Marjorie Pettit Meinel (born Marjorie Steele Pettit; May 13, 1922 – June 24, 2008) was an American astronomer and optical scientist. Her research focused on telescope design and optics, the technological applications of solar energy, atmospheric phenomena including the optical effects of volcanic eruptions, and the study of variable stars. For over 60 years she was the research collaborator and wife of fellow astronomer Aden Meinel.

Marjorie Steele Pettit was born in Pasadena, California May 13, 1922, to astronomers Edison Pettit and Hannah Steele Pettit who were based at the Mount Wilson Observatory. Meinel's older sister, Helen, also became an astronomer. As a teenager, Meinel assisted her father in making the first time-lapse photography of solar prominence eruptions, using a movie camera and a quartz-polarising monochromator attached to the 6-inch Alvan Clark refracting telescope at their home. As her mother's health declined Meinel increasingly assisted her father's research on the telescope at home and at the observatory.

While at Pasadena Junior College Marjorie met and began dating fellow pupil Aden Meinel, urging him to study astronomy.

In 1941 she began studying astronomy at the University of California, Berkeley, and in 1942 transferred to Pomona College in Claremont, California to be closer to family. She also began teaching some classes at the university, including training airmen in celestial navigation. For her masters research at Claremont Colleges she used the family telescope in 1943–1944 to study the variable red giant RT Cygni. Her thesis research was published by the Astronomical Society of the Pacific. In 1944 she married Aden. From 1944 to 1945 she worked as a research associate at the California Institute of Technology working on military rockets. She also worked in the editorial staff of the Office of Scientific Research and Development preparing and editing reports, including reports for the Manhattan Project. She resigned when the war ended.

In 1946 she moved with Aden to Berkeley, California, where he was to continue his studies. Later that year she gave birth to the first of seven children, the final child born in 1957. While raising their children she was not employed, but remained scientifically active, collaborating in Aden's research, attending conferences, and editing their work.

Meinel co-authored two papers during this period, both on the optical effects of volcanic eruptions on the atmosphere. The Meinel's 1967 paper was the first to suggest that the optical effects on sunsets and sunlight in the years after a volcanic eruption are caused by sulphate aerosols, rather than volcanic dust.

In 1949 the family relocated to Yerkes Observatory in Wisconsin, then in 1955 to Phoenix, Arizona while Aden established the new Kitt Peak National Observatory, and then in 1961 to Tucson where Aden began working at the University of Arizona.

In 1970 Aden took a sabbatical so the couple could spend their time exploring barriers to the uptake of solar energy technology. In 1972, Meinel returned to academic employment, working as a research associate at the University of Arizona Optical Sciences Centre, which Aden had founded in 1964. The Meinels continued their solar research throughout the 1970s as part of a team at the university and experimented in developing prototype solar collectors.