| Aphelion | The Aphelion occurs when a planet is furthest to the Sun during its slightly elliptical orbit. | 
Aphelion of Earth |
| Charon | A moon of the planet Pluto, also known as Pluto I. Charon is the largest of Pluto's Moons and was discovered in 1978 by James Christy. In Greek mythology Charon was the Ferryman who ferried souls to Pluto's underworld of the dead. |
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| Hydra | A moon of the planet Pluto, also known as Pluto III. Hydra was discovered by the "Pluto Companion Search Team" in June 2005 in images taken by the Hubble Space telescope. In Greek mythology Hydra was a multi-headed dragon who guarded Pluto's underworld of the dead. |
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| Nix | A moon of the planet Pluto, also known as Pluto II. Nix was discovered by Max Mutcher in June 2005 in images taken by the Hubble Space telescope. In Greek mythology Nix was the goddess of darkness and night and mother to Charon was the Ferryman who ferried souls to Pluto's underworld of the dead. |
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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 |
| Pluto | Pluto is usually the ninth planet from the Sun although it is sometimes 8th when Pluto moves within Neptune's orbit. Pluto's diameter is 2,300 km and takes 249 years to orbit the Sun. Pluto is named after the Roman god of the underworld. Pluto was discovered on February 18, 1930 by Clyde Tombaugh. |
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| Plutonian | Having to do with the planet Pluto. |
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