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Henrietta Leavitt, How She Loved the ‘Clouds’

Henrietta Swan Leavitt at about 30 years of age. Popular Astronomy, v. 30, no. 4, April 1922.

Henrietta Swan Leavitt (July 4, 1868-December 12, 1921) is not a name that’s familiar to most people.  Her name is certainly not as recognizable as Galileo, William Herschel, or Edwin Hubble, even though it ought to be.  I heard about Silent Sky, a play about her that’s opening at Ford’s Theater, which piqued my curiosity about this amazing woman.  Henrietta began her higher education at Oberlin College in Ohio before transferring to the Society for the Collegiate Instruction of Women, what is now Radcliffe Institute for Advanced Study, part of Harvard University, in Cambridge, Massachusetts.  While at the Society, Henrietta took a course in astronomy and was later an employee at the Harvard College Observatory where she performed stellar photometry under Edward Pickering.

While her time at the observatory was intermittent due to travel and illness, it was there, as a computer, that she thrived.  In her day-to-day tasks, Henrietta looked at photographic plates of stars to determine if their brightness had changed.  For those that had changed in brightness, or magnitude, she recorded the differences and over time determined their period.  These variable stars are known as Cepheid stars.  The plates that she looked at came from two observatories: the Harvard College Observatory and their outpost observatory in Arequipa, Peru.  The second allowed her to view the stars visible from the southern hemisphere, including the Magellanic Clouds.

Annie J. Cannon [and] Henrietta S. Leavitt [photograph]. Harvard University Archives.

These clouds, which are actually two dwarf galaxies that are visible in the southern hemisphere sky, are what Henrietta focused her attentions on while searching for variable stars.  In her landmark publication “1777 Variables in the Magellanic Clouds” she asserted that regarding these variable stars that the longer the cycle, the greater the brightness.  This period-luminosity relationship became known as Leavitt’s Law and though these words may not count for much, among astronomers it was a breakthrough (even though this work didn’t garner much attention upon release).  Her observations led to the Harvard Standard, which was a scale that measured the brightness of stars over 17 different magnitudes and was the standard to use when determining the magnitude of stars throughout the night sky.

Henrietta further locked in her law in her 1912 follow-up paper entitled “Periods of 25 Variable Stars in the Small Magellanic Cloud.”  In this paper, she called back to her previous publication: “In H.A. 60, No. 4, attention was called to the fact that the brighter variables have the longer periods, but at that time it was felt that the number was too small to warrant the drawing of general conclusions.  The periods of 8 additional variables which have been determined since that time, however, conform to the same law.”  This time, the astronomy community took note and began to use these variables as standards to determine distance in space to other stars that pulsated at the same rate using the inverse square law.

Henrietta Leavitt’s entry in the 1920 U.S. Census showing her occupation as “astronomer.”

For all her efforts, she was never known professionally as an astronomer, mainly due to her being a woman in very much a man’s world.  In the 1920 U.S. Census though, she did take a stand in having her occupation recorded as “astronomer.”  And there have been several posthumous honors such as having a crater on the far side of the moon, an asteroid, and a telescope named after her.  So the next time you look up at the stars on a clear night, just remember Henrietta made them just a bit clearer.


For more information regarding Henrietta Swan Leavitt and her field, check out these great books:

For more information on the Harvard Women Computers and their contribution to early 20th century astronomy, take a look at Project PHaEDRA.

And, for more information on women astronomers in general, consult the Women in Astronomy: A Comprehensive Bibliography by the Library of Congress.

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