Remembering Henrietta Swan Leavitt

Is our contemporary astronomical community very reluctant to honor the achievements of Miss Henrietta Swan Leavitt because it could reveal the very recent chauvinistic past of astronomy?


By: Vanessa Uy


Her mathematical works in astronomy were frequently rumored to have helped in the eventual discovery of the planet Pluto in 1930. But American astronomer Henrietta Swan Levitt (1868 – 1921) was better known in the astronomical community for her discovery of the relation between the absolute magnitudes of Cepheid variable stars and the periods of their light-variations. Which later provided as a method for “sounding” the depths of our cosmos. She is also credited with the discovery of a number of asteroids, four novas, and 2,400 variable stars.

Around 1912 – i.e. during the early years of stellar spectroscopy - particularly at the Harvard Astronomical Observatory, almost all of the data were catalogued and analyzed by female astronomers – called “computers” – who were forbidden because of their gender / sex to use the telescopes. It is quite ironic that during the “Dark Ages” – i.e. in the early 20th Century’s chauvinist-leaning sociological period of modern astronomy - that the work of such female astronomers as Henrietta Swan Leavitt and Annie Jump Cannon (1863 – 1941) came to be of greater significance. Especially when compared to the work of many of their male colleagues who saw these female astronomers to be nothing more than menial assistants.

Back in 1912, Miss Henrietta Swan Leavitt, an assistant at Harvard Astronomical Observatory had been assigned the task of studying 25 Cepheid variable stars in the Small Magellanic Cloud. Without any prior idea of the remoteness of the stars she was investigating, she noticed a peculiar characteristic that had since made all variable stars famous: The brighter they are, the more slowly they fluctuate. Miss Leavitt thought her find rather peculiar and published it. At once, because of her discovery, the great Danish astronomer at that time Ejnar Hertzprung – co-discoverer with Princeton’s H. N. Russel of the color-brightness graph and of the difference between dwarf and giant stars – took serious notice. The two of them soon realized that Miss Leavitt’s curious discovery might be the key to measuring the vast distances of the universe.

Before Henrietta Swan Leavitt’s discovery of Cepheid variable stars’ brightness fluctuations, astronomers previously used the geometric method of parallax in their attempts of measuring the vastness of our universe. Parallax is the apparent movement of stars, or in actuality a reflection of the Earth’s motion as viewed against the background of more distant stars. But parallax has one glaring drawback when used to measure the almost incomprehensible vastness of our universe. When measuring distances of 500 light-years and beyond, our base angle with respect of the diameter of the “circle” – actually it is an ellipse of low eccentricity - defining the Earth’s orbital path around the Sun could fall ridiculously low. When our base angle of parallax falls below 0.006 second, parallax as a means of geometrically measuring the vast distances of the universe becomes essentially useless.

This is where the beauty and brilliance of Henrietta Swan Leavitt’s discovery of using Cepheid variables to measure the vast distances of our universe comes into it’s own. If astronomers can see the distant galaxies and their retinue of Cepheid variables, those astronomers can measure how far away that galaxy is with certainty hitherto unobtainable previously via geometric parallax. Just imagine using geometric parallax to measure the distance of our nearest galactic neighbor, the Andromeda Galaxy. The parallax triangle that results in measuring the Great Galaxy of Andromeda’s 2 million or so light-year distance from us would look for all intents and purposes nothing more than a virtually straight line.