| Aphelion | The Aphelion occurs when a planet is furthest to the Sun during its slightly elliptical orbit. | 
Aphelion of Earth |
| Neptune | Neptune is usually the eight planet from the Sun although it is sometimes 9th when Pluto moves within it's orbit. Neptune's diameter is 49,500 km and takes 165 years to orbit the Sun. Neptune is named after the Roman god of the sea. Neptune was discovered on September 23, 1846 by French astronomer Urbain Le Verrier. |
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| Neptunian | Having to do with the planet Neptune. |
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| Nereid | The smallest moon of Neptune was discovered by Gerard Kuiper in May 1, 1949. Nereid is also designated as "Neptune II". In Greek mythology, the Nereid were sea-nymphs. |
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| Opposition | For planets outside the Earth's orbit (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto), the months around Oppositions are the best time to view these. An Opposition occurs when the planet is opposite from the Sun, relative to the Earth. At Opposition the planet will rise as the Sun sets and will set as the Sun rises providing an entire night of observation. Also at Opposition the planet comes physically closest to the Earth in it's orbit so it appears as large as possible. | 
Opposition |
| Perihelion | The Perihelion occurs when a planet is closest to the Sun during its slightly elliptical orbit. | 
Perihelion of Earth |
| Proteus | The second-largest, moon of Neptune was discovered by Stephen P. Synnott on June 16, 1989. Proteus is also designated as "Neptune VIII" and "S/19989 N1". In Greek mythology, Proteus was the shape-changing sea god. |
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| Triton | The largest moon of Neptune was discovered by William Lassell on October 10, 1846. Triton is also designated as "Neptune I". In Greek mythology, Triton was a sea god and the son of Neptune. |
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