US astronomer who worked on stellar distances and the radial motions of stars. He observed variable stars and classified them according to their spectra; he also determined the distance and direction of the centre of the Galaxy and attempted to calculate its rotation period.
Born in Greenville, Illinois, Joy attended Greenville College.
From 1904 to 1914 he worked at the American University of Beirut, Lebanon, becoming professor of astronomy and director of the observatory. He returned to the USA 1914, and worked at the Mount Wilson Observatory 1915-52.
Together with his colleagues Walter Adams and Milton Humason at Mount Wilson, Joy ascertained the spectral type, absolute magnitude, and stellar distance of more than 5,000 stars.
Joy and his colleagues studied the Doppler displacement of the spectral lines of some stars to determine their radial velocities. They showed that many stars are spectroscopic binary stars, and deduced not only their period and orbit, but also the absolute dimensions, masses, and orbital elements of some specific stars within eclipsing binary systems.
Joy later became interested in the parts of the Galaxy where dark, absorbing clouds of gas and dust exist, and in these areas he found examples of a particular kind of variable star, called a T Tauri star. Such stars appear to be in an early stage of their evolutionary history.