| Aphelion | The Aphelion occurs when a planet is furthest to the Sun during its slightly elliptical orbit. | 
Aphelion of Earth |
| Elongation | Elongations occur when an inner planet's position, in its orbital path, is at tangent to the view from Earth. Because these inner planets are inside the Earth's orbits their positions as viewed from the Earth are never very far from the position of the Sun. When a planet is at Elongation, it is furthest from the Sun as viewed from Earth, so it's view is best at that point. There are two kinds of Elongations: The Eastern Elongation occurs when the planet is in the evening sky and the Western Elongation Occurs when a planet is in the morning sky. | 
Elongation |
| Mercurian | Having to do with the planet Mercury. |
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| Mercury | Mercury is the closest planet to the Sun. Mercury's diameter is 4880 km and takes 88 days to orbit the Sun. Mercury is named after the Roman messenger god. In Eastern Asian cultures Mercury is also known as the 'Water Star.' |
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| Perihelion | The Perihelion occurs when a planet is closest to the Sun during its slightly elliptical orbit. | 
Perihelion of Earth |
