APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Comments and questions about the APOD on the main view screen.
User avatar
APOD Robot
Otto Posterman
Posts: 4076
Joined: Fri Dec 04, 2009 3:27 am

APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by APOD Robot » Wed Aug 05, 2020 4:05 am

Image Picture Rocks Sun Dagger

Explanation: Ancient sun daggers will not hurt you, but they may tell you the time. A sun dagger is a dagger-shaped gap in a shadow created by sunlight streaming through a crevice in a nearby rock. Starting over a thousand year ago, native people of the American southwest carved spiral petroglyphs into rocks that became illuminated by sun daggers in different ways as the Sun shifts in the sky. A type of sundial, where the end of the sundagger points in the spiral at high noon (for example) indicates a time of year, possibly illuminating a solstice or equinox. Sun daggers are thought to have been used by Sun Priests during lone vigils with prayers and offerings. Of the few known, the featured video discusses the historic Picture Rocks Sun Dagger near Tucson, Arizona, USA, likely created by a Hohokam Sun Priest around 1000 AD.

<< Previous APOD This Day in APOD Next APOD >>

User avatar
JohnD
Tea Time, Guv! Cheerio!
Posts: 1281
Joined: Wed Feb 16, 2005 2:11 pm
Location: Lancaster, England

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by JohnD » Wed Aug 05, 2020 9:30 am

As usual, when an archeologist uncovers an object of unknown function, it is labelled a "Ritual object", and presumed used for worship or date calculation. The video of this APOD shows that the "Sun Dagger" (!) is not centered on the spiral, at any time of day, and that at High Noon, the point is well away from the petroglyph. It appears to be randomly associated with the rock face, so it is a wild, unsupported hypothesis that it would have been used by prehistoric "Sun Priests"!

When archeologists really work on an object, it's true, and far more practical use may be found. Pierced bones with radial marks around the holes have been found in Europe and the UK, previously assumed to represent the Sun and to be 'sceptres' or the hackneyed 'ritual object'. Recently, it was shown by experimental archeology that these are tools for rope making! Engineering started, not in the 19th Century, but 40,000 years ago!

See: http://www.sci-news.com/archaeology/rop ... of%20Liege.

User avatar
orin stepanek
Plutopian
Posts: 5766
Joined: Wed Jul 27, 2005 3:41 pm
Location: Nebraska

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by orin stepanek » Wed Aug 05, 2020 11:29 am

Mt thought that when the spiral was made that maybe it was made around the tip of the sun dagger! I don't feel there was anything spiritual about it though! :? There is however; an arc of dots in the outer spiral, that the tip of the sun dagger does indeed touch! :wink:
Orin

Smile today; tomorrow's another day!

User avatar
Chris Peterson
Abominable Snowman
Posts: 15321
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by Chris Peterson » Wed Aug 05, 2020 1:18 pm

JohnD wrote:
Wed Aug 05, 2020 9:30 am
As usual, when an archeologist uncovers an object of unknown function, it is labelled a "Ritual object", and presumed used for worship or date calculation. The video of this APOD shows that the "Sun Dagger" (!) is not centered on the spiral, at any time of day, and that at High Noon, the point is well away from the petroglyph. It appears to be randomly associated with the rock face, so it is a wild, unsupported hypothesis that it would have been used by prehistoric "Sun Priests"!

When archeologists really work on an object, it's true, and far more practical use may be found. Pierced bones with radial marks around the holes have been found in Europe and the UK, previously assumed to represent the Sun and to be 'sceptres' or the hackneyed 'ritual object'. Recently, it was shown by experimental archeology that these are tools for rope making! Engineering started, not in the 19th Century, but 40,000 years ago!

See: http://www.sci-news.com/archaeology/rop ... of%20Liege.
It is wise to maintain a high degree of skepticism regarding all manner of ancient astronomical alignments, from Stonehenge right up to more recent things like these Indian petroglyphs. This is a culture that left no records or oral history. And, as you say, alignments can easily be coincidental.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

De58te
Science Officer
Posts: 320
Joined: Mon Sep 30, 2013 6:35 pm

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by De58te » Wed Aug 05, 2020 1:27 pm

Cool, what's even more interesting, and if anybody else noticed, there seems to be a white billy-goat in the shadow to the left, sticking its tongue out at the sun dagger. Strange that the goat disappears when the shadow is replaced by the sunlight.

NCTom

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by NCTom » Wed Aug 05, 2020 2:12 pm

The link to Picture Rocks Sun Dagger brings up an extensive article on this archaeological site. Unfortunately the APOD does not tell us what time of year the film was made. Apparently on the solstices the dagger does more approximately pass through the center of the spiral. Also might there be any significance to the object above the spiral toward which the dagger points on this particular day? Nothing is said about that object. That goat on the left is just plain rude!

User avatar
Ann
4725 Å
Posts: 10820
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by Ann » Wed Aug 05, 2020 2:15 pm

JohnD wrote:
Wed Aug 05, 2020 9:30 am
As usual, when an archeologist uncovers an object of unknown function, it is labelled a "Ritual object", and presumed used for worship or date calculation. The video of this APOD shows that the "Sun Dagger" (!) is not centered on the spiral, at any time of day, and that at High Noon, the point is well away from the petroglyph. It appears to be randomly associated with the rock face, so it is a wild, unsupported hypothesis that it would have been used by prehistoric "Sun Priests"!

When archeologists really work on an object, it's true, and far more practical use may be found. Pierced bones with radial marks around the holes have been found in Europe and the UK, previously assumed to represent the Sun and to be 'sceptres' or the hackneyed 'ritual object'. Recently, it was shown by experimental archeology that these are tools for rope making! Engineering started, not in the 19th Century, but 40,000 years ago!

See: http://www.sci-news.com/archaeology/rop ... of%20Liege.
Very good point, John!
Meanwhile, I can't help wondering about this little tease:
APOD Robot wrote:

Tomorrow's picture: 20 and 21
Is it possible? Could "20" be M20 and "21" be M21? In full RGB color, too?

Naaah. That's too much to ask for. :wink:

Ann
Color Commentator

Brad Schaefer

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by Brad Schaefer » Wed Aug 05, 2020 4:16 pm

Two of the people have expressed skepticism about the sun dagger, its 'ritual' use, and the 'priests'. Such skepticism is much-too-often warranted with much of the archaeoastronomy reported in the press and on the internet. But not in this case. For the intentionality of sun daggers, we have the large number (~100) of sunbeam/spiral connections that all operate the same way and that all operate only at the solstices and equinoxes. Such a pointing at specific predefined declinations in the sky is essentially impossible by chance, and the only reasonable way to do this is by the knowing intention of the petroglyph makers. And we have many, detailed, and early ethnographic reports (some by eye witnesses even in the 1880s) of Sun Priests going out on both solstices to Sun Shrines performing what can only be called rituals of planting rows of feathers, making offerings (often of corn meal), and chanting prayers (usually for a good harvest). These are known for many Pueblo tribes, direct descendants of the people who made the Fajada Butte Sun Dagger, all with very conservative religious practices, while such Sun Priests are also known for most groups within California and surrounding areas, all with many sun daggers. So, yes, we do have strong evidence that the sun daggers are a part of Sun Shrines and used for what we should call 'rituals' by 'priests'. The perceived-necessity for the point of the wedge to touch the spiral center is just a modern ideal. But the reality for the many operational sun daggers is only that the wedge-shaped sunbeam touches close to the center spiral, sometimes with the base doing the touching, sometimes the wedge center, and so on. This is showing us what the old Southwestern cultures actually valued, as opposed to our modern guesses. For the Picture Rocks Sun Dagger, near the end of the video, you can see that the center of the spiral does pass inside the middle of the wedge. So all is fine.

Skepticism is a good tool, especially for archaeoastronomy, where the majority of the claims in the press and internet are for known wrong anachronisms. Quickly popping to mind for the US Southwest are the Chaco Canyon (Penasco Blanco) "Crab Supernova" pictogram (a proven sun-watching station, not a depiction of any supernova), the lunar standstill alignments at many places (including the Fajada Butte Sun Dagger), claimed 'Cross Quarter Day' alignments for a number of sun daggers, and various alignments at Casa Rinconada. Unfortunately, the field is popularly known mainly for certainly-wrong spectacular claims, while the real and fun results are mostly known only to researchers. For the case of sun daggers in general, with their initial discovery in 1979, the evidence was too weak to convince anyone other than popular writers. This all changed dramatically in 1996, at the V Oxford Conference (the big international quadrennial meeting for archaeoastronomy). All in one session, three groups of researchers got up one after another and showed their results from their three massive surveys of sun daggers, with little overlap, all throughout the Southwest. They reported large numbers of sun daggers, and all operating in the same way. They proved sun daggers to be *intentional* constructs, and they proved how the sun daggers operated, and they proved that sun daggers were universally used by cultures from the Baja to Colorado, and from northern California to central Texas, all for times c.500-1300 AD. I was skeptical of sun daggers going in to the Oxford Conference, but I was completely converted by the strong evidences to knowing that sun daggers are real, and I even said so in the published conference summary. Around about that time, many workers (notably Zeilik and McCluskey) collected the ethnographic evidence supporting the details of the usages as a Sun Shrine, and worked out the details of Hohokam and Ancestral Pueblo astronomies. All of this is given in great detail with citations in the paper linked by today's APOD. So, yes, skepticism is good, but for the case of sun daggers, we can be very confident that they are real, intentional, and operate as a wedge of sunbeam (with apex angle of 3°-40°) that somehow covers the center of spirals on either the solstices or equinoxes, and that these were used by people we would call 'priests' for 'rituals'.

User avatar
Chris Peterson
Abominable Snowman
Posts: 15321
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by Chris Peterson » Wed Aug 05, 2020 4:22 pm

Brad Schaefer wrote:
Wed Aug 05, 2020 4:16 pm
Two of the people have expressed skepticism about the sun dagger, its 'ritual' use, and the 'priests'...
To be clear, I was making no comment one way or the other with respect to this particular case, only to the broader view that we know almost nothing about Indian culture (or many other ancient cultures) and it is very common to see archaeoastronomical claims made with far more confidence than is reasonably justified.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
JohnD
Tea Time, Guv! Cheerio!
Posts: 1281
Joined: Wed Feb 16, 2005 2:11 pm
Location: Lancaster, England

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by JohnD » Wed Aug 05, 2020 4:57 pm

Brad,
Thank you for your detailed answer, and polite acceptance of our cynicism. When YOU are the author of a paper cited in the blurb, many might have been offended! I'm sorry that I didn't click on it in the first place. I shall look forward to reading it, and to my further education.

John

User avatar
neufer
Vacationer at Tralfamadore
Posts: 17521
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by neufer » Wed Aug 05, 2020 8:16 pm

Brad Schaefer wrote:
Wed Aug 05, 2020 4:16 pm

Two of the people have expressed skepticism about the sun dagger, its 'ritual' use, and the 'priests'. Such skepticism is much-too-often warranted with much of the archaeoastronomy reported in the press and on the internet. But not in this case. For the intentionality of sun daggers, we have the large number (~100) of sunbeam/spiral connections that all operate the same way and that all operate only at the solstices and equinoxes. Such a pointing at specific predefined declinations in the sky is essentially impossible by chance, and the only reasonable way to do this is by the knowing intention of the petroglyph makers.
Art Neuendorffer

User avatar
MarkBour
Subtle Signal
Posts: 1100
Joined: Mon Aug 26, 2013 2:44 pm
Location: Illinois, USA

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by MarkBour » Thu Aug 06, 2020 2:47 am

Looking at today's APOD, I couldn't help but wonder if there are any nice sun daggers on Mars, just waiting to be put to good use.
Mark Goldfain

User avatar
alter-ego
Serendipitous Sleuthhound
Posts: 882
Joined: Mon Apr 21, 2008 4:51 am
Location: Redmond, WA

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by alter-ego » Thu Aug 06, 2020 3:54 am

JohnD wrote:
Wed Aug 05, 2020 9:30 am


The video of this APOD shows that the "Sun Dagger" (!) is not centered on the spiral, at any time of day, and that at High Noon, the point is well away from the petroglyph. It appears to be randomly associated with the rock face, so it is a wild, unsupported hypothesis that it would have been used by prehistoric "Sun Priests"!
...
I've no problem with the spiral being associated with the Sun. One thing that can't be ignored is the shadow position change after 1000 years of precession. Since the real-time angular velocity (degrees per unit time) of the shadow equals the negative of the of the Sun's velocity, and we know the scaling (60X and 30X) to real time, the distance between the dagger intersection points on the outer spiral = 1.75°. Next, the perpendicular bisector to that shadow path segment nominally intersects the center of the spiral at a distance of 0.7°. I the looked at the altitude variation at summer solstice from Tucson over 1000 years. Low and behold, the shadow would move 0.68°, and toward the spiral center.
I'm confident the Sun landed at the spiral center at the summer solstice 1000 years ago.
A pessimist is nothing more than an experienced optimist

Brad Schaefer

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by Brad Schaefer » Thu Aug 06, 2020 4:09 am

Wow, a Martian sun dagger, what a fun idea! Well, I'll bet that finding a wedge-shaped sunbeam shining on a flat rock panel on one of the Martian solstice days will be easy, and then a person on the Mars surface (in a 'space suit') could easily take a nearby rock and pound out a spiral petroglyph. I know this because I have systematically searched for wedge-shaped sunbeams shining on flat rock panels (such that a petroglyph can be made) on various small rocky hills here around Tucson. I have found that a single hours search amongst the rocks turns up a handful of potential sun daggers, all on any date or time of my choosing. Here on Earth, making a sun dagger is easy, just go to a hill with a jumble of rocks on your chosen target date, look around for a while, spot a nice wedge-shaped sunbeam on a flat rock panel, then pound out a spiral (or whatever) petroglyph. Further experimentation demonstrates that using a local rock as a hammer, it is easy to knock off the desert varnish, and this can be done to create any desired pattern. Or, with more work with the rock hammer (for example if there is no desert varnish on the rock), then shallow grooves can form the spiral. The American Southwest is full of hills and rock piles covered with desert varnish and with little in the way of vegetation to make shade, so sun daggers can be made throughout the area. All it takes is one hour to find a good wedge-shaped sunbeam, then a few hours to pound out the spiral, and voila, a new sun dagger. Nothing special is required. Making a sun dagger can be done by any ordinary person in one day on Earth. On Mars, I *think* that there is also extensive surface coloration of the rock that we would call 'desert varnish'. (Can anyone check me on this point?) And rover images show many rocks, rock piles, and hillsides, all unshaded. Still, a future astronaut would likely have access to steel chisels or equivalent, so making a Mars dagger should be easy. Gad, won't this confuse archaeologists from the far future?

Brad Schaefer

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by Brad Schaefer » Thu Aug 06, 2020 4:41 am

How much will any of the sun daggers change in the ~1000 years since they were made? The answer is that any changes made will be too small to measure. What is going on is that the sun daggers operate when the Sun is at the solstices (with extreme declinations of ±23.4°) or at the equinoxes (with declination of 0°), with the phenomena of the sunbeam passing over the rock panels depending solely on the declination of the Sun. The solstice and equinox declinations of the Sun is practically identical between now and a millennium ago. The 23.4° angle is the obliquity of the ecliptic. Jean Meeus' definitive "Astronomical Algorithms" gives the change in obliquity of 0.13° from 1000 AD to now. My detailed observations at the Picture Rocks Sun Dagger shows no discernible changes in the observed sun dagger from 27 May (solar declination 21.46°) to 20 June (23.36°) to 16 July (21.19°). Similar results have been made for the Fajada Butte Sun Dagger. If we cannot in practice use sun daggers to distinguish solar declination changes of around 2°, then there is no way to see the effects of variations in the obliquity of the ecliptic at 0.13°. The changes due to precession will make zero differences in the position of the Sun's path at the solstices or equinoxes. Another worry is about how much the rock surfaces might have changed in the last millennium, but for this, most all processes (other than rock slides and such) operate over much too long a time scale for one millennium of weathering to matter. So in all, we can be pretty confident that what we can see today is the same as was seen by the Hohokam and Ancestral Pueblo shamans.

User avatar
alter-ego
Serendipitous Sleuthhound
Posts: 882
Joined: Mon Apr 21, 2008 4:51 am
Location: Redmond, WA

Re: APOD: Picture Rocks Sun Dagger (2020 Aug 05)

Post by alter-ego » Thu Aug 06, 2020 5:24 am

Brad Schaefer wrote:
Thu Aug 06, 2020 4:41 am
How much will any of the sun daggers change in the ~1000 years since they were made? The answer is that any changes made will be too small to measure. What is going on is that the sun daggers operate when the Sun is at the solstices (with extreme declinations of ±23.4°) or at the equinoxes (with declination of 0°), with the phenomena of the sunbeam passing over the rock panels depending solely on the declination of the Sun. The solstice and equinox declinations of the Sun is practically identical between now and a millennium ago. The 23.4° angle is the obliquity of the ecliptic. Jean Meeus' definitive "Astronomical Algorithms" gives the change in obliquity of 0.13° from 1000 AD to now. My detailed observations at the Picture Rocks Sun Dagger shows no discernible changes in the observed sun dagger from 27 May (solar declination 21.46°) to 20 June (23.36°) to 16 July (21.19°). Similar results have been made for the Fajada Butte Sun Dagger. If we cannot in practice use sun daggers to distinguish solar declination changes of around 2°, then there is no way to see the effects of variations in the obliquity of the ecliptic at 0.13°. The changes due to precession will make zero differences in the position of the Sun's path at the solstices or equinoxes. Another worry is about how much the rock surfaces might have changed in the last millennium, but for this, most all processes (other than rock slides and such) operate over much too long a time scale for one millennium of weathering to matter. So in all, we can be pretty confident that what we can see today is the same as was seen by the Hohokam and Ancestral Pueblo shamans.
You right, the obliquity does change very slowly. The calculation I did assumes the obliquity doesn't change. The Earth's precession causes the Sun's declination to change 0.7° in 1000 years. If your scaling factor for the video is correct, the declination change in 1000 years in consistent with the shadow displacement and direction. 0.7° is a significant and easy to see given the clarity of the daggers' shadow edge. Of course my calculation does assume the rock is flat and it didn't move in a millennium.
Very cool Brad, nice job!

Edit: Ah, my bad. I was hasty with claim I was at the solstice. If I'm thinking clearly now :), I added equal yearly increments between each solstice which isn't right. 1000 years ago the solstice was at a different part of the Earth's orbit because of precession. Bottom line, the sun was not at maximum declination for my calculation. Ha! Looks like I fell into a false conclusion based on coincidence which is what started this.
A pessimist is nothing more than an experienced optimist