MAA Glossary

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Aberration, chromatical:
Imperfect correction of a compound lens, producing a colored fringe around the image of a star.

Aberration, spherical:
A lens imperfection in which the focal plane for rays passing near center of lens differs from that for rays close to the edge.

Aberration of light:
Shift in position of a star as the result of the compounding of motion of earth with the finite velocity of light.

Almagest:
One of the early star catalogs, by the Alexandrian astronomer Claudius Ptolemy. Its name, which signifies The Syntax or The Ordering of Nature, comes to us from the Arabs, who preserved it for posterity.

Altitude:
Angle of elevation of a star above horizon.

Aperture:
Diameter of a telescope lens or mirror.

Aphelion:
Point on a orbit farthest from the sun.

Apogee:
Point in orbit of the moon or artificial satellite farthest from the earth.

Apparent SolarTime (AST):
Sundial time. To derive the Local Solar Time (LST), add the Equation of Time (ET).

Asterism:
Distinctive group of stars not one of the 88 recognized constellations. (To see the list of Asterisms click here.)

Asteroid:
Minor Planet.

Astrology:
A pseudo-science, based on ignorance an superstition, wherein the stars are supposed to influence events on the earth.

Astronomical Unit:
Mean distance of the earth from the sun -- one half of the major axis of the earth's orbit.

Astronomy:
Science of the heavenly bodies and the universe.

Azimuth:
Angle measured from the south point of the horizon toward the west to the foot of the star's vertical circle. See also Bearing