Synastry compares two birth charts. A cross-chart aspect is an angular relationship between a position in one chart and a position in the other. The angle can be calculated; claims about attraction, conflict, or commitment are astrological interpretations, not measurements of a relationship.
Start with comparable positions
Use the same zodiac convention for both charts. Do not compare a tropical position with a sidereal position without converting them consistently. Record the planet, sign, and degree for each person. A birth time uncertainty can move the Moon and, especially, the angles and house placements.
Number the zodiac from 0° Aries to just below 360°. Each sign spans 30°: Aries starts at 0°, Taurus at 30°, Gemini at 60°, Cancer at 90°, Leo at 120°, Virgo at 150°, Libra at 180°, Scorpio at 210°, Sagittarius at 240°, Capricorn at 270°, Aquarius at 300°, and Pisces at 330°.
Calculate an aspect and its orb
An aspect angle is a target such as 120°. The orb is the difference between the measured separation and that target. It is not the whole separation between the planets.
For an illustrative example, person A's Venus is at 12° Aries and person B's Mars at 14° Leo:
- Convert to longitude: Venus = 12°; Mars = 120° + 14° = 134°.
- Subtract: 134° − 12° = 122°.
- Compare with a trine at 120°: the orb is 2°.
- Under a declared 3° allowance, this counts as a trine; under a 1° allowance, it does not.
Use the shorter distance around the circle. For 358° and 2°, the raw difference is 356°, but the shorter separation is 360° − 356° = 4°. That is a conjunction with a 4° orb, not a wide opposition.
Major aspect angles
- Conjunction, 0°: traditionally read as emphasis or blending.
- Sextile, 60°: traditionally associated with cooperation.
- Square, 90°: used to discuss tension or differing approaches.
- Trine, 120°: associated with ease or similarity.
- Opposition, 180°: interpreted through contrast and negotiation.
The quincunx, 150°, is also used by some readers, but is not one of the five major aspects listed above. Orb allowances vary by practitioner, planet, and software. The 3° allowance in the example is a teaching choice, not a universal rule or a claim about this site's calculator settings.
Interpreting different planet pairs
In modern Western astrological practice, Sun–Moon comparisons often prompt discussion of identity and emotional needs; Venus–Mars of affection and desire; Moon–Moon of habits and comfort; Saturn contacts of boundaries and responsibility; Pluto contacts of intensity and power. These are symbolic topics. An aspect cannot establish mutual attraction, consent, loyalty, or a relationship's duration.
For the Venus–Mars example, a useful question is “How do we each express affection?” An unjustified conclusion is “This person must find me irresistible.” Ask both people rather than treating a chart description as evidence about their feelings.
Avoid a compatibility score made from adjectives
A trine does not guarantee kindness, and a square does not prove incompatibility. Do not excuse controlling behaviour because an aspect is described as transformative. Consider actual communication, preferences, and boundaries. A Sun–Moon comparison illustrates how even one planet pair needs both directions checked.
Reference and next step
Astrodienst's aspect reference explains astrological aspect terminology. It is a practitioner reference, not evidence that the geometry predicts relationship outcomes. For the horizon angles, continue with Rising Sign Compatibility.