In astrology, an aspect is an angle that planets make to each other in the Horoscope; as well as to the Ascendant, Midheaven, Descendant, Lower Midheaven, and other points of astrological interest. As viewed from Earth, aspects are measured by the angular distance in degrees and minutes of ecliptic longitude between two points. According to astrological tradition, they indicate the timing of transitions and developmental changes in the lives of people and affairs relative to the Earth.
For example, if an astrologer creates a Horoscope that shows the apparent positions of the celestial bodies at the time of a person's birth (Natal Chart), and the angular distance between Mars and Venus is 92° ecliptic longitude, 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.
With Natal charts, other signs may take precedence over a Sun sign. For example, an Aries may have several other planets in Cancer or Pisces. Therefore, the two latter signs may be more influential.
History and approach
In medieval astrology, certain aspects and 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 is exemplified by research on astrological harmonics. In 1619, Johannes Kepler advocates this in his book Harmonice Mundi. Thereafter, John Addey was a major proponent. However, even in modern times, aspects are considered to be either easy (60° Sextile or 120° Trine) or hard (90° Square or 180° Opposition). Depending on the involved planets, a Conjunction (0°, which is a discounting orb) may be in either category.
Easy aspects may be positive, because they enhance opportunity for talent to grow. Hard aspects may be negative, because they enhance a challenge where an adjustment must be made to reach balance. Typically, manifestation may occur with a Conjunction, Square or Opposition.
Planets may be considered. Mars and Uranus tend to ignite while Saturn and Neptune inhibit. Whether a planet is direct or retrograde is of great significance. An eclipse of the Sun or Moon is even more significant. The South Node of the Moon denotes innate wisdom from past experience while the North Node denotes karma and evolution.
Astrological Signs may be considered. For example, the fire signs of Aries, Leo and Sagittarius are more compatible with the air signs of Gemini, Libra and Aquarius. The Earth signs of Taurus, Virgo and Capricorn are more compatible with the water signs of Cancer, Scorpio and Pisces. The mutable signs of Gemini, Virgo, Sagittarius and Pisces may be flexible. The cardinal signs of Aries, Cancer, Libra and Capricorn may change their mind. The fixed signs of Taurus, Leo, Scorpio and Aquarius may be difficult.
Astrological Houses may be considered.
Ptolemaic Aspects
Since they were defined and used by Ptolemy in the 1st Century AD, the traditional major aspects are sometimes called Ptolemaic Aspects. These aspects are the Conjunction (0°), Sextile (60°), Square (90°), Trine (120°), and Opposition (180°). Major aspects are those that are divisible by 10 and evenly divided in relation to 360° (with the exception of the Semisextile).
When calculating or using aspects, it is important to note that different astrologers and separate astrological systems/traditions utilize differing orbs, which is the degree of separation between exactitude. Orbs may also be subject to variation, depending on the need for detail and personal preferences. Although, when compared to other aspects, almost all astrologers use a larger orb for a Conjunction.
Kepler's Aspects
Collective astrological data along with Johannes Kepler described 13 aspects in his book Harmonice Mundi. Astrological data grouped together in five degrees of influentially picked from symbol ratios encountered in geometry and music: 0/2, 1/2, 1/5, 2/6, 1/3, 1/12 along with 1/5, 2/5, 15/5, 10, 10/3, 8, and 8/3. The general names for whole divisors are (Latin) n-ile for whole fractions 1/n, and m-n-ile for fraction m/n. A Semi-n-tile is a 2n-tile, 1/(2n), and Sesqui-n-tile is a Tri-2n-tile, 3/(2n).
All aspects can be seen as small whole number harmonics, (1/n of 360°). Multiples of m/n create new aspects where there are no common factors between n and m, gcd(n,m)=1.