Atlatl Rock Engraving Marks Equinoxes

Valley of Fire State Park, NV, USA

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Atlatl Rock News and Status of the Project

Definition of Terms Used:

time to/from eqinox:
E ± fraction of days
(e.g. E + 1.44 is 1 day and 10 hours and 34 minutes after equinox)

September 12, 2008 - first visit of the site:
(~ 10 days before autumnal equinox)

Picture: 2008-09-12-18:32 UT   (11:32 PDT);   time to/from eqinox:   E - 9d.88

Discovery of an alignment of the shadow of a rock with two petroglyphes (markings) that makes it probable that this alignment marks the autumnal equinox.

Equinox: 2008-09-22-15:44 UT   (08:44 PDT)

September 21, 2010 - second visit of the site to verify the assumed alignment.
(~ one day before autumnal equinox)
Picture: 2010-09-21-16:29 UT   (09:29 PDT);   time to/from eqinox:   E - 1d.35

At that time I had assumed that the alignment would be exact at local noon. This, however turned out to be not true, as the petroglyphes are in total shadow already before noon. Unfortunately no observations of the shadow movement were made in the time span between 06:54 PDT to 09:25 PDT as we were visiting some other parts of the park during that time.
Note added 2016: After the 2015 observations it was obvious that also in 2010 the upper shadow edge never touched the markers and they were always below the upper shadow edge!

Equinox: 2010-09-23-03:09 UT   (2010-09-22-20:09 PDT)

June 24, 2011 - third visit of the site
(summer solstice)

This time we were there shortly after summer solstice and documented the path of the shadow from sunrise until the petroglyphes were in total shadow.
As a first result it is clear that around summer solstice the shadow line never reaches the two marks.
The analysis is still pending, but a nice result is this animation:

© Helmut Steinle
Shadow movement a few days after summer solstice
Camera about 50 meter East of the rock;
pictures taken 2011-06-24; ~ 30 minutes apart; start 05:46 am - end 10:38 am (PDT).

September 22 to 25, 2015 - fourth visit of the site
(at and after autumnal equinox)

This time we were there four days around autumnal equinox and documented the path of the shadow from sunrise until the petroglyphes were in total shadow.

The marker were always totaly in the shadow, when the shadow tip reached them!

  2015-09-22-16:00 UT
  (09:00 PDT)
time to/from eqinox:
  E - 0d.68

partly overcast sky;
  2015-09-23-15:51 UT
  (08:51 PDT)
time to/from eqinox:
  E + 0d.31

autumnal equinox at
08:21 UT (01:21 PDT);
  2015-09-24-15:51 UT
  (08:51 PDT)
time to/from eqinox:
  E + 1d.31
  2015-09-25-15:52 UT
  (08:52 PDT)
time to/from eqinox:
  E + 2d.31
However: which way moves the upper shadow line until noon?
This was not observed, as the observations ended around 9 PDT when the shadow tip had moved beyond the cross. One should have waited until the whole rock face was in shadow (around noon)!

Preparation for a September 2017 Visit before and on Autumnal Equinox (Sept. 17 up to 23)
(autumnal equinox)

The following facts have to be considered:

  • in 2008 the pictures were taken on 2008-09-12 (10 days before autumnal equinox)
    at 11:30 PDT and 11:32 PDT.
    The alignment of the shadow edge and the two markers (concentric circles and cross) were almost perfect.
    No change in the shadow position is seen during this two minutes.
    Autumnal equinox: 2008-09-22-08:44 PDT (2008-09-22-15:44 UT)

  • in 2010 pictures were taken on 2010-09-21 (one day before autumnal equinox).
    The last picture before the shadow line/tip crossed the markers was taken at 06:54 PDT, the next one was taken at 9:25 PDT and the shadow had passed the markers already.
    Between 9:25 PDT and 12:00 PDT the shadow line did not move significantly. Then however, the whole rock was in total shadow very rapidly around 12:20 PDT (see analysis 2010 in German).
    Sunrise: 2010-09-21-06:28 PDT.
    Autumnal equinox: 2010-09-22-20:09 PDT (2010-09-23-03:09 UT)

  • in 2015 the shadow line was already above the markers (rings, cross) when they were reached.
    The observations on the first day (the day before the equinox) were made with many clouds present, but occasionally a shadow was visible.
    Unfortunately, the path of the upper shadow line after 9 PDT was not observed!

Observation plan 2017:

Autumnal Equinox 2017 is on September 22, 20:02 UT (13:02 PDT).
  1. Visit Atlatl Rock and document shadow movement
    - as many days before autumnal equinox 2017 as possible (travel plans however allow only 5 days before autumnal equinox)
    - up to three to four days after equinox.
  2. Determine sunrise point at the first day of observations and then each other day of the observations.
  3. Estimate the "speed" of sunrise point movement between the days.
  4. Take pictures of Atlatl Rock from due East every 10 minutes starting with sunrise, ending when the rock is in total shadow around noon. (After the first day of full observations, a breakfast break may be considered between Sunrise and 9 PDT.)
    The best position for the camera is on the stairs and a special point will be used as in 2015.

First Preliminary Results:

For testing purposes a first movie of the shadow movement on September 17, 2017 has been generated:

Thoughts and open Questions

  • All Atlatl Rock petroglyphes (exept one ) are "below" the equinox shadow line.
  • Is there an "alignment" for summer/winter solstice?
  • Are concentric circles and the type of cross as at Atlatl Rock known to be astronomical markers? E.g. the concentric circles are a symbol for the Sun and the cross with equal bars is a sign for equinox?
  • During summer/winter solstice and during spring/fall equinox: does the sun shine (at sunrise) through the gap in the rocks left of Atlatl Rock and illuminate a mark on the rocks behind Atlatl Rock?
  • At Summer solstice the shadow on Atlatl Rock has a shape that has some similarity with the profile of a face (see picture 6 and 7 in the sequence above).
  • In Fall (at least), the whole Atlatl Rock cast a shadow on the rock wall west of Atlatl Rock that looks like an human head (see picture).
  • In November, the shadow on the rock is not a straight line but has several kinks. On one picture published on the web (see here) it looks as two of the kinks may point to the concentric circles and the cross at a certain time. Can this be a winter solstice marker?
  • Sunrise at autumnal equinox is above the biggest of a group of 3 rocks about 200 m east of Atlatl Rock. Is there a mark on the rock?
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last update 2017-11-13 by   Helmut Steinle