The Baade-Wesselink method is a way of calculating the sizes and other properties of certain kinds of pulsating star, including Cepheid variables. Measurements of a star's color and light output at two different times, t1 and t2, in the pulsation cycle are used to find the ratio of the star's radii, R(t2)/R(t1), at these times. Spectra of the star throughout its pulsation period are then used to find the radial velocity of its surface. Knowing how fast the star's surface is moving, the difference R(t2) - R(t1) is found by adding the products of velocity and time during the interval t1 to t2. Given the values of both R(t2)/R(t1) and R(t2) - R(t1), it is easy to solve for the radii. The method is named after Walter Baade and the Dutch astronomer Adriaan Jan Wesselink (1909–1995).