Void-of-Course Moon (月空亡)

When the Moon Runs Void, Start Nothing New

Overview

Every two or three days, the Moon finishes her business in a sign: she perfects her last exact aspect to any planet, and from that moment until she crosses into the next sign she 'runs void of course.' The old horary masters treated the void Moon as dead air — William Lilly's verdict was that 'matters go hardly on' — and centuries of electional practice built the same fence: sign nothing, launch nothing, marry no one while the Moon is void; finish things instead, rest, tidy, reflect. Modern practitioners still consult void tables before scheduling anything that matters, which makes this one of astrology's most practical daily tools. askTIAN computes the windows to the minute on its own ephemeris — the current status, the exact aspect that opened the void, and the coming days' windows — with the sign-ingress moments cross-checked against the published tables and locked in tests.

Origin & history

The rule belongs to horary astrology's core machinery: the Moon is the universal significator of motion and negotiation, and a Moon that will complete no further aspect before leaving her sign has, in the judgment of the tradition, nothing left to negotiate — the question 'goes void.' Lilly gave the doctrine its most-quoted line and also its most-quoted exception: in Taurus, Cancer, Sagittarius and Pisces the void Moon 'performs somewhat' — signs of her dignity and joy — an exception askTIAN flags on each window rather than silently applying. Only the five Ptolemaic aspects count (conjunction, sextile, square, trine, opposition), and only their PERFECTION — an aspect applying within orb does not end a void; the aspect must go exact.

The tradition split quietly in the twentieth century over which planets count. Lilly's practice knew seven; the almanac tables modern readers actually consult — the Llewellyn lineage and its descendants — aspect the Moon to all ten, and because the outer planets add late aspects, the modern set SHORTENS void windows, sometimes dramatically. askTIAN implements both, defaulting to the modern set so its windows match the published tables users compare against, with the classical seven selectable for horary purists — and the test suite proves the structural property that the classical set never shortens a window. The engine's ingress moments were externally anchored before shipping: three published September 2026 sign entries reproduced within about two minutes each — the documented drift of the shared ephemeris — and those anchors are locked as tests.

How it works

Give a date, time and timezone. The engine walks the Moon sign by sign across the requested horizon (one to fourteen days): each sign-exit moment is found by bisecting her longitude against the next 30° boundary, and every aspect perfection inside the sign is found by a dense fifteen-minute scan of the Moon's separation from each planet, each crossing bisected to the minute — one ephemeris call per step, so a full week computes in under a second. The last perfection in each sign opens that sign's void window, which closes at ingress. The response reports whether the Moon is void at the query instant (with the window and its opening aspect), her current sign and longitude, and the upcoming windows — each with both timestamps in the caller's timezone, the aspect and planet that opened it, the next sign, and the Lilly-exception flag. Rare whole-sign voids — a transit with no perfection at all — are reported as exactly that.

Good for

• The scheduling question astrology users actually ask: is the Moon void right now?
• A week or two of void windows at a glance — for planning launches, signings, weddings
• Electional stacking: void windows beside almanac.electDate and bazi.day for cross-tradition date-picking
• Horary practice with the classical seven-planet set, one parameter away
• Calendar features: paint the void windows onto a scheduling UI, to the minute
• Seeing exactly which aspect opened each void — the astrology shown, not asserted
• Timezone-correct windows for any locale without client-side conversion

Use cases

The Void Banner

A single call powers the classic 'Moon is void until 14:32' banner — current status, the closing ingress, and the sign she enters next, in the user's own timezone.

The Launch Planner

Fourteen days of windows rendered as a calendar overlay: the clear stretches for starting things, the void stretches for finishing them — electional astrology as a scheduling feature.

Cross-Tradition Date Picking

Stack the void windows with almanac.electDate's 黃曆 ranking and bazi.day's personal workup — Western, almanac and 子平 timing on one date, three traditions agreeing or arguing visibly.

Horary Workbench

The classical seven-planet set with per-window opening aspects — the exact data a horary practitioner checks before judging a chart, computed instead of looked up.

Key terms

Void of Course

The Moon's state after perfecting her last Ptolemaic aspect in a sign, lasting until she enters the next — the tradition's no-new-business window.

Perfection

An aspect gone mathematically exact. Only perfection ends the counting — an aspect applying within orb does not; the void begins at the last exact hit.

The Ptolemaic Five

Conjunction, sextile, square, trine, opposition — the only aspects the void doctrine admits; minor aspects never count.

The Performing Signs

Taurus, Cancer, Sagittarius, Pisces — where Lilly allowed the void Moon to 'perform somewhat'; flagged per window, applied by the reader.

Modern vs Classical Sets

Ten planets (the published tables' practice, shorter voids) versus Lilly's seven (longer voids) — selectable, with the never-shortens property test-proven.

Whole-Sign Void

The rare transit in which the Moon perfects nothing at all — reported honestly as a full-transit void rather than forced into the usual shape.

Ingress

The Moon's entry into the next sign — the moment every void closes, computed by bisection and externally anchored against published tables.

Minute Precision

Windows are reported to the minute in the caller's timezone, inheriting the shared ephemeris's documented ~2-minute drift.

API

The askTIAN Void-of-Course API computes the horary tradition's most practical rule on the API's own ephemeris: current void status with the exact opening aspect, and one to fourteen days of upcoming windows to the minute in the caller's timezone — sign ingresses externally cross-checked against the published September 2026 tables (within ~2 minutes on all anchors, locked as tests), the modern/classical planet-set split selectable with the classical-never-shortens property test-proven, Lilly's performing-signs exception flagged per window, perfection-only semantics stated, and whole-sign voids reported honestly. Pure deterministic compute, no LLM, 3 TIAN.

Endpoint: POST /trpc/moon.voidOfCourse — 3 TIAN Points. See the API documentation and Playground.