pair of stars revolving about their common centre of mass in an orbit whose plane passes through or very near the Earth. An observer on the Earth thus sees one member of the binary pass periodically over the face of the other and diminish its light through an eclipse. The star Algol was the first recognized as an eclipsing binary, by John Goodricke (Goodricke, John), in 1782. Thousands are now known. By combining measurements of the brightness variations with spectroscopic information for both stars of the pair, astronomers can determine the mass and size of each star. See also variable star; binary star.