Thursday, December 19, 2019

The Kumeyaay Astronomical Calendar of Blue Sun Cave in Anza-Borrego Desert State Park

Our discovery

Some of you may know of the Blue Sun Cave in Anza-Borrego Desert State Park, which has various Kumeyaay pictographs inside it, including several suns, and one particular blue sun from which the cave gets its name. The surrounding area has ample evidence of native habitation including pottery shards, seed grinding holes, and an overhanging rock shelter with smoke marks from fires.

Last year we just happened to visit the cave around sunset in November and discovered that a shaft of sunlight penetrates the cave just before the sun sets behind the nearby hills. I took photos of this event, as shown below. The phenomenon only lasted for a few minutes.

Returning to the site a month and a half later and examining the situation further, I discovered that a small triangle of sky is formed between the cave mouth, a boulder in front of the cave, and the nearby hills, as shown below.

This arrangement appears to create a natural astronomical calendar that only allows the sun to penetrate the cave for a few minutes just before sunset, and only for several weeks either side of the winter solstice when the sun is correctly aligned with the triangular aperture between the rocks.

To me, it seemed obvious that this was why the Kumeyaay painted suns inside the cave. The cave served as a natural calendar for the Kumeyaay that marked the winter solstice. There is another "solstice cave" in Anza-Borrego with suns painted inside, and Native Americans have been known to mark such natural calendars in other parts of the southwest.

Sun daggers

I did some quick online research and found out that these rock calendars are known as "sun daggers", because the shaft of sunlight has a dagger-like shape. Often Native Americans mark these sites with rock art.

For example here are some "sun daggers" similar to the one I observed at Blue Sun Cave.

This one is in Colorado and marks the spring and autumn equinoxes.

Below is the famous Anasazi sun dagger at Chaco Canyon in New Mexico, which marks both solstices and equinoxes. This sun dagger appears to have been made by the Anasazi rather than being natural.

Here is the Wikipedia article on this sun dagger.

More info from another site

Here is an astronomical clock from Tübatulabal territory in Kern country, California.

And another sun dagger in West Texas.

Another Kumeyaay solstice calendar

After thinking about this for a while I realized there is another Kumeyaay astronomical calendar in this area.  It's located at the El Vallecito archeological site, just across the Mexican border near the town of La Rumorosa.  The site is the largest collection of Kumeyaay rock art in the region. One particular pictograph is known as El Diablito, which means "The Little Devil". Apparently in the early morning of the winter solstice a sun dagger enters the boulder cave where El Diablito resides and passes through his eyes.  I've been to the El Vallecito Archeological Site a few times and have seen the El Diablito pictograph, but I haven't seen it on the winter solstice.  I'll have to see if I che
ck that out some time

The realization!

Well it just so happens we are going camping in Anza-Borrego again this coming weekend and I noticed that Saturday December 21st, 2019 is the winter solstice. This means it would be a great time to check out the Blue Sun Cave's sun dagger again. The question however, was when would we need to be there?

Looking at the EXIF data for the photos I took of the sun dagger last year, I found out that I took the photos between 3:25 and 3:29pm on November 10th, 2018. So the phenomenon lasted no more than about 5 minutes Using I was able to determine that sunset on that day was at 4:46pm, and that sunset on the winter solstice of 2018 was at 4:42pm. I was also able to determine that those sunset times are the same in 2019. So there is only a four minute difference between the time of sunset on November 10th and December 21st.

So it is reasonable to assume that the sun dagger will appear in the Blue Sun Cave on December 21st, 2019 between about 3:20 and 3:25pm.

Let's see how I go checking it out. I wonder if I can get video? I'll post the results here next week.

Update, December 23rd, 2019.

Well that idea was thoroughly debunked. You can see the shadow on the cave entrance at sunset in this pic. On the winter solstice the sun is too low and therefore sets too far south to enter the cave at all. Wah waaah.  I was completely and unequivocally wrong.

Further resources

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