Neptune is the third most massive planet in the Solar System and, like Uranus, may be described as an ‘ice giant’. It is too faint to be seen with the naked eye, but binoculars show it easily, and a small telescope will reveal its pale bluish disc. Data are given in Table 12.1.

MOVEMENTS

Neptune is a slow mover; it takes almost 165 years to complete one journey round the Sun so that it was discovered less than one ‘Neptunian year’ ago. Opposition dates are given in Table 12.2. Some close planetary conjunctions involving Neptune are listed in Table 12.3: occultations by the Moon can of course occur.

EARLY OBSERVATIONS

Neptune was observed on several occasions before being identified as a planet. The first observation seems to have been made by Galileo on 27 December 1612. While drawing Jupiter and its four satellites, he recorded a ‘star’ which was certainly Neptune. He again saw it twice in January 1613, and noted its movement but, not surprisingly, mistook it for a star. His telescope had a magnification of ×18 and a resolving power of 190 arcsec, with a field of view 17 arcmin in diameter. Neptune's magnitude was 7.7, and Galileo often plotted stars fainter than that. Jupiter actually occulted Neptune in 1613.

The next telescopic observation was made in May 1795 by J. J. de Lalande, but again Neptune was mistaken for a star.