:30 PDT; view from parking lot) and
    2008-09-12-18:32 UT (11:32 PDT; at the engravings)

Time

Pacific daylight saving time (PDT) = UT - 7h;

Vernal* Equinox: 2008-03-20-05:48 UT    2008-09-19-22:48 PDT (Nevada)
Summer Solstice: 2008-06-20-23:59 UT    2008-06-20-16:59 PDT (Nevada)
Autumnal Equinox: 2008-09-22-15:44 UT    2008-09-22-08:44 PDT (Nevada)
Winter* Solstice: 2008-12-21-12:04 UT    2008-12-21-04:04 PST (Nevada)

*: daylight saving time 2008 started March 9 and ended November 2
    ( U.S. Naval Observatory (USNO): daylight time).

Analysis of Observation

Time of observation (taking close up pictures): 2008-09-12-18:32 UT

Using USNO software:
Local noon on 2008-09-12 was at 19:34 UT (see USNO printout), which is 12:34 PDT.
Elevation of the sun at noon (meridian transit) was ~58°. (see USNO printout)
Local time (apparent solar time) at 2008-09-12-18:32 UT was at ~ 10h58min


Using "Physics and Astronomy" tools:
(Time zone set to UT-7 as PDT is the "standard time")

Day of Vernal Equinox:
At 2008-03-20-19:45 UT, which is 12:45 PDT (local noon):
    - local time (apparent solar time) was 12:00h
    - sun azimuth (N->E->S->W) was 179.8° (i.e. on meridian)
    - sun elevation was 53.8°.

Day of Autumnal Equinox:
At 2008-09-22-19:31 UT, which is 12:31 PDT (local noon):
    - local time (apparent solar time) was 12:00h
    - sun azimuth (N->E->S->W) was 180.1° (i.e. on meridian)
    - sun elevation was 53.51°. Autumnal Equinox:

My observations close to Autumnal Equinox:
At 2008-09-12-18:32 UT, which is 11:32 PDT (pictures taken):
    - local time (apparent solar time) was 10h58min
    - sun azimuth (N->E->S->W) was 152.6° (i.e. east of meridian)
    - sun elevation was 54.39°
At 2008-09-12-19:34 UT, which is 12:34 PDT (local noon):
    - local time (apparent solar time) was 12:00h
    - sun azimuth (N->E->S->W) was 179.9° (i.e. on meridian)
    - sun elevation was 57.38°.

At 2008-09-22-19:31 UT, which is 12:31 PDT (local noon):
    - local time (apparent solar time) was 12:00h
    - sun azimuth (N->E->S->W) was 180.1° (i.e. on meridian)
    - sun elevation was 53.51°.

Summary

During my observation on September 12, 11:32 PDT the Sun altitude was almost exacty the same as on September 22, the date of the Autumnal Equinox: 54.39° vs. 53.51°. As the azimuth of the Sun was quite different (152.5° vs. 180.0°), the rock that casts the shadow must be aligned in such a way, that only the height of the sun, but not the azimuth is critical. Maybe there is a marker that signals the Sun is in due south?

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last update 2013-09-18 by   Helmut Steinle