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Astronomy & Telescopes Glossary - A
Aberration - Any optical defect that causes light to deviate from reaching the focal point of a lens, therefore reducing the quality of the image.
Absolute magnitude - The brightness a star would have if located away 10 parsecs from the Earth.
Achromatic Lens - A lens made of two or three separate elements which has the effect of bringing most colors to a focus, thus reducing chromatic aberration.
Altazimuth - A simple telescope mount that allows movement in altitude (north-south) and in azimuth (east-west). Also called alt-az mounts.
Altitude - Altitude is the angle of an object from the horizon. If an object is on the horizon, its altitude is 0 degrees. If it is at the zenith, its altitude os 90 degrees.
Amor asteroid - Amors are asteroids located between the orbits of Earth and Mars.
Analemma - The analemma is a figure 8-shaped diagram that shows the declination of the sun for each day in the year. If you took a snapshot of the the sun at the same time each day and from the same location, it would form this shape.
Annular eclipse - An annular eclipse is a type of solar eclipse. During an annular eclipse, the Sun looks like an "annulus" or ring. The ring is visible when the Moon does not entirely cover the disc of the Sun during the eclipse. This type of eclipse happens when the Sun is at perihelion (closest to the Earth) and the Moon is at apogee (farthest from the Earth).
Apastron - The point of greatest separation of two stars in a binary star system.
Aperture - The diameter of the primary mirror or lens of a telescope.
Aphelion - The aphelion is the point of orbit which is farthest from the sun.
Apoasis - For an object orbiting another object, the apoapsis is the point of orbit of one body which is farthest from the other body.
Apochromatic - A type of lens comprised of three elements of different materials, which has the effect of bringing most colors to a focus, thus reducing chromatic aberration. A step up from achromatic lenses.
Apogee - For an object orbiting the Earth, the point in orbit which is farthest from the Earth.
Apparent magnitude - Measure of the brightness of a celestial object as seen from Earth. The lower the number, the brighter the object. Negative numbers indicate extreme brightness.
Apparent motion - The observed motion of astronomical objects from the Earth.
Arc minute - A measure equal to one-sixtieth of a degree (there are 360 degrees in a circle).
Arc second - A measure equal to one-sixtieth of an arc-minute.
Asterism - A group of stars that appear to make a recognizable shape, such as the Big Dipper.
Asteroid - A large rocky object, also called a minor planet or planetoid. Most asteroids orbit the Sun in the asteroid belt between Mars and Jupiter.
Asteroid Belt - The asteroid belt is the concentration of asteroids orbiting the Sun between the orbits of Mars and Jupiter.
Astrometry - The precise measurement of the position and motion of astronomical objects.
Aten asteroid - Asteroids that are closer to the Sun than the Earth is.
Aurora - Auroras are undulating sheets of light in the northern sky. They are caused by gases that become excited after being hit by solar particles.
Averted vision - A method of detecting very faint objects using the sensitive low-light rod cells of your eyes that are located around the sides of your eyes. To see faint objects, don't look straight at them but rather look at them out of the corners of your eyes.
Azimuth - Azimuth is the angle of an object from an observer's north point projected onto the horizon. If an object is due north, its azimuth is 0 degrees. If it is due east, its azimuth is 90 degrees.