THE ASPECTS
 
In astronomy, the angular separation between two bodies, measured along a great circle connecting the bodies. In astrology, the angular separation between two bodies, measured along a great circle (not necessarily passing through the bodies). The term aspect is also used to refer to one of a set of particular angular separations. An Aspect is the relationship between two planets, in degrees and minutes, at the happening moment. The Aspect add an element of harmony or disharmony to the horoscope, depending on the Aspect. As the planets move in their elongated orbits around the Sun, they form various angular relationships with one another, using the Earth as the centre. These are called Aspects. The most popular Aspects result from dividing the zodiac circle by numbers like 1, 2, 3, 4, resulting in Aspects such as the 0 degrees, 180 degrees, 120 degrees, 90 degrees, and so forth. When two planets form an Aspect with one another, their energies and natures are said to combine and work in harmony or discord. For example, when two planets are exactly 0 degrees on circle, they are in Conjunction, or when two planets are 180 degrees on circle, on opposite sides of the Earth, they are in opposition. 
The path described by a heavenly body in its revolution around a center of attraction is called orb. "Orb" is ‘Aspect. When an aspect is exact, it has its greatest impact. Yet, the effect of most aspects can be felt for some time before and after the moment when it is exact. The range within which an Aspect is in operation is called its "Orb of influence". An Orb of one or two degrees of arc on either side of the exact Aspect is considered a close or tight Orb, while an Orb of 9 degrees is loose.
An Aspect is a certain angular relationships between the rays which reach the Earth from two celestial bodies, or between one ray and a given point, such as:(Aspects are “Angular relationships with one another sensitive point”, when looking at the planets involved. The Aspects to the Chart angles and Planets are:) AS = Ascendant <the begin>, the horizon; and MC = Midheaven <the attain>, are very important. Aspects to the Sun and Moon (the lights) are most important, followed by the planetary Aspects involvingMercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Others Sensitive Points are: DS = Descendent. IC = Imum-Coeli. MNN = Moon North Node. MSN = Moon South Node. Vertex. Equatorial Ascendant, and Part of Fortune. Many factors enter into the delineation of the effect of an aspect, the 5 mega factors by considering an Aspect between one ray and a given point, are: 1) their latitude. 2) the nature of the aspect. 3) character andrate of motion. 4) character of the planets involved. 5) virtue of their sign position.
After many experience most astrologer authorities have agree that orbs should vary with each Planet and Aspect, and that a larger orb should be allowed for an aspect that is forming than for one that is separating. For conjunction or opposition (my experience): max 9° when the Sun aspects the Moon, about 8° when either luminary aspects a planet, and 7° for aspects between planets. Observe whether either body is in retrograde motion. The faster moving applies to the slower.

According to Ptolemy, the following orbs apply to the different bodies: Sun 17°, Moon 12°, Mercury 7,, Venus 8°, Mars 7°, Jupiter 12°, Saturn 9°, Uranus 5°, Neptune 5°. When two planets are approaching conjunction or opposition, add their respective orbs and divide by two to ascertain the arc of separation within which the aspect is supposed to be effective. For the trine and square aspects reduce the arc by one-fourth, and for the minor aspects by one-half. In all cases the closer the aspect the more powerful it becomes; also the heavier and slower moving planets are more powerful than the smaller and faster. v. Celestial sphere.(-Nicholas de Vore's Encyclopedia of Astrology) »>.

Aspects Table
The Grad, Name, symbols for the Aspects are as follows:
degrees
 name
symbol
 explain
o 360°
Conjunction
Asy or Difficult - can be either harmonious or not.
15°
Untion
Variable
18°
Semidecile
Harmonious, but weak.
20°
Seminovil
Variable
24°
Quindecile
Harmonious
30°
Semisextile
Easy
32°7
Undecile
Easy, but weak.
36°
Decile
Mildly, harmonious
40°
Novile
Variable
45°
Semisquare
Hard, very Difficulty
51°4
Septile
Mildly, favorable
60°
Sextile
Easy, but weaker.
72°
Quintile
Mildly harmonious
75°
Bidecile
Favorable
80°
Binonile
Variable
90°
Square
Hard, very Difficulty
100°
Semipentanovile
favorable
102°7
Biseptile
Mildly, favorable
108
Tredecile
Difficulty
120
Trine
Harmony
135°
Sesquiquadrate
Hard, very Difficulty
140
Semieptanovile
favorable
144°
Biquintile
Mildly harmonious
150°
Quincunx
Mildly favorable
155°
Triseptile
favorable
160°
Quatronovile
Easy,
165°
Tao
Tao
Variable
180°
Opposition
Hard aspect. Difficult - can be either harmonious or not.
Parallel
favorable
Contra-Parallel
Variable