Step One: Obtain Both Natal Charts
Start with the birth dates and, when available, accurate birth times and birth locations of both individuals. Use an appropriate natal-chart calculation service to obtain planetary positions. A source such as Astrodienst provides astrology chart resources.
For this calculator, record the tropical zodiac sign and degree for the Sun, Moon, Mercury, Venus, and Mars. A birth time is particularly important when determining an accurate Moon position because the Moon moves relatively quickly. Birth time also affects angles and houses, which this simplified calculator does not evaluate.
Step Two: Enter Signs And Degrees
Each zodiac sign covers 30 degrees of celestial longitude. Aries begins at 0 degrees, Taurus at 30 degrees, Gemini at 60 degrees, and the sequence continues through Pisces, which begins at 330 degrees.
For example, Venus at 15 degrees Leo corresponds to 135 degrees of zodiac longitude. Mars at 15 degrees Sagittarius corresponds to 255 degrees. Their shortest angular separation is 120 degrees, forming an exact trine.
Step Three: Select An Orb
Choose a maximum orb according to how selective you want the results to be. Three degrees identifies relatively tight connections, while eight degrees produces a broader collection. The standard six-degree option offers a practical starting point for exploring the major aspects.
Using the same orb for every aspect makes this calculator's results straightforward to compare. Some professional astrological methods assign different orbs depending on the planets and aspects involved, so other software may report a different set of matches.
Step Four: Understand The Angular Formula
First, convert each planetary position into longitude using the sign's starting angle plus the degree within that sign. Next, calculate the absolute difference between the two planetary longitudes. If that difference exceeds 180 degrees, subtract it from 360 degrees to obtain the shorter separation.
Angular Separation: Δ = min(|L₁ − L₂|, 360° − |L₁ − L₂|).
Aspect Deviation: Orb = |Δ − A|, where A is 0°, 60°, 90°, 120°, or 180°.
An aspect is reported when its deviation does not exceed the selected maximum orb. Because the major aspect angles are sufficiently separated, the supported orb settings do not create ambiguous matches between these five aspects.
Step Five: Interpret The Planetary Pair
The planets provide context for the geometric aspect. The Sun traditionally represents identity and vitality; the Moon represents emotional needs and instinctive responses. Mercury relates to thinking and communication, Venus to affection and values, and Mars to action and assertiveness.
A Venus–Mars trine and a Mercury–Mars trine share the same 120-degree geometry, but astrologers interpret them differently because they connect different symbolic functions. The calculator identifies the pair so you can examine its meaning rather than reducing everything to a numerical rating.
Step Six: Consider The Wider Relationship
A Synastry Chart Calculator offers one perspective, not a complete assessment. The five-planet comparison excludes Jupiter, Saturn, outer planets, lunar nodes, angles, house overlays, and other techniques that some astrologers consider important.
It also cannot measure trust, safety, honesty, shared goals, or the ability to resolve disagreements. Those qualities must be evaluated through real experiences. Even a chart containing many traditionally harmonious aspects cannot establish that a relationship will be healthy or enduring.