Free Semi-Sextile Calculator

A semi-sextile is a 30° angle between two planets in a birth chart. It belongs to the 12th harmonic family of minor aspects, classified as mildly tense or quietly supportive depending on the planets, the orb, and whether the bodies share a sign or fall in adjacent signs. Standard orb is 1° to 3°.

Ptolemy left it out. The Hellenistic aspect set he canonized in Tetrabiblos used five figures (conjunction, sextile, square, trine, opposition), defined by which planets could see each other through the doctrine of aspectual rays. Adjacent signs were treated as in aversion (asyndetos), unable to make visible contact, so the 30° angle was excluded. Kepler put it back in 1619 by treating the zodiac as a 12-fold harmonic division in Harmonices Mundi, and the 30° angle survived as a minor aspect because it is the irreducible building unit of every other aspect that does work.

What that means in practice: two planets at 30° are usually one sign apart. They share no element, no modality, no obvious common ground. Mars at 8° Aries semi-sextile Venus at 8° Taurus is fire cardinal staring at earth fixed across a sign boundary. The energies do not blend, but they are close enough to keep nudging each other. The aspect rewards attention and frustrates anyone who wants planets to either fight or cooperate.

The three get conflated because all are 30° multiples that aren't squares or trines. They behave nothing alike.

A sextile (60°) connects bodies in the same elemental polarity: fire with air, earth with water. The signs are two apart. The classical reading is opportunity that arrives only if you reach for it. Cooperation is built into the geometry.

A semi-sextile (30°) connects bodies in adjacent signs that share neither element nor modality. Aries to Taurus is fire cardinal next to earth fixed. Cancer to Leo is water cardinal next to fire fixed. There is no elemental bridge. The energies must coexist without resembling each other.

A quincunx (150°) connects bodies five signs apart, again with mismatched element and modality. Aries to Virgo. Taurus to Libra. The classical reading is adjustment under pressure: the planets need to work together, but the geometry gives no obvious method.

Semi-sextile and quincunx share a structural tell. Both are derivatives of a 30° rotation, both produce element and modality mismatches, both ask the chart to integrate planets that have no obvious reason to work together. The difference is intensity: 30° is close range, 150° is the long arc back.

There is no consensus orb for the semi-sextile, which is why this tool gives you three.

1° (the partile-only school). Some practitioners only commit to a semi-sextile when it is exact or near-exact. The argument: minor aspects produce real effects only at tight contact, and a 30° orb wider than a degree starts overlapping the conjunction's range of influence on the next sign over.

2° (the modern Western default). What most chart software ships with for harmonic-12 work. The middle position. Tight enough to filter incidental contacts, loose enough to catch the ones a chart reading would actually use.

3° (the wider end). Used when a luminary (Sun or Moon) is involved or when surveying a chart for any harmonic-12 activity. The reasoning: luminaries pull more weight, and a 3° contact between a personal planet and the Sun reads more strongly than a 1° contact between two outer planets. Avoid this for transit work; a 3° semi-sextile transit is essentially a non-event.

Pick the orb that matches the question you are asking. If you want only the contacts that are unambiguous, run at 1°. If you want a working list, use 2°. If you want every potential connection including the borderline ones, run at 3° and read the partile flag to separate signal from noise. Robert Hand, Frank Hamblin, and Noel Tyl have each published recommendations within this 1° to 3° range; see Horoscope Symbols, Harmonic Charts, and Synthesis & Counseling in Astrology for the source positions.

A 30° aspect can land in one of two configurations, and they read differently.

In-sign semi-sextiles place the two planets in adjacent signs, exactly as the geometry suggests. Mercury at 5° Gemini and Venus at 5° Cancer is a clean in-sign 30°. Both planets sit comfortably inside their sign, the elemental and modal contrast is fully expressed, and the aspect does what the textbooks describe: friction without crisis.

Out-of-sign semi-sextiles happen when one or both planets sit near a sign boundary. Mercury at 28° Gemini and Venus at 28° Cancer is still a 30° angle, but Mercury is about to enter Cancer and Venus is about to enter Leo. The element and modality contrast that gives the semi-sextile its character starts to scramble. By the time you read the chart, the geometric aspect is in place but the sign-based interpretation is destabilized.

Tighter still: Mercury at 29° Gemini semi-sextile Venus at 29° Cancer. The aspect is partile, but Mercury sits at the anaretic degree of Gemini and Venus at the anaretic degree of Cancer. The reading inherits anaretic-degree weight on top of the semi-sextile pattern.

The calculator flags in-sign vs out-of-sign on every aspect row. Read the in-sign ones with the standard interpretations. Read the out-of-sign ones with one extra question: which sign is each planet actually expressing from?

Two planets at 30° are not the same aspect on Tuesday and Thursday.

If the faster planet is moving toward the slower planet, the aspect is applying. The contact is sharpening. Most traditional astrologers read applying aspects as more potent because the geometry is converging on exactness. Whatever the planets are doing together, they are doing more of it.

If the faster planet is moving away from the slower planet, the aspect is separating. The contact is fading. The energetic content is already present in the chart, but the convergence has happened and the planets are pulling apart. Some traditions read separating aspects as descriptions of recent past, not active current geometry.

For a semi-sextile this distinction matters more than for a sextile or trine, because the aspect is small and the orb tolerance is tight. A 2° applying semi-sextile has weeks or months left of tightening, depending on the planets involved. A 2° separating semi-sextile is on its way out the door. The classical reading of the same orb pulls in opposite temporal directions.

The calculator computes direction from the speed differential of the two bodies, the same method used by the Applying and Separating Aspects Calculator. Applying contacts are flagged on the result row. Separating contacts are unflagged. The flag is independent of the in-sign or partile status, so a single aspect can be partile, in-sign, and applying at once, or none of those things.

Partile means within one degree of exact. The 30° angle is precise to the arcminute.

Hellenistic astrology used partile to mark the contacts that actually carry weight in a chart. A partile aspect was treated as a guaranteed expression of the planetary combination involved, not a possibility or a tendency. Modern minor-aspect work has inherited the same logic. A 30° angle at 0°47' orb does work in the chart. A 30° angle at 2°50' orb might or might not.

The calculator flags partile semi-sextiles separately from the applying or separating direction. Both can be true at once. The partile flag is the single most predictive piece of metadata on a minor aspect: it tells you which contacts to read as active and which to file under background geometry.

If you are running the calculator at 3° orb and the partile flag is on, you have a contact that meets the strictest possible threshold. If you are running at 1° orb and the partile flag is off, the aspect is borderline by the orb you chose to use. The two filters interact, and the combination is what filters the result list down to the contacts that earn a sentence in the reading. Cross-check with the Partile Aspects Calculator to see partile flags applied across every aspect family.

A semi-sextile rarely stands alone in a chart that already has interesting geometry, but it is also not the load-bearing piece of any major aspect pattern. The textbook yod requires a sextile base (60°) and two quincunxes (150°) converging on an apex. Replace the sextile base with a semi-sextile and the geometry simply does not close: from a 30° base, no apex point exists that sits at 150° from both bases. The figure cannot exist.

What can exist is a partile semi-sextile that sits next to a larger configuration. Two planets one sign apart, both touching a separate quincunx or trine to a third body, will appear in the chart as a tight 30° contact braided into a wider geometric story. The semi-sextile contributes texture but does not anchor the figure.

Kites and grand crosses occasionally include a 30° relationship between adjacent corners as a structural connector. The interpretation flows through the major aspects. The semi-sextile is the connecting strut, not the headline.

For full pattern detection, run your chart through the Aspect Pattern Scanner, which picks up kite, grand trine, T-square, grand cross, and yod configurations directly. For yod-specific work, the Golden Yod Calculator handles the quintile-biquintile variant. This calculator focuses on the 30° contacts themselves.

Transits hit semi-sextiles the same way they hit any aspect: by ephemeris position.

If transiting Saturn is at 12° Aquarius and your natal Mercury is at 12° Capricorn, transiting Saturn is making a partile semi-sextile to your natal Mercury. The contact will hold within a 1° orb for several weeks, given Saturn's speed. The reading is a 30° aspect between Saturn and Mercury, expressed in the houses they occupy in your chart.

Run the calculator on your natal chart first to find the static 30° pairs. Those are the relationships that are always present in your chart's geometry, the patterns that come pre-loaded. Transits then add new 30° contacts on a moving timeline, layering the transiting planet's current position against your natal placements.

The interpretation method does not change between natal and transit. A 30° contact is a 30° contact. The difference is duration. Natal aspects are permanent. Transits are temporary windows that range from days (Moon) to years (outer planets). Saturn's semi-sextile to your Mercury is a season; the Moon's semi-sextile to your Mercury is an afternoon.

For a transit-aware view, run a transit chart and feed the transiting positions through the same orb and direction filters. The applying flag becomes especially load-bearing in transit work, because it tells you whether the contact is sharpening or already past peak.