Stephen Alexander (September 1, 1806 – June 25, 1883) was an American astronomer and educator.

[1] He was the brother-in-law of Joseph Henry, the first secretary of the Smithsonian, and worked closely with him.

[3] Alexander relied on the assistance of a free African American man named Alfred Scudder, who worked for him at Princeton during the 1850s.

[1] He was one of the original members of the National Academy of Sciences in 1862, and a member of the American Philosophical Society, and the American Association for the Advancement of Science.

His principal writings are "Physical Phenomena attendant upon Solar Eclipses", read before the American Philosophical Society in 1848; a paper on the "Fundamental Principles of Mathematics," read before the American Association for the Advancement of Science in 1848; another on the "Origin of the Forms and the Present Condition of some of the Clusters of Stars and several of the Nebulae", read before the American Association in 1850; others on the "Form and Equatorial Diameter of the Asteroid Planets" and "Harmonies in the Arrangement of the Solar System which seem to be Confirmatory of the Nebular Hypothesis of Laplace", presented to the National Academy of Science; and a "Statement and Exposition of Certain Harmonies of the Solar System", which was published by the Smithsonian Institution in 1875.