The Cosmic Year, Sagittarius A* and Ancient Ice Ages

Peter Winn

Image from a DesignBuildBLUFF presentation (2016?). Bluff is a Dark Sky town, with help from Sarah Burak.

I learned how to locate the North Star when I was a Boy Scout, but didn’t learn to recognize constellations until my first upper San Juan river trip in 1972.  River canyons have such a limited view of the night sky that you rarely see a whole constellation, but if you camp at Cottonwood Wash or Lime Creek, they’re everywhere. One of my passengers on a Museum of Northern Arizona trip in the early 1970s, Ollie (I can’t remember his last name), was an astrophotographer.  He was one of those folks who focused his camera on the North Star, set it on bulb (open shutter, predigital), and as the earth turned, the stars made hundreds of concentric circles around it – you might have seen a picture like this.  I met him at an MNA presentation and was really impressed by his first slide.  It showed the path of the full moon as being a beautiful sine curve.  Then he laughed, and told the audience he had set his camera on a tripod and as he moved the moon from one side of the field of view to the other, he slowly moved the camera up and down.  If he hadn’t told us, we would have believed the picture.  This may have been some of the first “astro fake news.”

This image was made by a computer and posted on the Universe Today website1.  Note the Galactic Bar in the center, the Solar System’s location (in red) and the Sun’s orbit (in yellow).

The Cosmic Year is the time it takes for the Sun to orbit around the center of the Milky Way, about 225 million earth years.  The last time the Sun was in its present location, all of Earth’s continents were in one huge mass near the equator (Pangea), dinosaurs hadn’t evolved and there were about 390 days per year.  225 million years from now, there will probably be a glaciated Himalayan type mountain range on the border between California and China, humans will have evaporated or be living underground, like the Worlocks in H.G. Wells “The Time Machine,” and there will be about 340 days per year1.

To put this in perspective, the Milky Way is believed to have formed about 13.8 billion years ago and our solar system is about 4.5 billion years old. This means it’s about twenty cosmic years old.  Our sun is projected to burn out in about ten billion years, so earth is equivalent to a thirty-year-old human1.  Our genus, Homo, is about two million years old, and our species, sapiens, is about 250,000 years old.  Neanderthals and other recent Homo species became extinct about this same age2.  What’s next?  Bionic smartphones?

Sagitarius A*, the massive black hole at the center of the Milky Way, rotates perpendicular to its Galactic Bar. Image recently taken by the Event Horizon Telescope (by orders of magnitude the largest on earth) and posted on the NASA website3.

Most astronomers believe that our sun (Sol) and solar system formed in the Milky Way, i.e. it didn’t form in another galaxy that was gobbled up by the Milky Way.  Sagittarius A* is surrounded by a dense mass of star forming nebulae.  A black hole is black because it’s so dense that light can’t escape.  The black hole is located behind the constellation Sagittarius from Earth’s point of view, hence the name.  Many stars are ejected from the bar and wind around the center as it spins in space-time, forming the spiral arms.  Some nebulas also ejected, which form stars within the arms.  Our sun probably formed in the Orion-Cygnus spiral arm where it is now located, so it doesn’t follow the yellow orbit through other spiral arms all by itself – Sol orbits with the entire spiral arm1.

While studying spiral arms in other galaxies such as Andromeda in the 1970s, Vera Rubin realized that the total mass of the stars wasn’t enough to explain the rotation rate of the galaxies and calculated the amount of dark matter needed. From The New York Times4: “On December 20, 2019, the Large Synoptic Survey Telescope was renamed the National Science Foundation Vera C. Rubin Observatory in recognition of Rubin’s contributions to the study of dark matter and her outspoken advocacy for the equal treatment and representation of women in science.  The observatory [is being] built on a mountain in Cerro Pachón, Chile and [will] focus on the study of dark matter and dark energy.”

Astronomers believe that the Universe is only 4% light matter, so this scope has a huge field of study, could take a few years….

The Navajo constellation Náhookos Bi’áadii, called Cassiopeia by the Greeks. Image from Sharing The Skies: Navajo Astronomy, by Dr. Nancy Maryboy, a former Bluff resident. Available at the San Juan County Library.

The stars in our spiral arm move slowly relative to Sol and the stars forming our constellations are in our spiral arm.  For a really cool short video showing how the Big Dipper (Ursa Major or the Big Bear, or Nahookos Bi’ka’, the Male Revolving One) changes over the next 50,000 years, see here.  The closest star in this constellation is 54 light years (over thirty trillion miles), the furthest is 509 light years (about three hundred trillion miles).  Sol is about 26,000 light years from the center of the Milky Way.  We also know the Pole Star (to which earth’s axis of rotation points) changes over time.  It’s currently Polaris (Nahookos Bi’ko’, the Central Fire), the last star in the handle of Ursa Minor (the Little Dipper).  Thurban (Alpha Draconis in the constellation Draco) was the pole star when the Egyptians built the pyramids and Deneb (Alpha Cygni in the constellation Cygnus) will be the pole star in 10,200 AD1.  Most likely my birth constellation, Scorpio, looks different now than it did to the Greeks, and like the Big Dipper, it will eventually become unrecognizable.  This may affect my horoscope.

Image from Chemical Geology5

A few astronomers think Sol and other stars may have formed in a galaxy that merged with the Milky Way, drift though the Orion-Cygnus spiral arm and pass through other spiral arms.  This theory explains why ice ages have occurred every one to two hundred million years.  They believe that the increase in cosmic radiation due to the proximity of other stars in the spiral arms causes more water vapor to condense, forming clouds that reflect solar radiation (cosmic radiation passes through clouds), causing cooling and increased snowfall.  These ice ages typically only last about ten million years, so earth didn’t have polar ice caps for about 90% of its history, when our solar system was in between spiral arms6.  We know that the angle of the Earth’s axis relative to its plane of revolution around the sun causes seasons due to changes in solar radiation.  It’s currently at 23.4 degrees, but fluctuates several degrees over tens of thousands of years, hence the change in the pole star.  The shape of Earth’s orbit around the sun fluctuates over hundreds of thousands of years, also affecting climate.  Climate is also affected by plate tectonics and major volcanic events, but these do not correlate well with ancient ice ages.  We also know that cosmic and solar radiation and radioactive elements can alter DNA, causing speciation and cancer.

Image created by Theresa Bresnau, a Bluff resident (

If you’re feeling lost in space-time, or if standing on a planet spinning at a thousand miles per hour makes you dizzy, or if you’re nervous about speeding around the Milky Way at 490,000 miles per hour without brakes or a steering wheel, listen to the gentle wind blowing on Mars on the NASA website here, and please join Sarah’s next star party.

“I don’t know what happened here, I probably stayed up too late and had too much caffeine.” – Paul Martini



2 Wood, Bernard; Richmond, Brian G. (July 2000). “Human evolution: taxonomy and             paleobiology”Journal of Anatomy197 (1): 19–60



5 Veizer, J. (1999). “87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater”Chemical   Geology161 (1–3): 59–88

6 Henrik Svensmark (2012).Evidence of nearby supernovae affecting life on Earth. Monthly Notes of the Royal Astronomical Society. doi:10.1111/j.1365-2966.2012.20953