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Archaeoastronomy


Equinox Observations at Atlatl Rock


Valley of Fire State Park, NV, USA

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My Visits to Atlatl Rock

  2008-09-12        10 days before autumnal equinox
  2010-09-21        one day before autumnal equinox
  2011-06-24        three days after summer solstice
  2015-09-22/25   around autumnal equinox
  2017-09-17/21   before autumnal equinox
  2018-09-13/16   well before autumnal equinox

As we live in Germany it is not so easy to do observations at the site as often as desired as there are more than 5800 miles in between. Also, the best travel time is in summer and fall and thus only the shadow movement before and at autumnal equinox has been observed in sufficient detail.
Observations of the shadow movement in spring to verify the predicted (inverse) sequence of events would be very useful. We had in mind to plan a visit of Atlatl Rock late March in 2022 to observe the vernal equinox and the days after, but it seems not very probable that the Covid-19 situation makes this possible. However, a visit in 2024 in connection with the total solar eclipse on April 08, 2024 seems probable!


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

Discovery of an alignment of the shadow of a rock with two petroglyphes (markings) that makes it probable that this alignment somehow marks the autumnal equinox.
The markers are four concentric circles (known to be a symbol for the Sun) and an outlined cross.
The shadow cast on the rock bisects the circles and the cross perfectly.

Camera: Canon IXUS 50 (2048 x 1536 pixel; time set to UT);
              no calibration of the timestamp of the camera has been made.

Astronomical autumnal equinox:
      2008-09-22-08:44 am PDT ;   JD  2454732.11389

Picture (left):
      2008-09-12-11:32 am PDT ;   JD  2454722.27222

time of picture to astronomical autumnal equinox:   E - 9d.84

Sun azimuth:   152°.63
Sun elevation:   54°.40

 

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2010, September 21 - second visit of the site to verify the assumed alignment.
(~ one day before astronomical autumnal equinox)
At the time of the visit I had assumed that the upper edge of the shadow would reach the center of the markers at the day of the equinox. As it turned out later (2017 and 2018 visits; see below), the last time this alignment occurs is already a couple of days before the date of the autumnal equinox. Later, like in this observation, the shadow edge no longer reaches the centers of the markers and is always above.
The picture shown was the last one taken with a well defined shadow edge before the whole rock was in self shadow (which was 6 minutes later at about 12:12 pm PDT).

Camera: Canon IXUS 50 (2048 x 1536 pixel; time set to UT);   time calibration (tc): no corretion necessary!

Astronomical autumnal equinox:   2010-09-22-20:09 pm PDT;   JD 2455462.63137

Picture:   2010-09-21-12:06 pm PDT;   JD 2455461.29583
time to/from eqinox:   E - 1d.35 ;
Sun:   azimuth   169°.33   ,   elevation   53°.64




Movie of the observations of the shadow; note that this are close-ups of the region of the petroglyps around the markers not taken from a "standard" position. The markers are always below the shadow line!

Pictures taken from 09:29 am PDT (16:29 UT) till 12:11 pm PDT (19:11 UT);
Sunrise has been observed on September 21, 2010 at 6:29 am PDT;   azimuth   89°.17   ,   elevation   0°.40
 

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

This visit was to check whether there is an alignment of the shadow with some marker(s) to indicate the summer solstice. Nothing obvious was found.
The upper shadow edge was always parallel but far below the connecting line of the two equinox markers (concentric circles and cross) until the whole rock was in shadow.

Camera: Canon IXUS 50 (2048 x 1536 pixel; time set to UT);
             time calibration (tc): no corretion necessary!

Summer solstice:   2011-06-21-10:16 am PDT;   JD 2455734.21944


Picture:   2011-06-24-10:44 am PDT;   JD 2455737.23889
Time from summer solstice:   SS + 3d.02
Sun:   azimuth   109°.30   ,   elevation   61°.70


movie of the shadow
from 05:46 am PDT (12:46 UT) until 10:38 am PDT (17:38 UT);
~ 30 minutes between pictures
Sunrise has been observed on June 24, 2011 at 5:40 am PDT;   azimuth   62.45 deg,   elevation   2.72 deg;
 

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2015, September 22 to 25 - fourth visit of the site
(around astronomical autumnal equinox)

This time we were there four days around autumnal equinox and documented the path of the shadow from sunrise until all petroglyphs were in the shadow around 9 am PDT when the shadow tip had moved beyond the cross.
I errorneously had assumed that at that time the shadow line would connect the centers of the markers to signal the equinox. This turned out to be wrong. Unfortunately the movement of the shadow line was not observed around noon.

It is very probable that like in the observations from 2010 the markers stay always in shadow until the wall is in total self shadow.

Camera: Canon IXUS 255 HS (2816 x 1880 pixel; time set to UT);
             time calibration: eight minutes have to be added to the time recorded in the data of the pictures!
                                       If times have been corrected, they are marked with "tc".

Astronomical autumnal equinox: 2015-09-23-01:21 am PDT ;

last picture:
    2015-09-22-09:08tc am PDT
time to/from eqinox:
    E - 0d.68
Sun:
    azimuth 115.66 deg,
    elevation 30.81 deg;

partly overcast sky;
last picture:
    2015-09-23-08:59tc am PDT
time to/from eqinox:
    E + 0d.31
Sun:
    azimuth 114.28 deg,
    elevation 29.97 deg;

astronomical autumnal equinox
at 01:21 am PDT;
last picture:
    2015-09-24-08:59tc am PDT
time to/from eqinox:
    E + 1d.31
Sun:
    azimuth 114.67 deg,
    elevation 28.77 deg;
last picture:
    2015-09-25-09:00tc am PDT
time to/from eqinox:
    E + 2d.31
Sun:
    azimuth 115.26 deg,
    elevation 28.75 deg;

As an example, the movement of the shadow on the day of the astronomical autumnal equinox on September 23, 2015
from sunrise to 08:59 am PDT is shown in this movie:


Sunrise has been observed:
    - on September 23, 2015 at 6:31tc am PDT,   azimuth   90°.12 ,   elevation   0°.49
    - on September 24, 2015 at 6:32tc am PDT,   azimuth   90°.63 ,   elevation   0°.53
    - on September 25, 2015 at 6:33tc am PDT,   azimuth   91°.14 ,   elevation   0°.56
 

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2017, September 17 to 21 - fifth visit of the site
(observations all five days before astronomical autumnal equinox (September 22, 2017 at 01:02 pm PDT))

After the experience from the first three visits, this time each day pictures were taken about every 15 minutes. The pictures were taken always from the same point on the stairs starting at least at about 10 am PDT. The point on the stairs from where the pictures were taken was choosen in such a way that the cross was directly visible above one of the struts of the platform to be able to check the proper position. By this it was possible to create a movie from each day that showed the movement of the shadow at least in the two hours before the wall with the Atlatl petroglyphs was in total self shadow.

The movies are shown here for each day together with the last picture taken on that day where the shadow edge was clearly defined.

Camera: Canon IXUS 285 HS (5184 x 3888 pixel; time set to UT);
              time calibration (tc): no corretion necessary!



2017-09-17:
5.04 days before astronomical autumnal equinox
3 days before autumnal solar quarter day;


On this first day of the 2017 observations I observed from sunrise until the wall with the pertoglyps was in total shadow. The resulting movie is shown here:
Note the finer time resolution after 12 pm PDT!



last picture with well defined upper shadow edge
12:10 pm PDT;   Sun: Az = 170.11 deg, El = 55.15 deg;


movement of shadow between 10:16 am and 12:13 pm PDT

Sunrise has been observed on September 17, 2017 at 6:27 am PDT;   azimuth   87°.52 ,   elevation   0°.55

2017-09-18:
4.04 days before astronomical autumnal equinox,
2 days before autumnal solar quarter day;


last picture with well defined upper shadow edge
12:02 pm PDT;   Sun: Az = 166.95 deg, El = 54.47 deg;




movement of shadow between 10:16 am and 12:11 pm PDT

2017-09-19:
3.04 days before astronomical autumnal equinox,
1 day before autumnal solar quarter day;


last picture with well defined upper shadow edge
12:08 pm PDT;   Sun: Az = 169.75 deg, El = 54.34 deg;




movement of shadow between 10:14 am and 12:09 pm PDT

2017-09-20:
2.04 days before astronomical autumnal equinox,
autumnal solar quarter day;


last picture with well defined upper shadow edge
12:08 pm PDT;   Sun: Az = 169.99 deg, El = 53.97 deg;




movement of shadow between 10:16 am and 12:08 pm PDT

Sunrise has been observed on September 20, 2017 at 6:29 am PDT;   azimuth   88°.91 ,   elevation   0°.49

2017-09-21:
1.04 days before astronomical autumnal equinox,
1 day after autumnal solar quarter day;


last picture with well defined upper shadow edge
12:05 pm PDT;   Sun: Az = 168.98 deg, El = 53.49 deg;




movement of shadow between 10:16 am and 12:07 pm PDT
 

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2018, September 13 to 16 - sixth visit of the site
(well before the astronomical autumnal equinox (September 22, 2018 at 18:54 pm PDT))

The 2017 observations started on September 17, five days prior to the astronomical autumnal equinox. As it turned out, this was almost too late to observe the shadow reaching the centers of the markes the last time before winter during the primary movement of the shadow shortly before the wall is in total self shadow.
Therefore the 2018 observations started 9 days before the equinox to complete the time span with the four days before the 2017 observations had started. Thus, 9 days before astronomical autumnal equinox are now covered which should it make possible to determine the date when the shadow edge reaches the centers of the markers the last time.
Like during the 2017 observations, each day pictures were taken about every 15 minutes from the same point on the stairs.

Again, the resulting movies are shown here for each day together with the last picture taken on that day where the shadow edge was clearly defined.

Camera: Canon IXUS 285 HS (3648 x 2432 pixel; time set to UT);
              time calibration: one minute has to be subtracted from the time recorded in the data of the pictures!
                                        If times have been corrected, they are marked with "tc".



2018-09-13:
9.29 days before astronomical autumnal equinox,
7 days before autumnal solar quarter day;


last picture with well defined upper shadow edge
11:59tc am PDT;   Sun: Az = 164.14 deg, El = 56.20 deg;




movement of shadow between 10:15 am and 12:16 pm PDT

Sunrise has been observed on September 13, 2018 at 6:23tc am PDT;   azimuth   85°.39 ,   elevation   0°.44

2018-09-14:
8.29 days before astronomical autumnal equinox,
6 days before autumnal solar quarter day;


last picture with well defined upper shadow edge
12:10tc pm PDT;   Sun: Az = 169.29 deg, El = 56.35 deg;




movement of shadow between 10:15 am and 12:16 pm PDT

2018-09-15:
7.29 days before astronomical autumnal equinox,
5 days before autumnal solar quarter day;


last picture with well defined upper shadow edge
12:09tc pm PDT;   Sun: Az = 169.10 deg, El = 55.94 deg;




movement of shadow between 10:15 am and 12:13 pm PDT

Sunrise has been observed on September 15, 2018 at 6:24tc am PDT;   azimuth   86°.41 ,   elevation   0°.51

2018-09-16:
6.29 days before astronomical autumnal equinox,
4 days before autumnal solar quarter day;


last picture with well defined upper shadow edge
12:10tc pm PDT;   Sun: Az = 169.80 deg, El = 55.61 deg;




movement of shadow between 10:14 am and 12:16 pm PDT


tc : Correction of the Clock Drift of the Camera
      The time of my camera always had been set to UT.
      Like almost each clock, this clock has a drift that has to be taken into accout if it is ≥ 1 minute to calculate the proper coordinates
      of the Sun (Az, El).
      If times quoted have been corrected, they are marked with "tc".

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last update 2021-05-01  
by Helmut Steinle