Alexander William Roberts (4 December 1857–27 January 1938) was Scottish-born, South African teacher and an amateur astronomer.
He was born in Farr, which lies in the county of Sutherland, Scotland. As a youth he developed an interest in astronomy, but was dissuaded from a career by Charles Piazzi Smyth, the Astronomer Royal for Scotland. From 1877 until 1881 he served as an assistant teacher at the North School, in Wick, Scotland.
He emigrated to South Africa in 1883, where he took a teaching position at the Lovedale Missionary Institution. (He would later serve as acting Principal at the institute, then Principal at Lovedale Training School.) The following year he was married to Elizabeth Dunnett. The couple would have three children.
In 1894 he received a F.R.A.S., then a F.R.S.E. in 1898. He was awarded a Doctor of Science from the University of the Cape of Good Hope in 1899.
In South Africa he pursued his interest in astronomy, first measuring the parallax and proper motion of Alpha Centauri and Beta Centauri. He then became a prolific observer of variable stars, particularly those that were members of a binary system. He continued his observations for over thirty years, and pioneered the study of close binary systems. He published over 100 works on these topics. Between 1891 and 1920 he made over 250,000 observations of 98 variable stars.
In 1913 he served as president of the S.A.A.A.S.
Much of his work was communicated through Edward C. Pickering, the director of the Harvard College Observatory. However, with the death of Pickering in 1919 he ceased further research. Instead he began to focus on politics and race relations in South Africa. The Prime Minister Jan Smuts appointed him as a senator to represent the interests of native Africans on the Native Affairs Commission. Later in 1925, however, he did serve as the South African delegate to the I.A.U. general assembly.
The crater Roberts on the Moon is co-named for him and Sir Isaac Roberts.