Bessel roughly estimated the period of the companion of Sirius to be about half a century;[15] C. A. F. Peters computed an orbit for it in 1851.[16] It was not until 31 January 1862 that Alvan Graham Clark observed a previously unseen star close to Sirius, later identified as the predicted companion.[16] Walter Adams announced in 1915 that he had found the spectrum of Sirius B to be similar to that of Sirius.[17]