What Is Synastry? Aspects and Orbs
Learn to read the angle between two chart positions before interpreting what it might mean.
By Love Calculator · Published · Updated
Comparing two natal charts
Synastry compares two natal charts, looking at how a position in one relates to a position in the other. An aspect table records their angular relationships; an interpretation assigns astrological meaning to them. Understanding that distinction lets you read the numbers without treating them as evidence about a couple. Astrodienst describes synastry as one approach to partner horoscopes.
Different astrologers and tools use different placements, tolerances, and interpretations. There is no single universal compatibility percentage.
What is an aspect?
An aspect is an angular relationship between two chart positions.
An orb is the permitted distance from an exact angle. With a six-degree orb, a 117-degree separation can count as a trine because it is three degrees from 120. A different orb can change which aspects appear.
The five aspect names describe geometry:
- A conjunction is near 0 degrees.
- A sextile is near 60 degrees.
- A square is near 90 degrees.
- A trine is near 120 degrees.
- An opposition is near 180 degrees.
A transparent tool states its convention and displays the separation or deviation rather than hiding everything behind a score.
How to read one row of a synastry table
Imagine Person 1's Venus and Person 2's Moon are 117 degrees apart. Under our six-degree rule, their row would show a trine, a 117-degree separation, and 3 degrees from exact. These are illustrative positions, not a reading for a real couple.
The two body names tell you what is being compared. Separation gives the measured angle; the aspect name identifies the nearby reference angle. The difference from exact tells you how closely the positions match that reference. It is not a rating of the strength of someone's feelings.
In our planetary comparison tool, the closest angular matches appear first. It uses unrounded positions for matching, even though the displayed degrees are rounded for readability.
Measuring an angle around the zodiac
The zodiac circle wraps at 360°. Imagine one longitude at 358° and another at 2°. Their shorter separation is 4°, not 356°. Under our fixed 6° orb, that pair meets the conjunction rule and is 4° from exact.
For a trine, a separation of 126° is 6° from the 120° reference and is included. At 126.1°, the difference is 6.1° and the row is excluded. These are invented positions for explaining the arithmetic, not claims about an actual birth moment. A missing row can mean a boundary was missed by a fraction of a degree; it does not identify a failed relationship.
Why the body labels matter
Person 1's Venus with Person 2's Moon is a different pair from Person 1's Moon with Person 2's Venus. Read both labels before comparing rows. Reversing the direction of the same measured pair does not change its smaller angle, but switching to different bodies can.
Our ten-body comparison checks 100 cross-chart pairs. It does not combine all those rows into a grade, rank the bodies by romantic importance, or include house overlays. A busy table mainly tells you that many pairs met the selected angle rules.
Calculation versus interpretation
Planetary positions can be calculated astronomically. Calling an aspect harmonious or challenging is an astrological interpretation. An accurate angle does not establish a scientific prediction about trust, communication, or a couple's future.
A count of aspects is just a count under the chosen rules. More aspects do not automatically mean more compatibility.
Check the scope
Look for the included planets, time-zone treatment, and handling of missing birth times. Houses and the Ascendant need additional calculations and should not appear as exact if they were never calculated.
Explore our local planetary comparison, or read about birth time uncertainty. Use an interpretation to open a conversation, not to define another person.
Synastry questions
Is synastry the same as Sun sign compatibility?
No. A basic Sun sign comparison uses one sign from each person. Synastry considers relationships between placements across two charts. Our Sun sign and birth chart guide explains the difference in inputs and scope.
Why do two tools list different aspects?
First compare their inputs and settings: birth time, UTC offset, included bodies, zodiac system, and orb. A wider orb can admit an aspect that a narrower one excludes. A different list is not, by itself, evidence that one relationship reading is better.
Is synastry the same as a composite chart?
No. Synastry compares placements across two separate charts. A midpoint composite chart constructs one chart from the midpoints between corresponding placements. Our tool performs the first kind of comparison: its rows connect a body from Person 1 with a body from Person 2. It does not calculate a composite chart.
Can one aspect confirm a soulmate connection?
A geometric match cannot establish that two people are meant to be together. Our tool does not attach soulmate labels or relationship verdicts to aspects. Use any symbolism you enjoy alongside, rather than in place of, what you learn from the person.