The Venusian clouds kept astronomers from seeing the planet's surface. Without visible landmarks, they could not measure how fast Venus turns on its axis. It took cloud-penetrating radar, first aimed at Venus in the early 1960s, for scientists to discover that Venus rotates backward as compared to the other planets in the solar system. This means that the Sun rises in the west and sets in the east. And Venus' rotation is extremely slow, so a "day" on Venus is longer than a year.