Observations From Earth
Our knowledge about Neptune came slowly before the 1989 Voyager 2 flyby, due to its remoteness. Even when it is closest, telescopic observations show Neptune as a small, featureless bluish-green disk of only 2.35" apparent diameter and 7.7 stellar magnitude. Spectroscopic observations showed that Neptune's color is produced by the absorption of sunlight by methane gas in its atmosphere; this is also true for Uranus. Observations of occultations (similar to eclipses) of stars by Neptune indicated that its atmosphere is mostly composed of molecular hydrogen and helium, which are also the main components of the other Jovian planets. These gases are inconspicuous in their visible spectra.
Soon after its discovery, it was found that Triton's orbit around Neptune is retrograde, meaning that, as seen from above Neptune's north pole, Triton revolves around
Neptune clockwise instead of counterclockwise (direct motion). If one views the solar system from above the Sun's north pole, all the planets have direct (counterclockwise) revolutions around the Sun, and most of them have direct rotations; the exceptions are Venus, Uranus, and possibly Pluto. Infrared (at wavelengths longer than those of red light) observations of Triton since 1980 indicated the presence of an atmosphere containing methane, and the presence of nitrogen in solid or liquid form on its surface. However, its size and mass remained poorly known.
G.P. Kuiper discovered a second satellite, Nereid, in 1949. It is a small, faint (nineteenth magnitude) object in an orbit around Neptune that is distant (3,423,821 mi [385,513,400 km] mean distance) from it, very elliptical (eccentricity 0.756), and highly inclined (29° to Neptune's equator). A third satellite was suspected at about a 45,954 mi (74,000 km) distance from Neptune's center because of a simultaneous 8.1 second decrease in the brightness of a star observed simultaneously from two locations 4 mi (6 km) apart in May 1981.
From 1982, Neptune was suspected to have rings like the other Jovian planets. However, confirmation of suspected occultations of stars by rings of Neptune was not definite; occultations by possible rings were only observed sometimes. This led to the theory that Neptune had a set of incomplete arc rights which were maintained by gravitational perturbations from one or more" shepherd satellites" which were still undiscovered.
Efforts to determine Neptune's rotation period from Earth-based observations using spectroscopy to find it from Doppler shifts of spectral lines across Neptune's disk and searches for periodic brightness changes gave conflicting and almost always incorrect results.
Earth-based infrared observations of Neptune indicated by 1974 that, unlike Uranus, Neptune emits 2.4 times as much energy at infrared wavelengths as it receives from sunlight (insolation). This led to the inference that, like Jupiter and Saturn, Neptune has a significant internal heat source. This may be produced by continuing gravitational contraction or by the settling of denser materials to the center of a planet.
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