Williamina Paton Fleming had a choice. She could let the laughter die down and allow herself to be the butt of the joke, or she could stand up, turning the joke on its head and using the moment to her advantage.

Her employer, Edward Charles Pickering, director of the Harvard College Observatory, had become so exasperated with the sloppy work of his male assistants — "computers," as they were called in those days — he'd quipped that his "Scotch maid" could do a better job.

It was true: Fleming was his housemaid. It was also true that she had come to America from Scotland, where she'd been an excellent math student and then, beginning at 14, a student teacher. And it was true, she believed: She could do a better job. While the personal circumstances that had led her to take on domestic work had been less than ideal, she had been thrilled to be working in the house of the Observatory director, in proximity to the most productive and groundbreaking astronomy research lab in the country. During the past two years of Mina's employment, Pickering, noting her aptitude for numbers and her quick mind, had even allowed her to do a little clerical work at the Observatory, outside of the housecleaning and other domestic work she performed. But she was ready for something more.

We don't know what the 24-year-old single mother's actual response was in that moment after the laughter subsided, whether it was a serious and bold, "Yes, I can do a better job," whether she met his frustrated, sarcastic suggestion with wit, whether she waited until later and approached him privately, quietly, after the workday was over; we don't even know for sure whether the "so easy my maid could do it" story of Pickering's reported pique is a faithful account or mere Harvard legend. What we do know is that in 1881, Mina Fleming took the opening her boss offered. It may have been suggested as a joke, but to her it was the opportunity of a lifetime, and in taking it she helped Pickering change the history of astronomy in America — and the opportunities available for women in that discipline — forever.

Mina had initially taken the position of housemaid out of necessity: Shortly after moving to America in 1878, her husband, James, left her. She was 22, in a new country, abandoned — but not alone. She was pregnant. It's not clear whether Pickering was aware of her condition when he hired her, but it quickly became obvious. What also became obvious to him was that Mina was industrious, dependable, highly attentive to detail, good with numbers, and in possession of a startling natural intellect. He invited her to work as a part-time copyist in addition to her domestic duties, and when she insisted upon traveling back home to Scotland for the birth of her baby, Pickering allowed her to return to her job upon her return. It is perhaps out of gratitude for this consideration, and her general esteem for her boss, that she named her son Edward Charles Pickering Fleming.

Mina wasn't the first woman to work in Pickering's lab. The Harvard College Observatory, founded in 1839, was in the beginning the province of men, young physicists and astronomers whose passion for the emerging science of stellar photography kept the Observatory running despite the constant plague of poor funding. But women were allowed to volunteer as "observers," and several applied to be student assistants. In 1875, a year before Pickering was appointed as its fourth director, the Observatory admitted three women as staff members, working as computers. At first, the women worked for free; eventually, by the time Mina was hired, they were awarded a salary of 25 cents an hour — roughly half that of the male computers.

Pickering had long been a supporter of women as amateur astronomers, and he believed that their participation in scientific research could provide them with opportunity previously denied their gender.

"The criticism is often made by the opponents of the higher education of women that, while they are capable of following others as far as men can, they originate almost nothing, so that human knowledge is not advanced by their work," Pickering wrote. "This reproach would be well answered could we point to a long series of such observations … made by women observers."

He had also struggled since the beginning of his tenure as director of the Observatory with funding. (Ironically, the Observatory's most enduring source of funds was due to the largesse of a woman: Mary Anna Draper, who established a memorial fund for her husband, the astronomer Henry Draper, that subsidized Pickering's research.) And so the hiring of women was a boon both financially and scientifically. As he wrote in one of his annual reports on the state of the Observatory, the women computers were "capable of doing as much and as good routine work as astronomers who would receive much larger salaries. Three or four times as many assistants can thus be employed."

When Pickering officially hired Mina as a computer in 1881, she was one of a handful of women computers at the lab, and the only one at that time with no scientific education. But that proved not to be a problem. She was a fast learner. She began by assisting Pickering with photometry, the process of measuring the brightness of stars, deciphering his notations, hastily scribbled in the dark of night as he and his assistants peered through telescopes, and then applying formulas to the notations he'd made during those nocturnal observations about the stars in order to calculate their magnitude.

Over the next few years, Pickering pioneered a new approach to photometry using photography, rather than purely direct observation, as a way to evaluate the brightness of stars. Photographs taken via telescope were developed on glass plates, about 8 by 10 inches. The stars appeared as tiny gray or black dots on the glass, and each plate was set on a special stand, which was designed to tilt the plate back slightly, with a mirror at the base of it that could capture daylight and reflect it up through the glass, illuminating the transparent plate.

Then, a computer would use a magnifying glass to examine the plate up close, enlarging the hundreds of spectra, or stars, on the plate, many of them only a half a centimeter across. With the magnifying glass, it was possible to discern tiny dark lines seemingly emanating from the bright pinpoints of stars, like a grayscale rainbow. These spectral lines were a kind of signature, indicating the presence of elements like hydrogen, iron, and calcium, among others. Certain kinds of stars had certain kinds of patterns, making it possible to identify and classify them based on those patterns. By examining the plates in this way, it was also possible to note the relative brightness of stars to other stars, which was a way of determining the distance between them, as well as to analyze the pattern of tiny lines that held clues to their chemical compositions and to chart their coordinates. All of this helped to better define the immense expanse and scope of our universe.

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