Measuring the Precise Diameter of the Sun Using Solar Eclipses: The May 20, 2012 Annular Eclipse

The moon is a convenient perfectly stable measuring stick who's motion we understand now to high precision. So well, in fact, that we can use it to measure the precise diameter of the sun to a matter of small fractions of a kilometer. These measurements are important as changes in the sun affect not only our understanding, but all life on our planet! Measuring the sun's diameter is not so easy! It's brilliant and seen against a turbulently heated Earth's atmosphere, measurements of it's diameter would be limited to on the order of several kilometers at best. However, during a solar eclipse, "Bailey' Beads" let you see bits of the edge of the sun shining through even the shallowest lunar valleys projected against the edge of the moon. The Japanese spacecraft Kaguya recent measured the precise topography of the edge of the moon to unprecedented precision. This data is now available to all, and allows us, with careful reductions, to make the solar diameter measurements.

Here's an accounting of just such a scientific expedition, which I participated in for the May 20, 2012 Annular Eclipse of the Sun, which passed only a little over 100 miles from Cabrillo College - a very rare treat not to be missed. I'll show you how simple equipment was used for the project. Accompanying me on the expedition, was astronomy major Rebecca S. as well as my former NASA colleague Professor Chris Kitting. We and our students would form one station gathering twin sets of data; Chris with a green narrow-band filter, and I with a red narrow-band filter. Temperature variations can make for different diameter measurements, at least potentially, at different wavelengths. This is an issue raised in past eclipses and one we set out to test at this eclipse.

(this page is still under construction - be patient!)


Photo of the camcorder screen with the Thousand Oaks aluminized glass filter. This produces an orange sun (but note the PC164C videocam is B/W)

Photo of the camcorder screen with the mylar solar filter I constructed. While there is light leakage around the edges of the filter in front of the corrector plate, note there's no hint of contrast degradation here vs. the snug fit Thousand Oaks filter - good!


Ted Swift got a nice recording of beads at the north end of our fence, where annularity was greatest. His YouTube video is here.