Once Uranus was discovered, astronomers set about charting its orbit and quickly discovered a small discrepancy between the planet's predicted position and where they actually observed it. Try as they might, astronomers could not find an elliptical orbit that fit the planet's trajectory to within the accuracy of their measurements. Half a century after Uranus's discovery, the discrepancy had grown to a quarter of an arc minute, far too big to be explained away as observational error.

The logical conclusion was that an unknown body must be exerting a gravitational force on Uranus—much weaker than that of the Sun, but still measurable. But what body could this be? Astronomers realized that there had to be another planet in the solar system perturbing Uranus's motion.

In the 1840s two mathematicians independently solved the difficult problem of determining this new planet's mass and orbit. A British astronomer, John Adams, reached the solution in September 1845; in June of the following year, the French mathematician Urbain Leverrier came up with essentially the same answer. British astronomers seeking the new planet found nothing during the summer of 1846. In September a German astronomer named Johann Galle began his own search from the Berlin Observatory, using a newly completed set of more accurate sky charts. He found the new planet within one or two degrees of the predicted position—on his first attempt. After some wrangling over names and credits, the new planet was named Neptune, and Adams and Leverrier (but not Galle!) are now jointly credited with its discovery.

Neptune's orbital and physical properties are listed in the Neptune Data box. With an orbital semi-major axis of 30.1 A.U. and an orbital period of 163.7 years, Neptune has not yet completed one revolution since its discovery. Unlike Uranus, distant Neptune cannot be seen with the naked eye, although it can be seen with a small telescope—in fact, according to his notes, Galileo might actually have seen Neptune, although he had no idea what it really was at the time. Through a large telescope, Neptune appears as a bluish disk, with a maximum angular diameter of 2.4" at opposition.

Figure 13.3 shows a long Earth-based exposure of Neptune and its largest moon, Triton. Neptune is so distant that surface features are virtually impossible to discern. Even under the best observing conditions, only a few markings can be seen. These are suggestive of multicolored cloud bands—light bluish hues seem to dominate. With Voyager 2's arrival, much more detail emerged, as shown in Figure 13.4. Superficially, at least, Neptune resembles a blue-tinted Jupiter, with atmospheric bands and spots clearly evident.

Figure 13.3 Neptune and two of its moons, Triton (large arrow) and Nereid (small arrow), imaged with a large Earth-based telescope.

Figure 13.4 (a) Neptune as seen by Voyager 2, from a distance of roughly 1 million km. A closer view (b), resolved to about 10 km, shows cloud streaks ranging in width from 50 to 200 km.