A good month for Einstein – gravitational starlight deflection during the Great American Eclipse

by Dr. Donald G. Bruns


Don Bruns and his wife Carol on eclipse day at the Lions Camp on Casper Mtn. The tripod is bolted to the custom mosaic designed and built by his cousin Steve Lang.

After much anticipation, two experiments had great successes last year.  On August 17 2017, the LIGO/VIRGO collaboration monitored the merger of two neutron stars millions of light years away.  Only four days later in Wyoming, an experiment to measure the gravitational bending of starlight by the Sun acquired the best data since the idea was first tested in 1919, by Sir Arthur Eddington, in Africa.  I published my results on that experiment in Classical and Quantum Gravity on March 6, 2018.  My solo project to repeat Eddington’s achievement, which made Einstein famous, required a lot less manpower than LIGO!

Early last century, Einstein published his General Theory of Relativity that contained some unusual predictions, including the idea that massive bodies bend light beams.  The only way to test this would be during a total eclipse, when the sky would be dark enough to see stars close to the Sun, where the effect just might be measurable.

I started planning Eddington’s re-enactment when I found out that no one had attempted it since 1973 (also in Africa) and that no one had ever succeeded in getting all the parts to work during those precious few minutes of totality.  I assumed that with modern charge-coupled device (CCD) cameras and computerized telescopes, the experiment would be much easier.  I was wrong!  While some aspects were simplified (the Gaia star catalog provided accurate star positions, for example, and modern weather predictions and the compact equipment eased many logistics problems), dealing with pixels, turbulence, and a limited sensor dynamic range presented new challenges.

The Great American Eclipse on August 21 2017 was viewed by citizens, scientists, and tourists from around the world, and it seemed most of them gravitated toward Wyoming.  I reserved lodging in Casper, WY a year in advance and had bad-weather back-up sites in Idaho and Nebraska.  Fortunately, the weather cooperated and clouds did not appear over Casper until an hour after totality.  Family and friends joined me in Wyoming, including my cousin Steve Lang, who built a custom platform with an eclipse mosaic so that I could bolt my telescope firmly to the ground.  I set up my telescope a few nights beforehand and didn’t want an accidental bump to destroy the critical alignment I needed to track the stars.

The other attempts in 1922, 1929, 1936, 1947, and 1952 were set up at inconvenient locations around the world and the astronomers couldn’t test everything ahead of time.  They tried using beamsplitters and other tricks to get the calibration images they needed, but never got results more accurate than about 10% because they always had to go back and use Plan B when they analyzed their data.


A total of 20 stars were used in the deflection calculations, including two stars very close to the sun. Eddington found only 5 stars in his photographic attempt in 1919. The circles here indicate the star locations.

In Wyoming last August, for the first time in nearly 100 years, all of the equipment performed perfectly under clear skies!  As a result, my data analysis matched Einstein’s predicted value to 0.01% accuracy, with a measurement error of 3%.  All of my efforts paid off even better than I expected.  And, because almost everything was computerized and automated, I was able to enjoy the eclipse spectacle while the data was being recorded.  At the next eclipse though, I will be a little more relaxed and let others repeat the experiment.  Maybe I’ll be just be reading about more neutron star mergers.

About the author: Don Bruns is a retired optical physicist living in San Diego, CA. He recently won the 2018 Chamblis Amateur Astronomy Achievement Award from the AAS for his work on this experiment.

Read his article, ‘Gravitational starlight deflection measurements during the 21 August 2017 total solar eclipse‘, in CQG to learn more about the experiment.

This work is licensed under a Creative Commons Attribution 3.0 Unported License.