Secondary Progressions & Solar Arc (推運與太陽弧)

Predictive Astrology — A Day for a Year, the Solar Arc & the Birthday Chart

Overview

A natal chart says who you are; predictive astrology asks what is being switched on now. This is the first forecasting engine in the askTIAN catalogue, and it ships the three techniques modern Western astrologers actually reach for to time a year. The first is secondary progressions — the elegant 'a day for a year' symbolism, where the real movement of the sky in the days after your birth is read as the unfolding of the years of your life, so the progressed Sun creeps about a degree a year and the progressed Moon, the fastest hand on the clock, walks a whole sign roughly every two and a half years. The second is solar arc directions, which takes that same yearly rate and advances the entire chart — every planet and angle — forward as one rigid body, so that any planet can be 'directed' into an aspect that perfects in a nameable year. The third is the solar return, the chart cast for the exact instant each year when the Sun comes back to its natal degree — your astronomical birthday — read as the keynote of the twelve months ahead. askTIAN computes all three with exact astronomy, scores how densely the year is loaded with timing contacts, and reads the result honestly as a symbolic tradition for reflection — never as a fixed forecast, and explicitly never as a calculation of lifespan.

Origin & history

Secondary progressions descend from a remark in Ptolemy's 2nd-century 'Tetrabiblos' and were given their modern 'day-for-a-year' form by Placidus de Titis in the 17th century; they are sometimes still called 'secondary directions' to distinguish them from the older primary directions. Solar arc directions are a simplification and generalisation of primary directions: rather than directing each point at its own speed, the whole chart is moved by a single arc equal to the Sun's own progressed motion — an idea systematised by Naibod in the 16th century (whose mean daily solar motion gives the 'Naibod key') and popularised in the 20th century by the German cosmobiologists, above all Reinhold Ebertin in 'The Combination of Stellar Influences'. The solar return is far older still — the 'anniversary horoscope' or revolutio of medieval and Renaissance astrology, treated at length by Persian and Arabic authors and carried into the Latin tradition by Bonatti and Morin de Villefranche.

These three methods became the working core of 20th-century predictive practice precisely because they are vivid and economical. Secondary progressions give a slow, psychological unfolding — the progressed Moon's phase cycle and its monthly sign-changes are the classic 'inner weather' of a life. Solar arc directions give the sharp, datable hits: because the arc is close to one degree per year, a directed planet a degree away from a natal aspect will perfect it in about a year, which makes solar arc the technique of choice for timing concrete turning points. The solar return supplies the annual chart whose rising sign and planetary emphasis colour the year between birthdays. Modern practice still differs on the conventions, and askTIAN surfaces every fork it takes: which length of year keys the progression (the tropical year of 365.2422 days is the Astrodienst default and the one used here); how the progressed angles are moved (the Naibod-in-right-ascension key, astro.com's default, versus the Quotidian or 'Q2' angles); whether the solar arc uses the true variable progressed-Sun motion (used here) or the constant Naibod mean; and whether the solar return is cast for the birthplace or the current residence, and left tropical or precession-corrected (askTIAN ships the non-precessed natal-place return, with the alternatives labelled).

How it works

askTIAN converts the birth date, time, timezone and place to a single instant in Universal Time and builds the natal chart — the ten planets, the lunar node, the Ascendant and the Midheaven on the tropical zodiac — with its own Meeus-precision ephemeris. For an evaluation date it finds the age in tropical years and advances the real ephemeris by that many DAYS: the planets at that 'progressed' instant are the secondary-progressed chart, and the progressed Sun's distance from its natal place is the solar arc. The progressed Ascendant and Midheaven are moved by the Naibod-in-right-ascension key — the natal right ascension of the Midheaven advanced by the Sun's mean motion of 0°59′08″ per year, then converted back to ecliptic Midheaven and Ascendant by spherical trigonometry. The progressed Moon's elongation from the progressed Sun gives the lunation phase, one of the eight octiles from New to Balsamic of the roughly twenty-nine-and-a-half-year cycle. Solar arc directions add the single arc to every natal body and angle, and the engine reports the directed-to-natal aspects within orb — with the year each perfects, since a degree of arc is about a year of time. The solar return is found by Newton's method: the moment the transiting Sun returns to its exact natal longitude in the governing year is solved to under an arc-second, and the return chart is cast for the birthplace. A transparent 0–100 emphasis score, computed in code rather than by the model, measures how densely the year is loaded with tight timing contacts — hard solar-arc hits to the luminaries or the angles weigh most; it is a symbolic measure of activity, not a probability of fortune.

Good for

• Timing a specific year — which directions and progressions perfect, and roughly when
• Reading the progressed Moon's sign and lunation phase as the year's 'inner weather'
• Finding solar-arc hits to the natal chart that fall close to exact this year
• Casting the solar-return chart for a birthday feature, with its rising sign and emphasis
• Building a forecasting product on top of a real, verifiable ephemeris rather than canned text
• Layering predictive timing over a natal or Hellenistic reading already in your app
• Teaching the three core Western predictive methods side by side from one consistent source

Use cases

This-Year Forecast

Send a birth chart and an evaluation date and the API returns the secondary-progressed planets and angles, the solar arc for the year, and the directed and progressed aspects within orb — each tagged applying or separating with the year it perfects. A complete, deterministic spine for an annual reading, no LLM required.

Progressed-Moon Mood Tracker

The response gives the progressed Moon's exact sign and degree and its lunation phase (New through Balsamic). Because the progressed Moon changes sign about every two and a half years and cycles phase over about twenty-nine, apps can render a slow 'inner weather' timeline that feels personal and updates meaningfully year over year.

Solar Arc Timing Hits

The whole chart is advanced by the solar arc and the directed-to-natal contacts are returned with the calendar date each perfects. Since a degree of arc is about a year, this surfaces the datable 'headline' turning points — directed Sun, Mars or Saturn reaching a natal angle or luminary — that solar arc is prized for.

Solar Return Birthday Chart

The API solves the exact moment the Sun returns to its natal degree each year and casts the chart for the birthplace, returning the solar-return Ascendant, Midheaven and planets. A natural engine for a birthday-return feature or a 'year ahead' product keyed to a real astronomical event.

Key terms

Secondary progression

The 'a day for a year' symbolic measure: the position of the real sky one day after birth represents the first year of life, two days the second, and so on. The progressed Sun moves about 1° per year and the progressed Moon about 13°, making the Moon the most dynamic point in the technique.

Progressed Moon & lunation phase

The progressed Moon changes sign roughly every two and a half years and completes its cycle of phases relative to the progressed Sun — New, Crescent, First Quarter, Gibbous, Full, Disseminating, Last Quarter, Balsamic — over about twenty-nine and a half years, a widely-used map of a life's developmental seasons.

Solar arc direction

A predictive method that advances every planet and angle of the chart forward by the same arc — the distance the secondary-progressed Sun has travelled, close to one degree per year of life. Because the arc is shared, any planet can be 'directed' to an aspect that perfects in a datable year.

Solar arc vs Naibod key

The arc added in solar-arc directions can be the TRUE variable motion of the progressed Sun (used here, so summer-born and winter-born charts differ slightly) or the constant Naibod mean of 0°59′08″ per year. The progressed angles use the Naibod-in-right-ascension key, the astro.com default.

Solar return (solar revolution)

The chart for the exact instant each year when the transiting Sun returns to its natal longitude — the astronomical birthday, within a day of the calendar one. Its rising sign and planetary emphasis are read as the keynote of the year ahead; askTIAN casts it for the birthplace on the tropical zodiac.

Applying vs separating

An aspect is applying when the moving point is still closing toward exactness and separating once it has passed. Applying contacts are read as the active, building influences of the year; the engine reports each contact's status and the calendar date it perfects.

The one-degree-a-year rule

Because both the progressed Sun and the solar arc advance at roughly a degree a year, the orb between a directed or progressed point and its natal target translates directly into time: a contact one degree from exact will perfect in about a year, which is what makes these methods so usable for dating events.

API

The askTIAN predictive API computes all three core Western timing techniques from one ephemeris: secondary progressions (the 'day-for-a-year' chart with progressed angles by the Naibod-in-right-ascension key and the progressed-Moon lunation phase), solar arc directions (the whole chart advanced by the true solar arc, with directed-to-natal contacts dated to the year they perfect) and the solar return (the birthday chart solved to the arc-second and cast for the birthplace). It returns a transparent 0–100 emphasis score measuring how densely the year is loaded with tight contacts, and a fixed-section LLM reading. Provenance is surfaced honestly — the day-for-a-year and solar-return methods are well-attested, the shipped conventions (tropical-year key, Naibod-in-RA angles, true arc, non-precessed natal-place return) are the common modern defaults with the legitimate alternatives labelled, and slow outer planets can drift about a degree, so very tight outer-planet contacts are flagged for cross-check. It deliberately does NOT calculate length of life. Set interpretation:false for structured data only.

Endpoint: POST /trpc/progressions.calculate — 7 TIAN Points. See the API documentation and Playground.