Archaeoastronomy

Equinox Observations at Atlatl Rock

Valley of Fire State Park, NV, USA

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Slope Change -
the Reason for the Downward Movement of the Shadow Before Noon on Each Day
(secondary shadow movement)

(The pictures are from differen years and have been selected to show the described observations best.)

The rock south of the Atlatl Rock wall that casts the shadow onto the wall where the petroglyps with the atlatl are has an almost straight upper edge which casts a shadow with an straight upper edge onto the wall.

The rock edge is almost parallel to the path of the rising Sun after about 9:30 am PDT and thus the shadow does not change very much until 10:50 am PDT. At that time, the shadow casting edge changes the slope a little bit and the Sun on its apparent path starts to move a little more above the egde causing the shadow edge to move downward after that time.

shadow casting rock as seen
from the platform near the markers

The movement of the shadow on September 15, 2018 (the above example) is shown in this movie that has been assembled from the single pictures taken on that day:

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The only picture where the shadows of the kink and the tower are visible

The 2010 observations are special due to the fact that the last picture taken at 12:06 pm PDT before the wall was in total self shadow is the only picture that shows the kink between the shadowedge and the "tower" (the part of Atlatl Rock that stands upright next (west) to the shadow casting flat edge).

Picture: 2010-09-21-12:06 pm PDT; JD 2455461.29583
time to/from eqinox: E - 1^d.35 ;
Sun: azimuth 169.33 deg , elevation 53.64 deg