About a century ago Henrietta Swan Leavitt made a discovery that revolutionized our ability to understand the cosmos, though most people wouldn't recognize her name.

Leavitt was born in Lancaster, Massachusetts on the Fourth of July 1868 to a Congregational minister and his wife. The family emphasized traditional New England Puritan values, embracing faith and service, shunning frivolity. It was also a well-educated family and supported Henrietta in her desire for higher education.

When the Leavitts moved to Ohio in 1885, Henrietta attended Oberlin College for three years. She then returned to Massachusetts to enroll at the Society for Collegiate Instruction of Women in Cambridge. It wasn't part of Harvard, but the entrance requirements were those of Harvard, as were the courses (taught by Harvard staff), exams and degree requirements. In 1894 the Society became Radcliffe College (now part of Harvard University).

Leavitt was a member of Phi Beta Kappa, the prestigious fraternity open only to those achieving academic distinction in an undergraduate degree. And yet when she completed the degree in 1892, she didn't exactly get a degree. She got a paper saying that if she had been enrolled at Harvard (i.e., had she been a man), she would have been awarded a Harvard degree.

In those days middle class women weren’t expected to have jobs. But for those who didn't marry and who were well-educated, the most promising career was teaching. Unfortunately, Leavitt often suffered ill health, and a severe illness left her deafened, so teaching wasn’t an option for her.

After graduation, she stayed in Cambridge for a time, doing graduate courses in astronomy and working at Harvard College Observatory as a volunteer. Then travel and family matters occupied her, but in 1902 the director of the Observatory, Edward Pickering, gave her a job as a computer.

The computers were human, in fact, well-educated women on low pay. They did many of the repetitive calculations that today would be done electronically. At Harvard Observatory, their main job was to study photographic plates from the observatory’s telescope in Peru. They would record in a logbook the contents of each plate, including the magnitude (apparent brightness) of each star.

The computers were also supposed to be on the lookout for variable stars, stars whose magnitude changed. Leavitt was particularly interested in variable stars and she discovered over 2400 of them. This was half of all of the variable stars known in her lifetime.

Because of her obvious ability, Pickering put Leavitt in charge of the department of photographic photometry. Photographic photometry is the science of determining stellar magnitudes from photographic images. Since the camera and the human eye respond differently, in order to classify stars from photographs you need a reference sequence. This is a sequence of stars whose magnitudes were analyzed so that they could be used for comparison purposes.

Leavitt's task was difficult and demanding, but she did it. Her sequence was known as the Harvard Standard and it was adopted internationally in 1913. Five years later she based an improved standard on a larger sample including stars as faint as the 21st magnitude. (The dimmer the star, the higher the magnitude.) It was in use until new technology superseded it several decades later.

Yet her greatest work was the discovery of the relationship between the period of a Cepheid variable star and its actual brightness. This was a tremendous breakthrough. Like the receding lights in a dark tunnel in the header image, if you know how luminous an object is and how bright it seems, you can can work out how far away it is. Before that, astronomers couldn't calculate distances beyond 100 light years. The new understanding would enable them to obtain the distance to any place where they could see a Cepheid, thereby extending the cosmic distance scale to 10 million light years.

Without distance indicators, astronomers disagreed about the extent of the universe. Was the Milky Way the whole universe and nebulae objects within it? Or were the nebulae other galaxies? Edwin Hubble used a Cepheid in the so-called Andromeda Nebula to show that it was a galaxy far beyond the Milky Way.

Henrietta Leavitt's body of work would be impressive as the career output of any astronomer, but she achieved it undeterred by deafness and the lowly status of women at Harvard. She also achieved it despite extended bouts of ill health and, sadly, an early death. She died of cancer on December 12, 1921, aged 52.

Leavitt was a valued colleague to other astronomers, a member of the precursor organization to the American Astronomical Society, a fellow of the American Association for the Advancement of Science and an honorary member of the American Association of Variable Star Observers. In 1925, unaware of her death, a prominent Swedish mathematician made inquiries about her to Harvard Observatory with a view to nominating her for a Nobel Prize.

Nonetheless her name was largely forgotten except for asteroid 5383 Leavitt and a crater on the far side of the Moon. However in the 21st century, women who were left out of the history of science are being rediscovered, among them Henrietta Leavitt. The relationship that makes it possible to use Cepheids to calculate cosmic distances was traditionally known as the period-luminosity relation, a name that bypassed its discoverer. At last, in 2009 the American Astronomical Society agreed to encourage people to refer to it as the “Leavitt Law” and I notice that this usage is becoming more common.