Neptune is the only giant planet that is not visible without a telescope. Having an apparent magnitude of 7.8, it is approximately one-fifth as bright as the faintest stars visible to the unaided eye. Hence, it is fairly certain that there were no observations of Neptune prior to the use of telescopes. Galileo is credited as the first person to view the heavens with a telescope in 1609. His sketches from a few years later, the first of which was made on Dec. 28, 1612, suggest that he saw Neptune when it passed near Jupiter but did not recognize it as a planet.

Prior to the discovery of Uranus by the English astronomer William Herschel in 1781, the consensus among scientists and philosophers alike was that the planets in the solar system were limited to six—Earth plus those five planets that had been observed in the sky since ancient times. Knowledge of a seventh planet almost immediately led astronomers and others to suspect the existence of still more planetary bodies. Additional impetus came from a mathematical curiosity that has come to be known as Bode’s law, or the Titius-Bode law. In 1766 Johann Daniel Titius of Germany noted that the then-known planets formed an orderly progression in mean distance from the Sun that could be expressed as a simple mathematical equation. In astronomical units (AU; the mean Sun-Earth distance), Mercury’s distance is very nearly 0.4; the distances of Venus, Earth, Mars, Jupiter, and Saturn are approximately 0.4 + (0.3 × 2ⁿ), in which n is 0, 1, 2, 4, and 5, respectively, for the five planets. The astronomer Johann Elert Bode, also of Germany, published the law in 1772 in a popular introductory astronomy book, proposing that the missing 3 in the progression might indicate an as-yet-undiscovered planet between Mars and Jupiter.

The suggestion was met with little enthusiasm until the mean distance of Uranus, at 19.2 AU, was noted to be very nearly equal to that predicted by Bode’s law (19.6 AU) for n = 6. Moreover, when the first asteroids, beginning with the discovery of Ceres in 1801, were found to be in orbit between Mars and Jupiter, they satisfied the n = 3 case of the equation.

Some astronomers were so impressed by the seeming success of Bode’s law that they proposed the name Ophion for the large planet that the law told them must lie beyond Uranus for the n = 7 case, at a distance of 38.8 AU. In addition to this scientifically unfounded prediction, observations of Uranus provided actual evidence for the existence of another planet. Uranus was not following the path predicted by Newton’s laws of motion and the gravitational forces exerted by the Sun and the known planets. Furthermore, more than 20 recorded prediscovery sightings of Uranus dating back as far as 1690 disagreed with the calculated positions of Uranus for the respective time at which each observation was made. It appeared possible that the gravitational attraction of an undiscovered planet was perturbing the orbit of Uranus.

In 1843 the British mathematician John Couch Adams began a serious study to see if he could predict the location of a more distant planet that would account for the strange motions of Uranus. Adams communicated his results to the astronomer royal, George B. Airy, at Greenwich Observatory, but they apparently were considered not precise enough to begin a reasonably concise search for the new planet. In 1845 Urbain-Jean-Joseph Le Verrier of France, unaware of Adams’s efforts in Britain, began a similar study of his own.

By mid-1846 the English astronomer John Herschel, son of William Herschel, had expressed his opinion that the mathematical studies under way could well lead to the discovery of a new planet. Airy, convinced by Herschel’s arguments, proposed a search based on Adams’s calculations to James Challis at Cambridge Observatory. Challis began a systematic examination of a large area of sky surrounding Adams’s predicted location. The search was slow and tedious because Challis had no detailed maps of the dim stars in the area where the new planet was predicted. He would draw charts of the stars he observed and then compare them with the same region several nights later to see if any had moved.

Le Verrier also had difficulty convincing astronomers in his country that a telescopic search of the skies in the area he predicted for the new planet was not a waste of time. On September 23, 1846, he communicated his results to the German astronomer Johann Gottfried Galle at the Berlin Observatory. Galle and his assistant Heinrich Louis d’Arrest had access to detailed star maps of the sky painstakingly constructed to aid in the search for new asteroids. Galle and d’Arrest identified Neptune as an uncharted star that same night and verified the next night that it had moved relative to the background stars.

Although Galle and d’Arrest have the distinction of having been the first individuals to identify Neptune in the night sky, credit for its “discovery” arguably belongs to Le Verrier for his calculations of Neptune’s direction in the sky. At first the French attempted to proclaim Le Verrier as the sole discoverer of the new planet and even suggested that the planet be named after him. The proposal was not favourably received outside France, both because of Adams’s reported contribution and because of the general reluctance to name a major planet after a living individual. Neptune’s discovery was eventually credited to both Adams and Le Verrier, although it now appears likely that Adams’s contribution was less substantial than earlier believed. It is nevertheless appropriate that the more traditional practice of using names from ancient mythology for planets eventually prevailed.

The discovery of Neptune finally laid Bode’s law to rest. Instead of being near the predicted 38.8 AU, Neptune was found to be only 30.1 AU from the Sun. This discrepancy, combined with the lack of any scientific explanation as to why the law should work, discredited it. The discovery in 1930 of Pluto, regarded as the ninth planet at the time, at a distance of 39.5 AU was even more at variance with the equation’s prediction of 77.2 AU for n = 8. Not even the proximity of Pluto’s mean distance to the 38.8 AU predicted for n = 7 could resurrect the credibility of Bode’s law.

Later observations from Earth

Earth-based observations of Neptune before Voyager 2’s flyby suffered greatly as a consequence of the planet’s enormous distance from both Earth and the Sun. Its average orbital radius of 30.1 AU means that the sunlight reaching its moons and its upper atmosphere is barely 0.1 percent as bright as that at Earth. Pre-Voyager telescopic viewing of Neptune through the full thickness of Earth’s atmosphere could not resolve features smaller than about one-tenth of Neptune’s diameter, even under the best observing conditions. Most such observations concentrated on determining Neptune’s size, mass, density, and orbital parameters and searching for moons. In the early 21st century specialized interferometric techniques have routinely improved spatial resolution of distant objects by factors of 10–100 over earlier surface-based observations.

From time to time astronomers reported seeing visual markings in the Neptunian atmosphere, but not until the use of high-resolution infrared charge-coupled device (CCD) cameras (see telescope: Charge-coupled devices) in the 1980s could such observations be repeated with enough consistency to permit determination of an approximate rotation period for Neptune. Spectroscopic observations from Earth revealed the presence of hydrogen and methane in the planet’s atmosphere. By analogy with the other giant planets, helium was also expected to be present. Infrared and visual studies revealed that Neptune has an internal heat source.

By the mid-1990s the fully operational Hubble Space Telescope (HST) was enabling images and other data concerning Neptune to be collected outside the filtering and distorting effects of Earth’s atmosphere. The orbiting infrared Spitzer Space Telescope also succeeded in imaging Neptune with a resolution much higher than those available from Earth’s surface in the 1980s. In addition, astronomers have developed techniques for minimizing the effects of atmospheric distortion from Earth-based observation. The most successful of these, known as adaptive optics, continually processes information from infrared star images and applies it nearly instantaneously to correct the shape of the telescope mirror and thereby compensate for the distortion. As a consequence, large Earth-based telescopes now routinely achieve resolutions better than those of the HST. Images of Neptune obtained with adaptive optics allow studies of this distant planet at resolutions approaching those from the Voyager 2 encounter.