You’ve Come a Ways, Henrietta

In looking up how distance is measured in astronomy, I came across something that tells about change in our times. The person who discovered that Cepheid variables can be used for measuring distance was Henrietta Swan Leavitt (1868-1921). I could not find an article on her in my (new) 1956 Encyclopedia Britannica but did find an article on her in my 1989 World Book Encyclopedia. I recently got my EB from the Magees as Christmas present. Some Christmases back, John also gave me a very fine biography of Rosalind Franklin (1920-1958). Does the name ring a bell? With James Watson and Francis Crick she was a co-discoverer of DNA; they got the Nobel and she did not. Okay. Nobel Prize rules insist that the award can only be given to living persons, and Rosalind had died by 1962…

Variable stars pulse in luminosity, and Leavitt discovered that those with longer periods between bright and dimmer phases are more luminous than those with shorter pulses. The earliest of these were discovered close enough to us so that we could measure their distance from us using simple geometry—the parallax method. How that’s accomplished is shown here. We could therefore correlate changes in measurable brightness, short and long pulses, and known distances. Then later, by extrapolation, just a reading of luminosity and period enabled us to use Cepheids for distance measurement even when they were too far away to discover their distance by parallax.

Franklin saw the DNA’s structure by taking an X-ray diffraction image of it—and that image then greatly helped Crick and Watson to zero in on the structure. The image, known as Photo 51, is available here.