| 1. | Aberration | The apparent angular displacement of the position of a celestial body in the direction of |
| 2. | Accretion | Accumulation of dust and gas into larger bodies. |
| 3. | Adrastea | A small satellite of Jupiter having an orbital radius of 80,140 milesand radial dimensions of 7,6, and 5 miles. |
| 4. | Advance of the perihelion | The slow rotation of the major axis of a planet's orbit in the direction of the planets revolution, due to gravitational interactions with other planets and other effects such as those More… |
| 5. | Albedo | Reflectivity of an object; ratio of reflected light to incident light. |
| 6. | Albedo feature | A dark or light marking on the surface of an object that might not be a geological or topographical feature. |
| 7. | Allocthonous | (1) Material that is formed or introduced from somewhere other than the place it is presently found. (2) Fragmented rock thrown out of the crater during its formation that either falls back More… |
| 8. | Alpha Centauri | The closest bright star to our solar system. |
| 9. | Angstrom | A unit of length = 1.0E-08cm. |
| 10. | Antipodal point | The point that is directly on the opposite side of the planet; e.g., the Earth's north pole is antipodal to its south pole. |
| 11. | Aphelion | The point in its orbit where a planet is farthest from the Sun. |
| 12. | Apoapsis | The point in orbit farthest from the planet. |
| 13. | Apogee | The point in orbit farthest from the Earth. |
| 14. | Ash | The fine-grained material produced by a pyroclastic eruption. An ash particle is defined to have a diameter of less than 2 millimeters. |
| 15. | Asteroid number | Asteroids are assigned a serial number when they are discovered; it has no particular meaning except that asteroid N+1 was discovered after asteroid N. |
| 16. | Astronomical unit (AU) | The average distance from the Earth to the Sun; 1 AU is 149,597,870 kilometers (92,960,116 miles). |
| 17. | Atmosphere | One atmosphere is 14.7 pounds per square inch (105 Newtons per square meter); the average atmospheric pressure at sea level on Earth. |
| 18. | Aurora | A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun. |
| 19. | Aurora borealis | The Northern Lights caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere; a similar effect happens in the southern hemisphere where it More… |
| 20. | Bar | A unit of pressure, equal to the sea-level pressure of Earth's atmosphere; 1 bar is equivalent to 0.987 atmosphere or 10,000 newtons per square meter. |
