Charlotte Emma Moore Sitterly was an American astronomer whose careful spectroscopic tables became a foundation for understanding the composition of the Sun and the structure of atoms. Born on September 24, 1898, in the small Quaker village of Ercildoun, Pennsylvania, near Coatesville, she grew up in modest circumstances as the daughter of George W. Moore and Elizabeth Walton Moore. Family and community expectations of the time did not point naturally toward a scientific career, yet she showed early ability in mathematics. To help pay for her education she tutored students from the early grades through high school, an experience that convinced her teaching would be too wearing as a lifelong profession. In 1920 she graduated from Swarthmore College with a bachelor of arts degree in mathematics.
That same year she joined the Princeton University Observatory as a computational assistant, one of the “human computers” who performed the lengthy numerical reductions that photographic astronomy then required. She worked under Henry Norris Russell, the observatory director and one of the leading astrophysicists of the era. Her early duties included calculations needed to determine the Moon’s position from photographic plates. The association with Russell soon broadened. Together they published papers on binary stars and stellar masses, work that culminated in the collaborative volume The Masses of the Stars in 1940. She also undertook a spectral classification of roughly 2,500 stars. Russell recognized her talent and treated her as a scientific colleague rather than merely a calculator, an unusual arrangement at a university that still barred women from its graduate programs and would continue to do so for decades.
After about five years at Princeton, Moore moved for a time to the Mount Wilson Observatory in California, continuing a collaboration between Russell and the solar spectroscopists there, especially Charles E. St. John and Harold D. Babcock. At Mount Wilson she concentrated on the solar spectrum. She helped revise Henry Rowland’s classic table of solar wavelengths and contributed to the redetermination of the international angstrom scale, the standard of wavelength measurement then used by spectroscopists. The Sun’s absorption lines, each produced by a particular atom or ion under particular physical conditions, became the central subject of her life’s work. She later remarked that opportunities for women at Mount Wilson were limited and that she found more room to grow in general astrophysics under Russell than she did as a computer on the mountain.
When Russell took a sabbatical in Europe, Moore saw a chance to earn a doctorate. Princeton would not admit her, so she enrolled at the University of California, Berkeley, supported by a Lick Fellowship. In 1931 she received her Ph.D. in astronomy for a thesis titled Atomic Lines in the Sun-Spot Spectrum. Using plates from the 150-foot solar telescope at Mount Wilson, she analyzed the atomic lines in sunspots and concluded that the temperature of those features was about 4,700 kelvin, cooler than the surrounding photosphere. While completing the degree she continued to collect and organize laboratory and solar data on the spectra of elements and molecules.
She returned to Princeton as a research assistant and remained there until 1945. During those years she produced the first comprehensive multiplet tables of astrophysical interest, organizing thousands of spectral lines into related groups according to the quantum transitions that produce them. The multiplet arrangement was far more useful to astronomers than a simple wavelength list, because it linked observed lines to atomic energy levels and allowed abundances and physical conditions to be inferred. A major version, A Multiplet Table of Astrophysical Interest, appeared in revised form in 1945 and remained a standard reference for decades. In the mid-1930s she also announced the identification of additional chemical elements in the solar spectrum; contemporary reports credited her with bringing the number of elements known both on Earth and in the Sun to sixty-one. Later spectroscopic work associated with her identified technetium in sunlight, an element that does not occur naturally in stable form on Earth and whose presence in stellar atmospheres bears on theories of nucleosynthesis.
In 1937 she married Bancroft W. Sitterly, a Princeton astronomer and mathematician. She continued to publish largely under the name Charlotte E. Moore, though she is widely known as Charlotte Moore Sitterly. That year the American Astronomical Society awarded her the Annie Jump Cannon Award in Astronomy. In 1949 she became the first woman elected an associate of the Royal Astronomical Society.
Russell’s impending retirement led her, in 1945, to the National Bureau of Standards in Washington, now the National Institute of Standards and Technology. She insisted, as a condition of the appointment, that she be allowed to continue solar work alongside laboratory spectroscopy. At the Bureau she directed the compilation of atomic energy levels derived from analyses of optical spectra. The multi-volume Atomic Energy Levels, issued from the late 1940s into the 1950s, and the related ultraviolet multiplet tables, became indispensable references for physicists and astronomers. Her standards of critical evaluation were exacting; colleagues and later users praised the reliability of the tables long after electronic data sets began to replace printed volumes.
The arrival of rocket astronomy extended her reach beyond the atmosphere. Beginning in 1946, spectra obtained by Richard Tousey and collaborators at the Naval Research Laboratory, first with captured V-2 rockets and later with other vehicles, recorded the Sun’s ultraviolet radiation that ground-based telescopes could not see. Sitterly analyzed these data, identified lines, and incorporated them into her multiplet and energy-level compilations. The collaboration continued for decades and later included observations from Skylab. She also worked on the infrared solar spectrum. In all she authored or co-authored more than one hundred papers and attended major international meetings, including the 1958 General Assembly of the International Astronomical Union in Moscow.
She retired from the Bureau of Standards in 1968 at the age of seventy, but she did not stop. From the early 1970s she worked with Tousey’s group at the Naval Research Laboratory while keeping an office at the Bureau, publishing further tables into the mid-1980s. Her last major collection of atomic data appeared in 1985, when she was nearly eighty-seven. Honors accumulated late as well as early: the Federal Woman’s Award in 1961, an honorary doctorate from Swarthmore in 1962, the Optical Society of America’s William F. Meggers Award in 1972 (she was the first woman to receive it), and a commemorative issue of the Journal of the Optical Society of America in 1988. The Astronomical Society of the Pacific awarded her the Bruce Medal for lifetime achievement in 1990; she died in Washington, D.C., on March 3 of that year, at the age of ninety-one, before she could receive it in person.
Sitterly’s achievement was not a single discovery but a sustained act of scientific infrastructure. By reducing chaotic laboratory and solar measurements to critically evaluated wavelengths, multiplets, and energy levels, she gave other astronomers the means to determine what stars are made of and under what conditions their atmospheres radiate. The work was painstaking, often anonymous in the sense that users consulted “the Moore tables” without always recalling the person behind them, and she described it without glamour: the interest of the subject itself carried the labor along. That labor remains visible whenever a modern spectrum of the Sun or a star is interpreted with atomic data whose lineage runs through her compilations.