Synastry
Chart Calculator

Synastry is the comparison of two birth charts. Each chart stays exactly as it was cast for its owner's birth; what synastry studies is the angle between a planet in one chart and a planet in the other. Those angles are called cross-aspects, and most of a synastry reading is a reading of them.

A Sun-sign comparison looks at one placement per person. Synastry looks at all of them. This calculator compares 20 points in each chart, 398 pairs in all, and in the worked example below 130 of those pairs fall within orb. A reader rarely uses every one: the usual approach is to start from the few tightest aspects between the ten planets and work outward.

Nothing in the method assumes romance. The arithmetic is identical for business partners, friends, a parent and child or two siblings; only the reading changes. That is why the example on this page compares two brothers.

Both charts are cast separately, by the same routine as the birth chart calculator. The API looks up each birthplace's offset from Universal Time for that date in the IANA zone tables, summer time included, and its ephemeris supplies the positions. The results page asks the API for Placidus houses for both people.

Each chart contributes 20 points: the ten planets from the Sun to Pluto; the true lunar node and its opposite point, the South Node; Chiron; Black Moon Lilith at the mean apogee of the Moon's orbit; the asteroids Juno, Ceres, Pallas and Vesta; and two points that depend on the birth minute, the Part of Fortune and the Vertex. Every point in the first chart is measured against every point in the second, 398 pairs in all; the two pairings of one person's North Node with the other's South Node are skipped because they repeat the node-to-node contact. The Ascendant and Midheaven are not on the list, so aspects to the other person's angles are not reported. The Vertex and Part of Fortune, just as sensitive to the birth minute, are included.

For each pair the API takes the shorter distance around the zodiac, between 0° and 180°, and tests it against nine angles. The orb is the gap between that distance and the exact angle. The orb allowed starts from a base figure for the aspect and is multiplied by the average of the two planets' factors, then rounded to a tenth of a degree:

The range column runs from two points such as the nodes (×0.6) to the Sun and Moon together (×1.25). A Sun–Moon conjunction is allowed 10°, a Venus–Mars sextile 5°, and a Sun–Saturn square 7.4°: 7° times the average of 1.25 and 0.85. The Vertex and Part of Fortune have no factor of their own in the code and take ×1. Each aspect found is labeled exact when the orb is 1° or less, tight within half the allowed orb and wide beyond that. The same table governs aspects inside a single chart; the methodology page lists it with the site's other conventions.

The test is made in degrees, not signs. Two planets late in one sign and early in another can form an aspect their signs alone would not suggest, which astrologers call an out-of-sign or dissociate aspect. The calculator lists these like any other. In the worked example 4 of the major aspects between planets are out of sign; one is Prince William's Sun at 0°06′ Cancer and Prince Harry's Neptune at 28°39′ Sagittarius, 178°33′ apart: an opposition with an orb of 1°26′, between two signs that are not in that relation.

The percentage at the top of the results page is computed by the API from the major cross-aspects only; semi-sextiles, semi-squares, sesquiquadrates and quincunxes don't count. Every major aspect goes into one of two piles, or neither:

• Harmonious: every trine and sextile, plus each conjunction that involves the Sun, Moon, Venus, Jupiter or North Node.
• Challenging: every square and opposition, plus each conjunction that involves Mars, Saturn or Pluto. This test runs first, so a Venus–Saturn conjunction lands here.
• Neither: conjunctions between other points, such as Mercury on Chiron or Uranus on Neptune, are left out.

Each aspect is then weighted by closeness: weight = 1 − orb ÷ allowed orb, never less than 0.1. An exact aspect counts nearly 1 and one at the edge of its orb 0.1. The score is the harmonious weight divided by the two piles together. A pair with no major aspects at all would get 50%.

Treat it as a ratio and nothing more. It does not grow with the number of contacts, so a pair with ten aspects can score the same as a pair with eighty. It weighs all 20 points alike, so a trine between two asteroids counts as much as a Sun–Moon trine at the same orb. And it follows the familiar soft-versus-hard convention, while many astrologers argue that squares and oppositions are part of what keeps two people engaged. The list of aspects tells you more than the number.

These two charts have reliable times, both from the official birth announcements, and both births took place at the same hospital in London. The example stays with the mechanics: positions, aspects, orbs and how to read a list like this. It makes no claims about the brothers.

Of the 398 pairs, 130 are within orb: 99 major aspects and 31 minor ones. Restricted to the ten planets, the list has 25 major aspects. The six tightest:

1. Check whether the date or the person explains it. The tightest planet contact is Prince William's Jupiter at 0°29′ Scorpio on Prince Harry's Pluto at 0°33′ Scorpio, 0°04′ apart. Jupiter spends about a year in each sign and Pluto many years, so every pair born on these two dates shares this contact, whatever the hour or place. Slow-planet aspects like it describe the two birth dates more than the two people.
2. Then look at the fast points. Prince William's Moon at 4°57′ Cancer and Prince Harry's Mercury at 5°12′ Virgo are 60°14′ apart, a sextile with an orb of 0°14′. At roughly half a degree per hour, the Moon makes this aspect depend on the birth times as well as the dates. A Moon–Mercury sextile is usually read as ease in talking about feelings and day-to-day practicalities.
3. Note the Sun contacts. Prince William's Sun at 0°06′ Cancer is in trine to Prince Harry's Pluto at 0°33′ Scorpio, orb 0°27′. Aspects to either Sun are the ones most readings put first, because the Sun stands for the person's identity in the chart.
4. Score. Of the 99 major aspects, 45 are trines or sextiles and 35 squares or oppositions. The remaining 19 are conjunctions: 8 count as harmonious, 4 as challenging and 7 as neither. Weighted, the harmonious pile comes to 28.70 and the challenging pile to 21.12, so the score is 28.70 ÷ (28.70 + 21.12) = 58%. The Jupiter–Pluto conjunction, with a weight of 0.99, sits in the challenging pile: Pluto is on the hard list, and that rule is checked before Jupiter's place on the soft one.
5. Weigh the minor points last. The tightest aspect of all is not between planets. It joins Prince William's Chiron at 25°16′ Taurus and Prince Harry's Ceres at 25°16′ Taurus, a conjunction with an orb of 0°00′. Asteroid and Chiron contacts are worth a look, but readers usually rank them below aspects between the planets.