THE 25 GREATEST ASTRONOMICAL FINDINGS of all time, according to the editors of Astronomy magazine (October 1998) are as follows: the discovery of quasars (1963); the cosmic microwave background (1965-66); pulsars (1967); Galileo's observations of the phases of Venus, Jupiter's moons, and craters on the moon (c 1609); extrasolar planets (1992); supermassive black holes (early 1990s); Newton's Principia, formulating the mathematics of our heliocentric system (1687); the discovery of Uranus (1781); the first known asteroid (1801); discovery of Pluto (1930); Neptune (1846); spectroscopic proof that nebulae are gaseous in nature (1864); recognition of galaxies beyond our own (1923); the advent of radio astronomy (1931-32); studies of globular clusters help to map the Milky Way (1918); cometary explosion over Siberia (1908); an accurate measurement of the speed of light (1675); Southern Hemisphere celestial objects cataloged (1834-38); Cepheid-variable period- luminosity relationship worked out (1912); Copernicus' De Revolutionibus sets forth the heliocentric system (1543); Laplace's theory on how the solar system formed (1796); a transit of Venus suggests Venus has an atmosphere (1761); the Hertzsprung-Russell diagram for understanding how stars age (1913); scheme for classifying star types (1890); the use of parallax for finding a star's distance from Earth (1838).