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Archaeoastronomy


Equinox Observations at Atlatl Rock


Valley of Fire State Park, NV, USA

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Summary of the Observations Around Autumnal Equinox                     Zusammenfassung in deutscher Sprache

A slightly modified summary has been published as a printable pdf file (~4 MB, in letter format) and can be downloaded here.
    Location and Description of the Site
    The Markers
    The Sun Around Autumnal Equinox
    Shadow at Atlatl Rock
    What is Marked?
    The Resulting General Picture Over the Year
    Timetable for Future Observations

    Remarks, Open Questions, Thoughts, and Proposed Observations

 Location and Description of the Site

  • Atlatl Rock is a very easily accessible petroglyph site. It is located in the Valley of Fire State Park about 80 miles NE of Las Vegas, Nevada, USA.

  • The petroglyphs of interest are on the left (south) of a straight, smooth rock wall that is almost perfectly facing east. They are at a height of about 20 meters (estimated), easily visible from the ground.
    No climbing on rocks is necessary (one should never do that anyway!) as there is a platform mounted at the height of the petroglyps.

  • The shadow at Atlatl Rock is cast by a rock south of the wall that has an almost perfect straight upper edge with an inclination that is almost parallel to the path of the rising Sun in the morning (March and September) until noon. Thus, once the shadow has reached the wall, a straight upper shadow line is visible that changes its position slowly between following days. There is a movement of the shadow line each day that is described in more detail below.

  • In the vicinity around Atlatl Rock are plenty of other petroglyph panels, some on ground level and others up on the rocks northwest of Atlatl Rock.
    Sometimes the site can be very crowded when a bus from Las Vegas arrives and delivers many interested people.

  • More details on the location are available in section "The Site" of this web site.





view from NNE (parking lot)
about one hour after sunrise in September
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The Markers

  • At the Atlatl Rock site there are two outstanding petroglyphs which are proposed to indicate the time around the equinoxes. They belong to a group of petroglyphs that, among other pictures, show an atlatl that gives the rock its name.

  • The supposed equinox markers are four concentric circles (known to be a sun marker), an outlined cross (proposed here to be an equinox marker at least at this site), and a petroglyph that I call "foot" as I could not find it in any list of named petroglyphs.

      concentric circles         outlined cross         foot
     concentric circles          outlined cross                     "foot"
    estimated diameter          estimated size              estimated size
                30 cm                   50 cm x 50 cm                     15 cm


petroglyps in full sunlight after sunrise
(in summer - for better visibility)
  • The "foot" is the only petroglyph that is above a line connecting the upper edges of the concentric circles and the cross.
    The shadow edge moves across the "foot" seven to two days before (autumnal equinox) or two to seven days after (vernal equinox) when the other markers and petroglyphs are already (autumnal equinox) or still (vernal equinox) in shadow.
    This makes the "foot" the only petroglyph that is in contact with the shadow edge closest to the equinoxes!

  • The alignment and the dimensions of the concentric circles and the cross are such that the shadow edge is parallel to the markers. The shadow edge is tangent to the upper edges of the two markers at the same time. This is true also for the centers and the lower edges of the markers (see section in "analysis").

  • It has to be noted that all petroglyphs (except the "foot" (and part of an atlatl)) are to the left of an imaginary line tangent to the upper edge of the concentric circles and the outlined cross.
Concentric circles like the ones at Atlatl Rock are known to be a symbol for the Sun as described e.g. by Patterson (1992: 66 ff, [B05]) and Zoll (2014: 7 ff; references therein, [B08]).
No clear association of the cross with equal bars like the one at Atlatl Rock as a sign for equinox is found in the literature although it seems obvious to me that this is the case especially here at Atlatl Rock. An outlined cross is mentioned in Patterson 1992: pp. 76 ff, [B05]) but its meaning in the American Southwest is less certain.

A search in the literature for rock art symbols similar to the "foot" revealed no clear identification. There is one picture in Patterson (1992: p. 3, [B05], left picture of footprints) that shows footprints which have a slight similarity to the "foot" but no further information on the purpose/function of this "marker" is available.
In 2008 we visited the Newspaper Rock in San Juan County, UT (about 500 miles NE from Atlatl Rock). At the right upper corner of the rock are some petroglyphs that represent footprints and one of them (the only one that has five toes and not six!) has some similarity with the foot here at Atlatl Rock.

The markers and the interaction with the shadow that indicates certain dates around the equinoxes can be viewed easily from any position in front of the rock. They do not require a special position of the observer and thus can be viewed by many people at the same time.
This is contrary to markers in the landscape or at the horizon that always require the observer to view them from a specific spot.
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The Sun Around Autumnal Equinox

Observing the shadow change around the equinoxes gives a tool to determine the position in time during the year that may be most useful at the change from winter to spring/summer when the planting season starts.
But probably equal important is the change to the fall/winter season which may be the time for harvest celebrations and ceremonies. This is described in detail in the literature at many places.

Around the equinoxes, the daily sunrise point of the Sun shows the most significant changes on the horizon during a year.

The Changing Path of the Sun Around Autumnal Equinox




The azimuth of the point at the horizon where the Sun is visible the first time at sunrise changes significantly by about 0.5 deg per day around the days of the equinoxes. (0.5 deg is almost exactly the apparent diameter of the Moon and the Sun.) This makes the observation of the sunrise point a very good and exact tool to mark certain dates if structures on the horizon or alignments with structures in that direction are available.

The elevation of the Sun around noon also changes significantly by 0.3 deg per day during that period and causes the daily change in the shadow. This is called the primary shadow movement.

Sunrise Observed Around Autumnal Equinox




The location of the sunrise point around autumnal equinox has been determined on eight days (in different years) with different distance to the astronomical autumnal equinox date. The distance in days to the astronomical autumnal equinox is indicated
(negative: before , positive: after).

A summary is the above picture with the distances to the astronomical equinox in days indicated.

More details of the sunrise observations can be found in the corresponding section in "Analysis & Results".
Depending on the location from where the sunrise is observed, the rocks in the foreground (280 meters away) of Atlatl Rock may be used to indicate the location of the sunrise at a desired date.
(I am pretty sure that investigations of the area now under the platform, where old steps cut into the rock are visible, will reveal a special point (perhaps marked like the one mentioned in Zoll (2014:14-15; references therein, [B08]) from which the sunrise position on the horizon is to be viewed using alignment "tools" in the foreground like the rocks.)
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Shadow at Atlatl Rock

There are two reasons that cause the movement of the shadow edge across the Atlatl Rock wall during the year and during a day:

      -   the primary shadow movement is caused by the change in elevation of the Sun between following days

      -   the secondary shadow movement is caused the changing azimuth and elevation of the Sun during one day (part 1) and
           the effect of the change of the slope of the shadow casting rock in the ~ 1.5 hours before noon (part 2).

Primary Shadow Movement :

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

The decreasing elevation of the Sun after the summer solstice causes the upper shadow edge at Atlatl Rock to move upward between each following day. In the first couple of weeks after solstice, the change in elevation is small.
Around equinox, however the change in shadow position has a maximum of about 0.36 deg each day and this causes the upper shadow edge around the autumnal equinox to move upward significantly between following days.

This is shown in a movie that shows the shadow position for each of the nine days before astronomical autumnal equinox at the same time at ~ 10:45 am PDT. In the time span between 9:30 am PDT and 10:50 am PDT each day the shadow edge is very straight and almost constant in position and thus 10:45 am PDT is a good reference point in time for the description of the behavior of the shadow edge before the downward movement of the shadow starts.

Secondary Shadow Movement:
(the downward movement of the shadow edge after 10:50 am PDT)

A detailed definition, description and the reason for final conclusion about the secondary shadow movement is available on the "analysis" page!

All the detailed observations of the secondary shadow movement lead to the conclusion, that the interplay of the shadow edge and the markers is too vague and too uncertain to establish a clear signature.

It therefore has been decided to only consider the primary shadow movement to have a clear connection to the solar quarter days and the equinoxes.
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What is Marked?

During the 12 years since my chance discovery of the alignment of the shadow at Atlatl Rock with the concentric circles and the cross and its connection to the equinoxes, my ideas about the meaning of the timing and the markers have changed a lot.

The first idea was that at the date of the equinox, the shadow line would connect the centers of the concentric rings and the cross. This turned out not to be the case as this happens about 12 days before autumnal equinox (and 12 days after vernal equinox).

The next idea was that the last day during the secondary shadow movement, on which it was possible to observe the shadow to cross the centers of the markers, is the equinox. However, this turned out to be not very well defined and also far off the date of the equinox.

Finally, it turned out that the interaction of the shadow line with the "foot" during the primary shadow movement is the closest event to the equinox. It is even coincident with the solar quarter day!

It seems that the time in spring, when the vernal equinox (or vernal solar quarter day) has been reached, a couple of days (12) later, the crossing of the shadow line across the circles and the cross, bisecting them, may have had a special meaning and may have marked a certain date. As in other cultures, it may have been the date to start planting.
The offset of almost half a month from the equinox/solar quarter day may be due to the geographical location of the site and the climatic circumstances.

There is no doubt, that the shadow at Atlatl Rock and its interplay with the markers has some connection to the equinoxes, but the purpose of the various other alignments remain unexplained so far and are open to educated guesses.
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The Resulting General Picture Over the Year

As explained above, only the primary shadow movement is considered to be of importance as the secondary shadow movement does not provide a clear signature to determine any date.
From the description of the observations in 2017 and 2018 during the nine days before and one day after autumnal equinox the following general picture over the year (starting in summer) can be derived.

Note for the following description:
  • due to the variable dates of the astronomical equinoxes caused by the calendar (e.g.leap years) the date of the equinoxes can be March 19/20/21 and September 21/22/23.
    Also, the given das of the events are good +/- one day as the exact time of the equinox can be at any time between the start of a day and the end.

Summer / Fall

During "summer", i.e. the time between vernal equinox and autumnal equinox, the upper shadow edge cast by the rock south of the Atlatl Rock wall is always below the markers that are proposed to indicate the dates near the equinoxes. Most of the petroglyphs are always in sunlight until the wall is in self shadow each day. Starting already soon after summer solstice, the decreasing elevation of the Sun causes the shadow line to move upward between each following day and more and more petroglyphs are in the shadow zone.

Around the autumnal equinox the interaction of the shadow line and the markers is as follows (equinox at September 22 assumed):

About 18 days prior to the equinox (~Sept. 4th), the shadow line approaching the markers closer each day from below touches for the first time the lower part of the concentric circles and the cross. Between the following days it continues to move upward.
Around 14 days prior to the equinox (~Sept. 8th), the shadow edge connects the centers of the markers. The upward movement of the shadow line on each following day continues until about 10 days prior to the equinox (~Sept. 12th), when the shadow line touches the circles and the cross during the primary shadow movement for the last time.
The only marker then left in sun light above the shadow line is the "foot"!
The shdow line continues to move upward and reaches the "foot" three days later (about 7 days before the equinox) (~Sept. 15th). In the following days it crosses the foot until two days before equinox (~Sept. 20th), when also the "foot" is completely in the shadow zone below the shadow line.

Two days before autumnal equinox is the autumnal solar quarter day!

Winter / Spring

During "winter", i.e. the time between the autumnal equinox and the vernal equinox, the petroglyphs stay in shadow all the time once the shadow tip has reached them shortly after sunrise. This holds until the middle of March (~ March 20th), when the shadow line approaches the markers from above and the observations of the movement of the shadow line before autumnal equinox happen in reverse order (equinox at March 20 assumed):
  - the "foot" is touched the first time (~ March 23rd)
  - shadow line below "foot", but above circles and cross (~ March 28th)
  - shadow line at upper edge of circles and cross (~ March 30th)
  - shadow line connects center of circles and cross (~ April 3rd)
  - shadow line at lower edge of circles and cross (~ April 7th)
  - more and more Atlatl Rock petroglyphs are always in sunlight

Note: Due to the equation of time, local noon and all derived times are different in March and September so that times in March
         are later by almost 10 minutes (see here) as compared to the observed times in September.

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Timetable for Future Observations
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Remarks, Open Questions, Thoughts, and Proposed Observations

  • An intensive search in the internet for pictures of Atlatl Rock with the shadow visible and with information on date and time of the picture taking has been made 2015 (see |X02|).

  • On the wall at and below the height of the platform there are many more petroglyphs including concentric circles. Their significance in marking other events like solstices has not been checked. In addition, the seem to be of a different style and to be of a more recent origin (including modern graffiti).

  • The direction to the sunrise point on the horizon around equinox may be indicated by the large rocks in that direction. It has to be checked if there is a marked position (like e.g. in |B08|:pages 14-15) at Atlatl Rock from where an observation of an alignment is probable.
    Also, if the rocks are used as an alignment tool, there may be marks on the rocks.

  • 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?

  • In November, the shadow on the rock about one hour after sunrise is not a straight line but has several kinks. On one picture published on the internet (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(come) a winter solstice marker in December?

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by Helmut Steinle