---
title: "Synastry: How Two Birth Charts Are Compared"
description: "What synastry is, which placements the couples reading compares, the exact orbs it uses, what a missing birth time changes, and how to read the result."
canonical: https://unifiedreading.com/synastry
date-published: 2026-11-09
last-updated: 2026-11-09
publisher: Unified Reading
---

# Synastry: How Two Birth Charts Are Compared

Synastry compares two birth charts by measuring the angles, called aspects, between one person’s placements and the other’s. Unified Reading’s couples reading measures each person’s Sun, Moon, Venus and Mars, plus each Ascendant whose birth time and place are known, with fixed orbs, and names every contact without a score.

[Open this page](https://unifiedreading.com/synastry)

By the Unified Reading team

## What synastry is, and what it is not

A birth chart records where the Sun, Moon and planets stood in the zodiac at one moment, seen from one place. Synastry takes two of those charts and looks across them: how far one person’s Moon sits from the other person’s Sun, or one person’s Venus from the other’s Mars. When the distance lands near one of a few traditional angles, astrologers call it an aspect and read it as a point of contact between the two people.

Synastry is different from a composite chart, which averages the two charts into a third chart for the relationship itself. Unified Reading does not draw a composite. It is also different from the compatibility pages in this library, which compare whole signs: two Sun signs, two Moon signs or two rising signs by element, modality and ruler. A sign comparison works from birth dates alone. Synastry needs positions in degrees, which is why the couples reading calculates both charts with the same ephemeris as the single reading.

## Which placements this engine compares, and why

The couples reading compares four bodies from each chart: the Sun, the Moon, Venus and Mars. The Sun and Moon are the two lights that carry most of a chart’s character in modern Western practice. Venus and Mars are the planets most often read for attraction, affection and how each person goes after what they want. Together they cover the questions people usually bring to a reading for two.

The Ascendant, or rising sign, is added for each person whose birth time and birthplace are both known, because it moves through the whole zodiac in a day and cannot be estimated from a date. If only one person knows their time and place, that person’s Ascendant is still compared with the other person’s four planets. Mercury, Jupiter, Saturn and the outer planets are left out on purpose. They move slowly or sit close to the Sun, so many people born in the same weeks share them, and adding them would fill the table with contacts that say little about these two people in particular.

## Aspects and orbs

Five aspects are measured: the conjunction at 0°, the sextile at 60°, the square at 90°, the trine at 120° and the opposition at 180°. Real positions rarely land on an exact angle, so each aspect has an orb, the distance from exact that still counts. Here the conjunction and opposition allow 8°, the square and trine 7°, and the sextile 5°. Contacts between one person’s Venus and the other’s Mars use an orb 2° tighter, because those two planets are often read with special weight and a loose orb would overstate them.

The angle between two positions is always the shorter way round the circle, so it runs from 0° to 180°. A contact within 1° of exact is marked exact. The reading lists at most ten aspects, tightest first, so the closest contacts lead. These orbs are this engine’s conventions, chosen to sit inside common practice. Other astrologers use wider or narrower orbs, so another site may list a contact that this one leaves out, or the reverse. The table below is printed from the same constants the couples reading uses.

## What a missing birth time or place changes

The Moon moves about 13° a day, which is wider than most orbs. Without a birth time, the engine calculates that chart for noon, and the Moon can be several degrees from where it really was. A time without a resolved birthplace is read as UTC, which can also move the Moon by several degrees. In both cases every aspect involving that Moon is marked approximate. Treat those rows as tendencies to test against experience, not as measurements.

The Sun, Venus and Mars move slowly enough that a date alone usually places them well within an orb, although a planet near the edge of a sign may be flagged when its sign could change across the day. The Ascendant needs both a time and a place. Without them that person has no Ascendant in the comparison, while the other person’s Ascendant still counts if theirs is known. The worked example shows the same pair calculated both ways.

## Element balance and the Venus–Mars pair

Beside the aspects, the reading counts the elements across both charts: each person’s Sun, Moon, Venus and Mars, eight placements in all, sorted into Fire, Earth, Air and Water. The Ascendants are left out of this count so that knowing a birth time does not tilt the balance. A pair heavy in one element shares a temperament; an element with no placements at all is worth noticing, since neither person brings it by default.

The reading also compares each person’s Venus with the other person’s Mars by element, in both directions. Venus and Mars in the same element are called the same element; Fire with Air, or Earth with Water, are complementary; any other pairing is contrasting. This is a sign-level comparison, so it holds even when no Venus–Mars aspect falls within orb, and it is often the plainest way into a conversation about how each of you shows interest and how each likes to be approached.

## A worked example: two fictional charts

The tables below come from two invented people. Person A was born in London on March 10, 1990 at 06:00, at 51.5074, -0.1278, on Europe/London time, UTC+00:00 that day; it is the same fictional chart the chart ruler and empty houses guides use. Person B was born in Lisbon on September 21, 1992 at 19:45, at 38.7223, -9.1393, on Europe/Lisbon time, UTC+01:00 that day. Both charts are calculated when this page is built, with the code that writes the couples reading.

The second table repeats the comparison as if Person B did not know a birth time or place. Some contacts disappear, some move, and the ones involving Person B’s Moon become approximate. That difference is the practical case for looking up a birth time when you can: it decides which rows you can lean on.

## What is not calculated

The couples reading does not calculate transits, progressions, a composite or Davison chart, either person’s planets in the other person’s houses, midpoints, the lunar nodes, or aspects within one person’s own chart. It never produces a compatibility percentage or a verdict. If another reading you have seen relies on any of those, it is answering a different question from this one.

## How to read aspects as conversation prompts

Each aspect names a kind of contact, not an outcome. A conjunction suggests two parts of you that blend, for better and worse. A trine or sextile describes something that tends to flow without effort, which can also mean it goes unnoticed. A square points at friction that asks for adjustment, and an opposition at two pulls that each of you may expect the other to carry. None of them says whether a relationship works.

A useful way to read the table is together, one row at a time. For each aspect, each of you gives one recent example where it fits and one where it does not. Keep the examples ordinary: a weekend plan, a disagreement about timing, how you each recover after a long day. The rows that produce the most recognition are the ones worth keeping, and the ones that produce none can be set aside.

## The aspects and orbs used here

| Aspect | Angle | Orb |
| --- | --- | --- |
| Conjunction | 0° | 8° (Venus–Mars 6°) |
| Sextile | 60° | 5° (Venus–Mars 3°) |
| Square | 90° | 7° (Venus–Mars 5°) |
| Trine | 120° | 7° (Venus–Mars 5°) |
| Opposition | 180° | 8° (Venus–Mars 6°) |

An aspect within 1° of exact is marked exact. At most 10 aspects are listed, tightest first. Angles are the shorter arc between the two ecliptic longitudes.

## The fictional pair, calculated

These are invented sample charts, not customers. Person A: London, March 10, 1990 at 06:00 (Europe/London, UTC+00:00; 51.5074, -0.1278). Person B: Lisbon, September 21, 1992 at 19:45 (Europe/Lisbon, UTC+01:00; 38.7223, -9.1393).

Person A: Sun Pisces 19.4°, Moon Virgo 5.4°, Venus Aquarius 4.8°, Mars Capricorn 29.0°, Ascendant Pisces 1.4°. Person B: Sun Virgo 29.0°, Moon Cancer 24.3°, Venus Libra 25.9°, Mars Cancer 5.3°, Ascendant Aries 5.0°.

9 aspects between the two charts, tightest first:

| Person A | Aspect | Person B | Orb |
| --- | --- | --- | --- |
| A’s Mars | Trine | B’s Sun | exact (0.1°) |
| A’s Moon | Sextile | B’s Mars | exact (0.1°) |
| A’s Venus | Sextile | B’s Ascendant | exact (0.2°) |
| A’s Mars | Square | B’s Venus | 3.1° |
| A’s Ascendant | Trine | B’s Mars | 3.9° |
| A’s Mars | Opposition | B’s Moon | 4.7° |
| A’s Sun | Trine | B’s Moon | 4.9° |
| A’s Ascendant | Trine | B’s Venus | 5.5° |
| A’s Venus | Trine | B’s Sun | 5.8° |

Element balance across both charts’ Sun, Moon, Venus and Mars: Fire 0, Earth 3, Air 2, Water 3. Person A’s Venus and Person B’s Mars are contrasting elements; Person B’s Venus and Person A’s Mars are contrasting elements.

The same pair if Person B’s birth time and place were unknown:

| Person A | Aspect | Person B | Orb |
| --- | --- | --- | --- |
| A’s Mars | Trine | B’s Sun | exact (0.2°) |
| A’s Moon | Sextile | B’s Mars | exact (0.3°) |
| A’s Sun | Trine | B’s Moon | exact (0.9°), approximate |
| A’s Mars | Square | B’s Venus | 3.4° |
| A’s Ascendant | Trine | B’s Mars | 3.8° |
| A’s Ascendant | Trine | B’s Venus | 5.8° |
| A’s Venus | Trine | B’s Sun | 6.1° |

Without Person B’s time and place there is no Ascendant for Person B, and Person B’s Moon is a noon position, so every aspect to it is marked approximate. Person A’s Ascendant still counts, because Person A’s time and place are known.

## Read the two of you together

Want these aspects measured for the two of you? The couples reading takes both birth dates, and birth times when you know them, and reads your numbers, signs and Chinese zodiac together with the aspects between your charts. No score, no verdict.

[Start a couples reading from two birth dates](https://unifiedreading.com/couples)

## Frequently asked questions

### Is synastry the same as compatibility?

Not quite. Compatibility pages often compare whole signs, such as two Sun signs by element. Synastry measures the exact angles between placements in two birth charts. The couples reading uses both: sign-level facts about the pair and the measured aspects between the two charts. Neither produces a score.

### Do we both need our birth times?

No. Without a birth time the reading still compares the Sun, Venus and Mars, and marks any aspect to that person’s Moon as approximate. An Ascendant is included for each person whose birth time and birthplace are both known, so one known time already adds that person’s Ascendant.

### What does an orb mean?

An orb is how far from an exact angle a contact can be and still count as an aspect. Here the conjunction and opposition allow 8°, the square and trine 7°, and the sextile 5°, with Venus to Mars 2° tighter. A contact within 1° is marked exact.

### Does a square mean we are a bad match?

No. A square describes a kind of friction between two placements: two ways of doing things that do not fit without adjustment. Many people read squares as the places a relationship grows. The reading names the friction so you can talk about it, not to grade the pair.

[How a combined reading works](https://unifiedreading.com/numerology-and-astrology)

[Our method and sources](https://unifiedreading.com/method)

## Sources and calculation references

- [Astronomy Engine, the ephemeris behind both charts](https://github.com/cosinekitty/astronomy)
- [Astrodienst astrowiki: Synastry](https://www.astro.com/astrowiki/en/Synastry)
- [Astrodienst astrowiki: Composite chart](https://www.astro.com/astrowiki/en/Composite_Chart)
- [Unified Reading method and sources](https://unifiedreading.com/method)

## Read next

- [What a compatibility reading for two contains](https://unifiedreading.com/compatibility-reading)
- [Zodiac compatibility: all 78 Sun sign pairings](https://unifiedreading.com/compatibility)
- [Moon sign compatibility](https://unifiedreading.com/compatibility/moon)
- [Your Sun with their Moon](https://unifiedreading.com/compatibility/sun-moon)
- [Rising sign compatibility](https://unifiedreading.com/compatibility/rising)
- [Venus sign calculator](https://unifiedreading.com/venus-sign-calculator)
- [Mars sign calculator](https://unifiedreading.com/mars-sign-calculator)
- [Moon sign calculator](https://unifiedreading.com/moon-sign-calculator)
- [Rising sign calculator](https://unifiedreading.com/rising-sign-calculator)

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