In his 1979 science book ', astronomer Carl Sagan wrote that the high surface temperature of Venus was known prior to Velikovsky, and that Velikovsky misunderstood the mechanism for this heat. Velikovsky believed that Venus was heated by its close encounter with the Earth and Mars. He also did not understand the greenhouse effect caused by Venus' atmosphere, which had earlier been elucidated by astronomer Rupert Wildt. Ultimately, Venus is hot due to its proximity to the Sun; it does not emit more heat than it receives from the Sun, and any heat produced by its celestial movements would have long since dissipated. Sagan concludes: "(1) the temperature in question was never specified Velikovsky; (2) the mechanism proposed for providing this temperature is grossly inadequate; (3) the surface of the planet does not cool off with time as advertised; and (4) the idea of a high surface temperature on Venus was published in the dominant astronomical journal of its time and with an essentially correct argument ten years before the publication of "Worlds in Collision"" (p. 118).