The Discovery of Neptune

In 1846, the planet Neptune was discovered after its existence was predicted because of discrepancies between calculations and data for the planet Uranus. Astronomers found the new planet almost exactly at the position predicted by the calculations of Urbain Jean Leverrier. (John Couch Adams had also calculated the position independently; see the box below.) We illustrate the situation schematically in the adjacent diagram.

The dominant interaction between Uranus and the Sun is indicated by the broad line, but some perturbations associated with other masses are indicated by thinner lines. (However, do not be misled by the relative widths of the lines in this schematic diagram. The perturbations on the orbit of Uranus by other planets are tiny compared with the dominant interaction with the Sun.) Using Newton's laws to calculate the perturbations on the orbit of Uranus by an hypothesized new planet, Leverrier and Adams were able to predict where the planet had to be in order to cause the observed deviations in the position of Uranus. Once astronomers took this calculation seriously, they found the new planet within minutes of turning their telescopes on the region of the sky implicated by the calculations.



How Neptune Was Discovered

John Couch Adams (1819-1892) was an English astronomer. Upon completing his degree in 1843, he set about trying to account for the motion of the planet Uranus, which was known to exhibit deviations from the motion expected from Newton's laws. Adams assumed that these deviations were caused by an undiscovered planet lying outside the orbit of Uranus and used Newton's laws to work out the properties of this new planet. He sent his calculations to the Astronomer Royal, who gave it to an observer, but apparently neither took the calculation seriously since Adams was young and not yet well-established. Therefore, the observer did not search immediately for the new planet.

Meanwhile, French astronomer Urbain Leverrier (1811-1877) made the same calculations independently. He sent his calculations to the Berlin Observatory, where they were taken more seriously: on the very night that the calculations arrived, Johann Galle found Neptune after only a few minutes of searching. It was only two degrees away from where Adams had predicted the new planet would be. When Adams' calculations were announced after the discovery of Neptune, a bitter war of words broke out between the English, who hailed the accomplishment of the young scientist Adams, and the French, who suspected that Adams had plagiarized Leverrier's work. In fact, both men had correctly predicted the new planet, but the dispute went on for many years. The story of the discovery of Neptune illustrates an important truth. Science strives for objectivity, but it is practiced by humans with human frailties and these sometimes interfere with scientific objectivity.