Accidental (house) Ruler: a planet ruling a house by virtue of it’s ruling the sign on the cusp of that house (see "Ruler" below).

Accidentally Dignified: said of a planet conjunct an Angle. Such a planet is most projected, manifest or noticeable, and is often the most influential planet in the chart.

Adaptive Optics (AO): A primary issue confronting large, modern telescopes is the turbulence of the Earth's atmosphere through which their images must necessarily pass before reaching the telescope. The larger the telescope, the more extreme this problem becomes. This is why nowadays all large telescopes are put on the tops of mountains particularly selected for the lack of turbulence in the atmosphere above them, as well as far from light pollution. It is also why the Hubble Space Telescope (and its planned successors) is in outer space.

Another way to address this problem, however, is referred to by some as the most revolutionary advance in astronomy since Galileo's 1609 introduction of the telescope itself. Known as adaptive optics, it is a procedure of measuring the atmospheric distortion in a celestial object's light beam, and then sending signals to deform a multitude of mirrors in the light path to exactly compensate for the atmospheric turbulence. This can be accomplished by dividing the light reflected from the telescope mirror into hundreds of smaller beams, each of which is examined for deviation due to turbulence from a standard. The system's electronic circuitry computes the shape of many small mirrors, so that when each is altered their individual distorted beams realign in the same direction. Then these deformable mirrors are sent a signal that in fact does change their shapes, producing a composite, undistorted beam.

In order to know how to deform the deformable mirror, the shape of both the distorted and undistorted image need to be known. The undistorted shape for stars is usually just a point without detail. In fact, since stars are so far away anything more than just a geometrical point must be an artifact of the optical system. To determine the final distorted shape of an ideal point source once it reaches the Earth’s surface, adaptive optics systems initially used a bright star near the telescope’s target as a comparison source for the distorted wavefront. This, however, required that the object being observed was close to a relatively bright star, which is usually not the case.

Subsequent adaptive optical techniques use a laser beam instead of a guide star. There's a naturally occurring layer of sodium atoms 60 miles above the Earth's surface (in the mesospheric layer) whose atoms fluoresce when excited by a laser beam. A laser directed into the sky very close to the direction of a celestial object will cause the mesosphere's sodium atoms to fluoresce, and that directional location, if you will, will become a "guide star" for the celestial object of interest. This allows any object to be observed even when no bright stars are nearby.

Afflicted: said of a planet in a sign contrary to it’s nature, as Moon in Capricorn or Scorpio; Mars in Libra, Taurus or Cancer; or Venus in Aries, Virgo or Scorpio; OR with a hard aspect from Mars, Saturn, Uranus, Neptune or Pluto. Jupiter afflictions are not as severe, and denote excesses or poor judgment.

Air: one of the four elements (earth, air, fire and water) by which the signs and the houses are categorized. The element of air represents thought, relationship, and the breath. For its application in the signs and houses, see below.

Air house: the third, seventh and eleventh houses; houses with the same ordinal numbers as the three air signs.

Air sign: The element of thought, knowledge, relationship, and all aspects of the mind, as exemplified by the Air signs Gemini, Libra and Aquarius. If a liquid is continuously heated, there comes a point at which the average vibrational energy of the constituent particles so exceeds their bonding energy that they repel each other. At this point the liquid boils and becomes a gas—the counterpart of the astrological element "air." See "plasma" for an explanation of temperature, absolute zero, and the four states of matter.

Angle(s): the first, fourth, seventh or tenth house cusps, formed from the intersection of the local horizon and meridian circles with the ecliptic.

Angular: the first, fourth, seventh or tenth houses, or said of a planet therein; a planet in these houses is very strong, and if conjunct an angle (orb about 3° - 6°) its energy is probably the most projected or noticeable in the chart; the closer to the angle, the stronger this effect is.

Aphelion: the point in its orbit where an object is furthest from the Sun (opposite of perihelion); the term apogee is used for objects orbiting the Earth; "apoapsis" is used for the furthest point in orbit around other bodies.

APOD: "Astronomy Picture of the Day;" an archive of popular astronomical photographs (from Hubble, space missions and other sources) updated daily (http://antwrp.gsfc.nasa.gov/apod/archivepix.html) on the internet and maintained since June 16, 1995 by Robert Nemiroff and Jerry Bonnell of NASA.

Apogee: the point in its orbit where an object is furthest from the Earth (opposite of perigee).

Applying: aspects that become closer or more exact with time; stronger than a separating aspect.

Apsis: that point in the orbit of a planet or moon nearest to (lower apsis) or farthest from (higher apsis) the center of attraction; the "line of apsides" is the major (or long) axis of an elliptical orbit.

Arc second ("): a unit of angular measure dividing a minute of arc into 60 equal parts, and the apparent diameter of a dime as seen from 2.3 miles away. Since there are 60 angular minutes (symbol ' ) in one degree, and 60 angular seconds (symbol ") in one angular minute, there are therefore 3600 seconds in one degree. As seen from the Earth, Saturn and Uranus have angular diameters of 20" and 3" respectively. The closest star in the sky, Alpha Centauri, and the brightest star in the sky, Sirius, both have angular diameters of 0.007".

Ascendant (Asc): the intersection of the local eastern horizon with the ecliptic; where planets rise or ascend in the east; the beginning or cusp of the first house; that part of our self with which we most identify in our ego; where we project our personality.