Synastry is the part of astrology that compares two people's birth charts. Where a single chart is read as a portrait of one person, synastry looks at the angles between one person's planets and the other's, and astrologers read those angles as the texture of a relationship: where two people fit easily, where they rub, and what each one brings out in the other.
This guide separates the two things synastry is made of. One is astronomy: where the Sun, Moon and planets were at two moments, seen from two places, which can be computed to a fraction of a degree and checked against NASA data. The other is interpretation: what a Venus-Mars square is supposed to mean, which is astrological tradition and has not held up in controlled tests. Knowing where one ends and the other begins is the most useful thing to bring to any synastry chart.
TL;DR: Synastry compares two birth charts by measuring the angle between each of one person's placements and each of the other's, and flagging angles close to 0°, 60°, 90°, 120° or 180° as aspects. How close counts is the orb: the synastry chart calculator uses 8° for conjunctions and oppositions, 7° for trines and squares, 5° for sextiles, plus 2° when the Sun or Moon is involved. It lists every aspect in both directions, sorted by exactness, client-side, free, no signup. The positions are real astronomy; the readings are tradition, and a double-blind test published in Nature in 1985 found astrologers matched charts to people no better than chance.
What is synastry in astrology?
Synastry is the comparison of two birth charts to describe a relationship between the two people. The word comes from the Greek syn, together, and astron, star. It is used for romantic partners most often, but astrologers apply it to friends, business partners, parents and children, or any pair.
A birth chart, also called a natal chart, is a map of where the Sun, Moon and planets stood along the zodiac at the moment and place of a birth. Each placement has an ecliptic longitude, a position from 0° to 360° around the sky. The zodiac signs are twelve 30° slices of that circle, starting at 0° Aries.
Synastry takes two such charts and asks one question over and over: how far apart is this placement in chart A from that placement in chart B? If the gap is close to one of a few special angles, the pair is said to form an aspect, and the aspect is what gets interpreted.
This is different from a composite chart, which averages the two charts into a third, single chart. It is also different from sun sign compatibility, which compares only which of the twelve signs each person's Sun falls in. Synastry works with the exact degrees of many placements at once, which is why it needs full birth data.
How are inter-chart aspects calculated?
An inter-chart aspect is found by taking the angular distance between two longitudes, one from each chart, and checking it against a set of target angles. The distance is always measured the short way around the circle, so it runs from 0° to 180°.
The steps, for any pair:
- Subtract one longitude from the other.
- Fold the result into 0° to 360°.
- If it is over 180°, subtract it from 360° to get the shorter arc.
- Compare that separation with each aspect angle. The difference is the orb.
- If the orb is within the allowance for that aspect, the pair is in aspect. If more than one aspect fits, keep the closest.
The five major aspects divide the circle by whole numbers:
| Aspect | Angle | Circle divided by | Traditional reading |
|---|---|---|---|
| Conjunction | 0° | 1 | The two act as one |
| Opposition | 180° | 2 | Tension that needs balancing |
| Trine | 120° | 3 | Easy, mutual support |
| Square | 90° | 4 | Friction, drive and frustration |
| Sextile | 60° | 6 | Opportunity that has to be taken up |
The right-hand column is tradition, shown because it is what the calculator prints in its Reading column. The geometry on the left is plain arithmetic.
A worked example with real positions
The calculator's page uses two sample charts: 15 June 1990 at 14:30 in London, and 3 March 1992 at 08:15 in New York. Call them the first and second person. Their computed positions include:
- First person's Mars: 11°05' Aries, a longitude of 11.086°
- First person's Moon: 16°12' Pisces, 346.194°
- First person's Sun: 24°12' Gemini, 84.192°
- Second person's Mars: 11°02' Aquarius, 311.027°
- Second person's Sun: 13°13' Pisces, 343.221°
- Second person's Venus: 17°06' Aquarius, 317.101°
Mars to Mars. 311.027 - 11.086 = 299.941. That is over 180, so the short arc is 360 - 299.941 = 60.059°. The nearest aspect is the sextile at 60°, with an orb of 0.059°, well inside the 5° allowance. The calculator shows it as First person's Mars sextile Second person's Mars, orb 0.1°, and it is the top row of the list.
Moon to Sun. 346.194 - 343.221 = 2.973°. That is a conjunction with an orb of about 3°. A conjunction normally allows 8°, and because the Moon is involved it gets 2° more, so the allowance is 10°.
Sun to Venus. 317.101 - 84.192 = 232.909, so the short arc is 127.091°. That is 7.09° from a trine. A trine allows 7°, which this would miss, but the Sun is involved, so the allowance is 9° and the aspect counts. Without the luminary allowance, this row would not appear.
Across all pairings, these two charts produce 32 major aspects. With minor aspects included, the count rises to 44.
What orbs does the calculator use?
An orb is how far from exact an aspect can be and still count. Orbs are not standardized in astrology, and two calculators disagreeing about whether an aspect exists is almost always an orb disagreement rather than an arithmetic one. The Toolz calculator uses fixed orbs and prints the orb of every aspect it finds, so you can apply a tighter standard yourself.
| Aspect | Angle | Base orb | With Sun or Moon |
|---|---|---|---|
| Conjunction | 0° | 8° | 10° |
| Opposition | 180° | 8° | 10° |
| Trine | 120° | 7° | 9° |
| Square | 90° | 7° | 9° |
| Sextile | 60° | 5° | 7° |
| Quincunx (minor) | 150° | 3° | 3° |
| Semi-sextile (minor) | 30° | 2° | 2° |
| Semi-square (minor) | 45° | 2° | 2° |
| Sesquiquadrate (minor) | 135° | 2° | 2° |
The extra 2° for the Sun and Moon, the two luminaries, is common practice on the reasoning that they dominate a chart; the exact size varies by author, and 2° is the middle of the range. Minor aspects do not get it. Widening a 2° orb by 2° would double it, and the result would be a grid where nearly everything aspects everything.
Minor aspects are off by default. Tick Include minor aspects to add the four minor ones. The orbs cannot be changed on the page.
Which placements does the calculator compare?
The calculator compares ten placements from each chart: the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, the Ascendant, the Midheaven and the North Node. Every placement in the first chart is checked against every placement in the second, which is 100 ordered pairs.
Uranus, Neptune and Pluto are left out on purpose. They move so slowly that two people born a few years apart have them in nearly the same place:
| Planet | Approximate time in one sign | Effect in synastry |
|---|---|---|
| Uranus | About 7 years | People born close together share it |
| Neptune | About 14 years | Shared across most of a generation |
| Pluto | 12 to 30 years, varying with its orbit | Shared across whole generations |
A Neptune-Neptune conjunction between two people born two years apart says nothing about the pair; it would show up for almost anyone their age. Including those contacts pads the list with rows that carry no information about the relationship.
The North Node used is the mean node. Software that reports the true node can differ by up to about 1.6°, so a North Node contact near the edge of its orb may appear in one program and not another.
Why does direction matter in synastry?
In synastry, your Venus on their Mars and their Venus on your Mars are two separate contacts, and astrologers read them differently. The tradition treats the first as describing how you are drawn to them, and the second as how they are drawn to you.
That is why the calculator lists both directions, a full cross-product rather than each pairing once. Every row is read left to right: the first body belongs to the first person, the second body to the second person. If you enter names, the rows use them.
When the same pair of bodies forms an aspect in both directions, astrologers call it a double whammy and treat it as stronger than either contact alone. In the sample charts, the first person's Sun squares the second person's Mercury at an orb of 5.3°, and the first person's Mercury squares the second person's Sun at 7.4°. Look for the same two bodies appearing twice with the names swapped.
How is the aspect list sorted?
The list is sorted by exactness relative to the allowance, not by raw orb. Each aspect gets a strength score of 1 minus orb divided by allowed orb, so an exact aspect scores 1 and one at the edge of its orb scores 0.
This is why a wide aspect can rank above a tighter one. In the sample, the first person's Sun square the second person's Mercury has an orb of 5.25° out of an allowed 9°, a score of 0.42. The first person's Mars square the second person's North Node has a smaller orb, 4.60°, but only 7° is allowed, so it scores 0.34 and sits lower.
The top of the list is where the tradition puts the weight. Astrologers generally treat aspects within 1° as far more significant than the same aspect at 6°, which is why the orb column matters more than the count at the top.
What does the calculator show?
After you fill in both forms and press Compare the charts, the result has four parts:
- A header with both names and each chart's local date, time and place.
- The inter-chart aspects, with a count of how many are within orb and a table of every one: the two placements with their owners, the aspect, its orb to a tenth of a degree, and a one-line traditional reading.
- Both birth charts in full, side by side: each placement's sign, degree and house in that person's own chart.
- Notices about either chart, such as a mean node close to a sign boundary or a birth at a latitude where the Placidus house system fails and equal houses are used instead.
Each person's houses are calculated with Placidus, the system most modern software reports. The calculator does not list house overlays, meaning where one person's planets fall in the other person's houses. You can read those off the two tables by comparing a placement's degree with the other chart's house cusps.
Both birth times and places are required. The Ascendant and Midheaven are among the most-read contacts in synastry, and both depend entirely on the exact time and place. The Ascendant moves through all twelve signs in about a day, roughly 1° every 4 minutes on average, so a birth time off by half an hour can shift it by several degrees. The Moon moves about 13° a day, roughly half a degree an hour. For a comparison without birth times, the love compatibility calculator accepts a missing time and says so in its result.
Are the planetary positions accurate?
Yes. The positions are astronomy, and they are computed with published methods and checked against NASA's own reference ephemeris.
The Sun and Moon follow the algorithms in Jean Meeus's Astronomical Algorithms (2nd edition, 1998), including the full 60-term series for the Moon's longitude. The planets use a truncated form of VSOP87, the planetary theory published by Pierre Bretagnon and Gérard Francou in Astronomy and Astrophysics in 1988, with corrections for light travel time, aberration and nutation. The tool's test suite pins positions against JPL Horizons, NASA's ephemeris service, and the errors are a matter of arcseconds. A tenth of a degree, the precision the orb column shows, is 360 arcseconds.
The input side is handled carefully too. Birth times are converted to universal time using the IANA time zone database's historical rules, with an offset override for older births where those records are approximate, and a Delta T correction bridges clock time and the uniform time the orbital formulas need. Births from 1600 to 2100 are accepted.
Signs are tropical, measured from the vernal equinox, which is the convention in Western astrology. In practice, the biggest source of error in any synastry chart is the birth time, not the arithmetic.
Why do two synastry calculators give different results?
Two calculators given the same birth data can produce different aspect lists without either one being wrong. When they disagree, the cause is almost always one of a handful of choices rather than the astronomy:
- Orbs. A calculator with an 8° trine will list contacts that a 7° calculator drops. This is the most common reason by far.
- Which placements are included. Some programs add Uranus, Neptune, Pluto, Chiron or Lilith, which multiplies the number of rows.
- Mean or true node. The two can differ by up to about 1.6°, enough to move a North Node contact in or out of orb.
- Minor aspects. Some tools include the quincunx and semi-square by default; this one leaves them off until you tick the box.
- Birth time and place. A different time zone rule for an older birth, or a city chosen a few hundred miles away, moves the Ascendant and Midheaven.
The planetary positions themselves should agree closely across any modern program. If a Sun or Venus position differs by more than a few arcminutes, check the birth date, time and time zone before anything else. The orb printed on every row in this calculator lets you apply your own cutoff: if you prefer a 6° trine, ignore trines with an orb over 6.
Is there any scientific evidence for synastry?
No controlled study has shown that astrological chart readings, including synastry, describe people or relationships better than chance. The positions in a synastry chart are real; the meanings attached to them are tradition.
The best-known test is Shawn Carlson's "A double-blind test of astrology", published in Nature in 1985. Carlson worked out the protocol with astrologers in advance so they would accept it as fair. Astrologers were given a person's natal chart and three personality profiles from the California Psychological Inventory, one belonging to that person and two chosen at random, and asked which one matched. The astrologers had predicted they would pick the right profile more than half the time. They picked it about one time in three, the rate expected from guessing among three options.
Carlson's study tested natal chart interpretation, not synastry specifically. Synastry, though, is built from the same chart factors: the same planets, signs and aspects, read across two charts instead of within one. If the factors do not describe one person better than chance, there is no demonstrated basis for them describing two.
That is why the Toolz astrology tools separate the two layers explicitly. Each result carries a note that the positions are astronomy and the readings are traditional attributions, offered for interest, and that nothing on the page is advice about relationships or any other decision. People use synastry as a vocabulary for talking about a relationship, and that is a reasonable way to treat it. A synastry chart cannot tell you whether a relationship will last.
How should you read a synastry chart?
If you use synastry as the tradition does, a few principles keep the reading grounded:
- Start at the top of the list. The tightest few aspects are where the tradition puts the weight.
- Read the direction. Your Moon on their Saturn and their Moon on your Saturn are separate contacts.
- Do not treat squares and oppositions as failures. In the tradition they are where the friction and drive are, and a list of nothing but trines is usually described as comfortable but flat.
- Weigh the pairings traditionally given most attention: Sun-Moon, Venus-Mars, anything touching the Ascendant, then Moon-Venus and Moon-Saturn.
- Check both birth times before trusting any Ascendant or Midheaven contact.
To go deeper on either person's chart on its own, the birth chart guide explains every placement and house, and the love compatibility guide covers the simpler sign-based comparison. The birth chart calculator and the rest of the set are listed on the astrology tools hub.
Is my birth information stored?
No. Both charts are computed client-side, and neither set of birth details is transmitted, saved or logged. The page makes a daily usage check and records that a comparison was run, which carries the tool name and a device identifier, never the birth data. The calculator is free, no signup.
When you have both sets of birth details, compare two birth charts, read the top of the list first, and keep the astronomy and the tradition in their separate columns.
