| Aphelion | The Aphelion occurs when a planet is furthest to the Sun during its slightly elliptical orbit. | 
Aphelion of Earth |
| Deimos | The smaller and outermost of the planet Mars' two moons. Deimos was discovered by Asaph Hall on August 11, 1877. In Greek mythology, Deimos was the son of Ares (Mars) and Aphrodite (Venus) and was the personification of "Panic". |
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| Mars | Mars is the fourth planet from the Sun. Mars' diameter is 6,790 km and takes 687 days to orbit the Sun. Mars is named after the Roman god of war. In Eastern Asian cultures Mars is also known as the 'Fire Star.' |
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| Martian | Having to do with the planet Mars. |
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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 |
| Phobos | The larger and innermost of the planet Mars' two moons. Phobos was discovered by Asaph Hall on August 17, 1877. In Greek mythology, Phobos was the son of Ares (Mars) and Aphrodite (Venus) and was the personification of "Fear". |
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