This image, taken from the Voyager 2 flyby, shows Neptune as a blue planet, similar in color to Uranus but with more visible surface features. Although usually considered the eighth planet from the Sun, Neptune sometimes lies beyond Pluto, whose eccentric orbit brings that planet closer. 
    Because Neptune is so far from the Sun—about 4.5 billion kilo- meters—it was long a mystery to astronomers. Seen from Earth, even through the most powerful telescopes, it is only a faint blue disk. The planet was not dis- covered until 1846, by the Prussian astronomer Johann Galle at the Berlin Observatory. Its discovery was a triumph for Newtonian celestial mechanics, for its position had been predicted to within 1