GRE Reading Comprehension: Maggoosh-GRE阅读Maggoosh - 1CXH8MG7Q550578FN

Galileo's drawings show that he first observed Neptune in 1612, and again in 1613. On both occasions, Galileo mistook Neptune for a fixed star when it appeared very close – in conjunction – to Jupiter in the night sky; hence, he is not credited with Neptune's discovery. During the period of his first observation, Neptune was stationary in the sky because it had just turned retrograde that very day (retrograde refers to the apparent backward motion created when the orbit of the Earth takes it past an outer planet). Since Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileo's small telescope.