HYBRID – General Meeting – August 2022

6:30 pm – Main Presentation

TITLE:  Sun Daggers and Simple Noontime Sun Observations; From the Hohokam and Ptolemy to Kepler and Einstein

PRESENTATION: All cultures have specialists and laypeople observing the Sun, for purposes ranging from calendars (for planting and ritual days) and timekeeping, to astronomy research (e.g., sunspot counts, Earth’s orbit, and the gravitational bending of light).  For TAAA, Dr. Schaefer will share two sets of his research with naked-eye observations of the Sun from the Tucson area that are of interest to amateur astronomers, professional astronomers, and historians of astronomy. One set of research concerns the many sun daggers—dagger-shaped gaps in shadow created by sunlight steaming through crevices in rocks—etched onto rocks by the Hohokam Indians (c. 1000 AD) as markers for solstices and equinoxes for ritual purposes.  His research (with Jim Stamm) on sun daggers from spiral petroglyphs at Picture Rocks, Signal Hill, and Cerro Prieto prove the solstitial alignments were intentional.  

Another local observing program has been to accurately measure the Sun’s analemma; a diagram that shows the position of the Sun as viewed from a fixed position on Earth over the course of a year. With these observations for the Astronomical League Observing Award on the Analemma, he can define Earth’s orbit, and derive Tucson’s latitude, the obliquity of the ecliptic, the dates of solstice and equinox, the eccentricity of the Earth’s orbit, and the date of perihelion. This is a modern empirical test of the ancient Greek discoveries that led them to introduce epicycles. Dr. Schaefer’s modern data can also be used to test and prove the three Kepler Laws.  And all this, he says, is possible “by making quick naked-eye observations, with simple and cheap equipment, from my front yard.”

BIO:  Dr. Bradley Schaefer received his Ph.D in 1983 from Massachusetts Institute of Technology and is Professor Emeritus in the Department of Physics & Astronomy at Louisiana State University. His wide range of interests include many areas of astrophysics, as well as many aspects of astronomical events in history (e.g. the Crucifixion and the Star of Bethlehem) and in literature. For his work on the Supernova Cosmology Project which led to the discovery of dark energy and for which its leader, Saul Perlmutter won the 2011 Nobel Prize in Physics, Dr. Schaefer received a share of the 2007 Gruber Cosmology Prize, and the 2015 Breakthrough Prize in Fundamental Physics.

7:15 pm – Seasonal Star Information

Presented by Mary Turner.