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In astrology, an aspect is an angle the planets make to each other in the horoscope, 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 being a 90° aspect). The more exact an aspect, 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 (but still unscientific) approach to astrological aspects 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. But even in modern times, the aspects are divided into hard aspects or easy aspects. Major hard aspects being the square and opposition and the easy aspects as trine and sextile. The conjunction can be in either category depending on the planets that are connected.
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.
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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 (0°), 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 have a maximum distance 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 (abbrev. Tri) is an angle of 120° (1/3 of the 360° ecliptic), an orb of somewhere between 5° and 10° . The trine indicates harmony and ease. The trine is a source of artistic and creative talent, which is innate. The trine has been traditionally assumed to be extremely beneficial.
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.
A semi-sextile is an angle of 30° (1/12 of the 360° ecliptic). An orb of 30±1.5° is allowed. It indicates constant interaction between the planets involved and one building on the energy of the other, but planets and signs involved must be considered.
A quincunx is an angle of 150° (5/12 of the 360° ecliptic). An orb of 150±2° is allowed. It indicates a strong flow of energy between dimensions of life represented by the planets involved, often in a compulsive or annoying manner.
The parallel and antiparallel (or contraparallel) are two other aspects, which refer to degrees of declination above or below the celestial equator. 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.
- Hard aspect (astrology)
- Soft aspect (astrology)
- Applying aspect (astrology)
- Hamburg School of Astrology