In a birth chart
Look for a skill the person is known for publicly. Sun tredecile Mars might belong to a fitness instructor whose classes fill because her energy is part of the show.
Refinement, harmonic aspect
Tredecile
108°, 3/10
The tredecile is a 108° aspect, three tenths of the circle, also called the sesquiquintile. Harmonic astrologers read it as talent that wants an audience: a skill two planets develop together and then present with confidence. Not to be confused with the 13th-harmonic tridecile.
The essential reading
The tredecile (108°, 3/10) is the third member of the tenth harmonic series and is sometimes called the sesquiquintile. It sits between the quintile and bi-quintile in the creative 5th/10th-harmonic family and shares their association with talent — but at a more refined, mature register. It describes a creative or structural intelligence that has been worked and polished toward authority.
- Angle
- 108°3/10 of the circle
- Nature
- FlowingHarmonic aspect
- Harmonic
- 10thwith the Decile
- Tradition
- KeplerianOrb 1.5° standard
- Sign relationship
- None fixed: the angle does not divide the zodiac into whole signs
Developmental arc
Creative Mastery
Refining a creative capability into genuine craft authority. The tredecile marks the move from having talent to commanding it — the patient shaping of ability into a mature, recognisable form.
Well integrated
A polished, authoritative command of craft or structure. The native tends to produce work with a noticeable quality of refinement and considered design.
Unresolved
Over-identification with technique at the expense of inspiration, or a perfectionism that withholds work because it is never finished enough. Can become sterile if the creative spark is not fed.
Interpretive contexts
A lasting pattern in one person
Natal chart
A high-order creative signature, especially meaningful at tight orbs and when combined with quintile-series aspects. Suggests a capacity for refined, authoritative work.
A moving planet meets a natal one
Transit
Minor on its own; most significant when activating a natal quintile or decile-series configuration.
One person’s planet meets another’s
Synastry
A refined creative partnership — two people whose collaboration has a quality of considered, mature craft rather than raw inspiration.
Technical data
104°exact 108°112°
- Standard orb
- 1.5°
- Tight orb
- 0.5°
- Signs apart
- Not whole signs
- Compatible elements
- No
- Shared modality
- No
- Harmonic
- 10th
Tradition
Keplerian harmonic astrology (10th harmonic)
Also known as
- T10
- Sesquiquintile
- 3/10 Harmonic
Related patterns
The 10th-harmonic family:
- Creative Mastery
- Refinement
- Authority
- Artistry
- Polish
In the sky now
Live tredeciles
Pairs among the Sun, Moon and planets within 3° of an exact tredecile, measured from Earth right now.
What the tredecile means
A tredecile joins two planets 108° apart. It is the third step of the decile series (3 × 36°) and also one and a half quintiles (1.5 × 72°), which is why it is sometimes called the sesquiquintile. Like the decile and quintile, it belongs to the family of aspects built on the number five, which harmonic astrologers link to making, composing and personal style.
Writers who use the tredecile tend to describe it as craft that wants to be seen. Where the quintile is the private workshop, the tredecile is the moment the work is put on the table. Take Moon tredecile Mercury in a primary-school teacher: she reads a room instinctively and turns that instinct into lessons that hold thirty children’s attention. To use the aspect, ask where the person is at their best when performing, presenting or explaining something they have practised.
- Keywords
- Presentation, flair, applied talent, confidence in a skill, showmanship
- Nature
- Soft in this site’s catalog; it supports expression rather than guaranteeing it
- Asks
- What have you practised enough to show, and who needs to see it?
- Typical feel
- Coming alive in front of others when doing the thing you know well
- A talent that travels. Venus tredecile Jupiter: a florist whose arrangements get photographed and shared, bringing in wedding commissions from other towns.
- Explaining well. Mercury tredecile Saturn: an accountant who can walk nervous clients through a tax return without jargon.
- Confident improvisation. Mars tredecile Uranus: a session drummer who can join an unfamiliar band and lift the set on the first night.
The geometry of the tredecile
- 1
Convert to longitude
Venus at 7° Leo: the sign starts at 120°, so add 7°.
127°
- 2
Convert to longitude
Pluto at 26° Scorpio: the sign starts at 210°, so add 26°.
236°
- 3
Measure the gap
236° − 127° = 109°, already the shorter way round.
109°
- 4
Compare with exact
The exact tredecile is 108°; the gap is 1° away from it.
orb 1°
- 5
Check the allowance
The standard allowance is 1.5°.
It counts
104°exact 108°112°
Divide the circle into ten steps of 36°. Three steps make 108°, the tredecile, so it is a 10th-harmonic aspect. The same series contains the decile (one step), quintile (two), biquintile (four) and opposition (five). 108° is also the interior angle of a regular pentagon, a neat link to the five-based geometry Kepler admired.
Sign relationship. 108° is three signs and 18°, so the planets fall either three signs apart, a square by sign, or four apart, a trine by sign. Our example is the first kind: Leo and Scorpio are both fixed, but Fire and Water, yang and yin. Move Venus to 20° Leo and Pluto to 8° Sagittarius and the pair is still 108° apart, now four signs apart in the same element. Read the signs separately.
| Step | Working | Result |
|---|---|---|
| Convert Venus | Leo starts at 120°; 120 + 7 | 127° |
| Convert Pluto | Scorpio starts at 210°; 210 + 26 | 236° |
| Subtract, shorter way round | 236 − 127 (under 180, no wrap needed) | 109° |
| Compare with 108° | 109 − 108 | Orb 1°00′ |
| Check the allowance | Standard 1°30′; tight 0°30′ | Valid at the standard orb only |
Applying or separating? Venus moves roughly a degree a day; Pluto moves a few arcminutes. Venus sits behind Pluto and is catching up, so the gap shrinks from 109° toward 108°: the aspect is applying and should perfect in about a day. After that the gap keeps closing, and around three weeks later Venus reaches a square to Pluto. If Venus were retrograde, the gap would widen instead and the tredecile would be separating.
Orbs and how schools differ
- Tradition
- Keplerian harmonic astrology (10th harmonic)
- What that means
- It belongs to the five-based aspect family Kepler argued for in the early seventeenth century and was given its place as a 10th-harmonic angle by twentieth-century harmonic astrologers
- Also called
- Sesquiquintile; occasionally spelled tridecile, which causes the confusion described above
In Hellenistic whole-sign practice the tredecile does not exist; our Leo–Scorpio pair would simply be read as a square by sign. Lilly-style moieties were applied to Ptolemy’s five configurations, not to this one. Modern astrologers who use it generally allow 1–2°, and most software leaves it off by default. Harmonic astrologers read it in the 10th-harmonic chart, where a 1°30′ natal orb is stretched to 15°, so they prefer narrow natal orbs. Cosmobiology, built on 45° multiples, does not use it.
Natal, transit and synastry at a glance
As a transit
A short window to present work. While transiting Mercury is tredecile natal Venus, send the portfolio, post the finished piece or give the talk you have rehearsed.
Between two charts
Two people who show each other off well. Her Jupiter tredecile his Mercury: she introduces his ideas to the right people, and he gives her plans words that persuade.
Common questions about the tredecile
No. On this site the tredecile is 108° and the tridecile is about 27°41′, one thirteenth of the circle. Some authors swap or merge the names, so always confirm the angle.
“Sesqui” means one and a half: 108° is one and a half times the 72° quintile.
Both are 10th-harmonic aspects. The decile (36°) is usually read as getting a job finished; the tredecile is more often associated with presenting and expressing a skill.
Orbit
Reading a tredecile step by step
A seven-step method for reading a tredecile, with a fully worked example.
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Orbit
The tredecile in relationships, work and inner life
Concrete scenes, one action and one pitfall for using a tredecile well.
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Orbit
Timing tredecile transits
How long tredecile transits last and how to use their short windows.
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Astral
Advanced methods
Harmonic charts, sign aversion, reception, declination and directions for the tredecile.
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Astral
Case studies
Three fictional charts that test tredecile readings, including a naming trap.
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Astral
History, debates and sources
Where the tredecile came from, how its names got tangled and what is unproven.
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