| Term | Meaning |
| Abberation of starlight | The tiny apparent displacement of stars resulting from the motion of the Earth though space. |
| Absolute magnitude | The magnitude that would be assigned to a star if it were placed at a distance of 10 parsecs (or 32.6 light years) from the observer. The sun, for example, has an absolute magnitude of 4.7. |
| Absolute visual magnitude | The absolute magnitude of an object measured through a special yellowish filter that approximates the visual range of the human eye. |
| Absorption nebula | A nebula seen in silhouette as it absorbs light from behind; also called a dark nebula. |
| Achromatic lens | A lens that transmits white light without dispersing it into a color spectrum. It usually consists of two component parts, cemented together to form one unit. |
| Albedo | Percentage of light reflected by a body, such as a planet, from the total amount of light falling on it. |
| Altitude | Angular distance between the horizon and given object, measured along a vertical circle. |
| Analemma | The figure 8 representing the equation of time and the variation of the sun’s altitude in the sky during the course of a year. |
| Angstrom | A unit of wavelength or distance, equivalent to 1/10,000 micrometer or 1/10,000,000,000 meter. |
| Annular eclipse | An eclipse of the central portion of the solar disk; an outer ring shows. |
| Aphelion | The point on a planet’s orbit farthest from the Sun. |
| Apogee | Point on the moon’s orbit farthest from the Earth. |
| Apollo | The name assigned to the U.S. project whose mission was to land men on the moon. Also the name of the vehicles used. Apollo II landed Neil Armstrong and Edwin Aldrin on the moon on July 20, 1969. |
| Apparent magnitude | Magnitude as seen by an observer. |
| Apparent solar time | Time determined by the actual position of the sun in the sky; corresponds to time on most sundials. |
| Ascending Node | The point in the Moon's orbit where it rises above the ecliptic. |
| Asterism | A noticeable pattern of stars that makes up part of one or more constellations; not a constellation itself. |
| Asteroid | A minor planet, smaller than any major planet in our solar system; not one of the satellites (moons) of a major planet such as the Earth or Jupiter. |
| Astronomical unit | The average distance between the Earth and the sun 93 million miles or, more exactly, 92,955,700 |
| Aurora | A diffused glow of light in the form of curtains, or bands, seen at high latitudes (70ºN or 70ºS). The glow is due to the interaction between the solar wind and the particles in the Earth’s atmosphere. The Aurora in the northern hemisphere is known as the Aurora Borealis, or northern lights; in the southern hemisphere it is known as the Aurora Australis, or southern lights. |
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