Sergey Nikolaevich Blazhko (Сергей Николаевич Блажко in Russian) (November 5(17), 1870 - February 11, 1956, Moscow) was a Russian and Soviet astronomer, a corresponding member of the Soviet Academy of Sciences (1929). He was a graduate of Moscow University and held a number of positions there including head of the Moscow Observatory from 1920-1931. He discovered a secondary variation of the amplitude and period of some RR Lyrae stars and related pulsating variables, now known as the Blazhko effect.
Sergey Blazhko was awarded the USSR State Prize (1952), two Orders of Lenin, two other orders and numerous medals. The crater Blazhko on the Moon is named after him.