In astrology, an aspect is an angle the planets make to each other in the horoscope, and also to the ascendant, midheaven, descendant, lower midheaven, and other points of astrological interest. Aspects are measured by the angular distance in degrees and minutes of ecliptic longitude between two points, as viewed from Earth. According to astrological tradition, they indicate the timing of transitions and developmental changes in the lives of people and affairs relative to the Earth.
As an example, if an astrologer creates a horoscope that shows the apparent positions of the celestial bodies at the time of a person's birth (a natal chart), and the angular distance between Mars and Venus is 92° of arc, the chart is said to have the aspect "Venus square Mars" with an orb of 2° (i.e., it is 2° away from being an exact square; a square is a 90° aspect). The more exact that an aspect is, the stronger or more dominant it is said to be in shaping character or manifesting change.
In medieval astrology, certain aspects, like certain planets, were considered to be either favorable (benefic) or unfavorable (malefic). Modern usage places less emphasis on these fatalistic distinctions. The more modern approach to astrological aspects, grounded more on psychology and tendencies, is exemplified by research on astrological harmonics, of which John Addey was a major proponent, and which Johannes Kepler earlier advocated in his book Harmonice Mundi in 1619.
A list of aspects below presents their angular values and a recommended orb for each aspect. The orbs are subject to variation, depending on the need for detail and personal preferences.
Major aspects 
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The traditional major aspects are sometimes called Ptolemaic aspects since they were defined and used by Ptolemy in the 1st Century, AD. These aspects are the conjunction (approx. 0-10°), sextile (60°), square (90°), trine (120°), and opposition (180°). It is important to note that different astrologers and separate astrological systems/traditional utilize differing orbs (the degree of separation between exactitude) when calculating and using the aspects, though almost all use a larger orb for a conjunction when compared to the other aspects. The major aspects are those that can be used to divide 360 evenly and are divisible by 10 (with the exception of the semi-sextile).
A conjunction (abrv. Con) is an angle of approximately 0-10°. An orb of approximately 10° is usually considered a conjunction, however if neither the Sun or Moon is involved, some consider the conjunction to be a separation (orb) of only about 0±08°. This is said to be the most powerful aspect, intensifying the effects of the involved planets mutually — and being a major point in the chart.
Whether the union is to be regarded as "positive" or "negative" depends upon what planets are involved: Venus, Jupiter and the Sun, in any possible combination, is considered the most favourable scenario (and all three actually met on November 9–10, 1970, for example), while the most unfavourable configurations involve Mars, Saturn, and/or the Moon (with all three conjoining on March 10 in that same year). If the planets are under stress from other configurations, then the conjunction will be said to intensify the stress. When a planet is in very close conjunction to the Sun it is referred to as cazimi; when a planet is moderately close to the Sun, it is said to be combust. The Sun and Moon are in conjunction monthly during the New Moon.
Sextile — intermediate major/minor aspect 
A sextile (abrv. SXt or Sex) is an angle of 60° (1/6 of the 360° ecliptic, or 1/2 of a trine [120°]). A separation (orb) of 60±04° is considered a sextile. The sextile has been traditionally said to be similar in influence to the trine, but of less significance. It indicates ease of communication between the two elements involved, with compatibility and harmony between them, but only provides opportunity, requiring effort to gain its benefits. See information on the semisextile below.
A square (abrv. SQr or Squ) is an angle of 90° (1/4 of the 360° ecliptic, or 1/2 of an opposition [180°]). An orb of somewhere between 5° and 10° is usually allowed. As with the trine and the sextile, in the square, it is usually the outer or superior planet that has an effect on the inner or inferior one. Basically, the square's energy is similar to that of a trine but it is intensified to such an extent that the energy is said to be stressful.
A trine (abrv. Tri) is an angle of 120° (1/3 of the 360° ecliptic). A separation (orb) of 120±04° is considered a trine. The trine indicates harmony, and ease of expression, with the two elements reinforcing each other. The trine is a source of artistic and creative talent, which is innate. The trine has been traditionally assumed to be extremely beneficial, providing ease even if undeserved, but it can be a 'line of least resistance' to a person of weak character.
An opposition (abrv. Opp) is an angle of 180° (1/2 of the 360° ecliptic). An orb of somewhere between 5° and 10° is usually allowed. Oppositions are said to be the second most powerful aspect. It resembles the conjunction although the difference between them is that the opposition causes exaggeration as it is not unifying like the conjunction but instead exalted.
The parallel and antiparallel (or contraparallel) are two other aspects, which refer to degrees of declination above or below the ecliptic. They are considered strong influences, though not much research has gone into studying these particular aspects.
- Parallel: same degree± 1-degree 12-minutes of arc. This is similar to a conjunction, but usually provides benefits.
- Contraparallel: opposite degree± 1-degree 12-minute of arc. Said to be similar to the opposition, but weaker.
See also 
- Hard aspect (astrology)
- Soft aspect (astrology)
- Applying aspect (astrology)
- Hamburg School of Astrology
- An articles on the use of the aspects
- Listing of Daily Aspects -- For Astro Trading Purposes
- Online Ephemeris from Khaldea.com -- 600BC to 2400AD—Calculated for Midnight GMT; also with an Aspectarian included for years 1900 to 2005