A method of determining the distances of certain types of pulsating variable star, particularly Cepheids and RR Lyraes. It is jointly named after W. Baade, who first proposed it in 1926, and the Dutch astronomer Adriaan Jan Wesselink (1909–95), who refined it in 1946. In the original method, the variation in angular diameter of a Cepheid as it pulsates is inferred by means of model atmosphere calculations from the measured changes in its brightness. Spectroscopy is used to measure the corresponding changes in radial velocity, providing the actual distance over which the star's surface has moved. By dividing the measures of angular and linear diameter, the distance to the star is obtained. More recently, it has proved possible to measure the angular diameter of the pulsating star directly using optical interferometers, thus allowing a more accurate measurement of the star's distance. This newer approach is known as the geometric Baade–Wesselink method.
Subjects: Astronomy and Astrophysics.