equinox
Enable an AI agent to identify an equinox, assess the reliability and local relevance of its timing, and decide which seasonal labels, observations or schedules it can support.
Research draft, second pass
A second pass drafted this model: the structure a model of this thing needs, and what is known about it in the world. The line under this one says how the second half was obtained - researched against sources, or recalled without web access, in which case nothing here was read anywhere and every claim is a lead to verify. Unreviewed either way.
Researched by: Codex + Grok
Purpose and description
Enable an AI agent to identify an equinox, assess the reliability and local relevance of its timing, and decide which seasonal labels, observations or schedules it can support.
An equinox is either of the two instants in a tropical year when the apparent geocentric ecliptic longitude of the Sun is 0° or 180°, so the Sun's centre lies in Earth's equatorial plane and the ecliptic intersects the celestial equator at the Sun.
It can be Resolve a year and crossing direction to a particular equinox occurrence.; Compare candidate event times and retain a supported estimate at a precision appropriate to the task.; Translate the event instant into local dates and create reminders using an explicit offset or scheduling window.; Assign a seasonal label when the hemisphere and seasonal convention support it.; Assess an equinox-related observation plan using separately supplied location, horizon and daylight information.; Flag claims that confuse the equinox with equal daylight, a weather transition or an associated celebration..
Distinguishing features
An equinox concerns a solar crossing of the celestial equator; a solstice concerns an extremum in the Sun's north-south seasonal position.
An equinox is an event instant under a stated astronomical convention, rather than an entire calendar day or season.
An equinox does not require exactly equal daylight and darkness at an observer's location; that local condition belongs to equilux.
The same equinox receives opposite spring or autumn labels in the Northern and Southern Hemispheres, so a season name alone cannot identify it.
A celebration named for an equinox is a separate event whose scheduled date need not coincide with the astronomical instant.
Scope
+ Identification of a particular equinox by year and direction of solar crossing
+ Predicted or retrospectively calculated event time, with its reference conventions and uncertainty
+ Evidence supporting an equinox identification and reconciliation of competing time estimates
+ Translation of the event instant into local dates and hemisphere-dependent seasonal labels
+ Assessment of whether the event supports a proposed observation, seasonal boundary or scheduling rule
- Solstices and other astronomical events with different defining conditions
- Equilux dates and the full calculation of local sunrise, sunset and daylight duration
- Weather, climate and ecological season transitions
- Religious festivals, civic holidays and cultural observances associated with an equinox
- General orbital mechanics, ephemeris production and timekeeping infrastructure
Characteristics
- Crossing direction
- northward across the celestial equator | southward across the celestial equator | unresolved Distinguishes the two equinoxes without depending on a hemisphere-specific season name.
- Occurrence year
- year with an explicitly named calendar Separates a particular occurrence from the recurring class of equinox events.
- Event instant
- timestamp with declared time scale, precision and uncertainty where available Supports comparison, countdowns and scheduling without confusing formatting precision with timing accuracy.
- Astronomical definition used
- documented event criterion, reference frame and treatment of solar position Makes it possible to determine whether different published times refer to equivalent calculations.
- Timing evidence
- links to a consulted ephemeris, published event table or documented calculation Makes the event time traceable and allows an agent to assess its suitability for a particular use.
- Temporal status
- upcoming | within the adopted event-time tolerance | elapsed | unresolved Supports action decisions without treating an instantaneous event as naturally lasting a whole day.
- Local calendar expression
- event instant mapped to a named time zone and calendar Explains why one occurrence can have different local dates.
- Seasonal interpretation
- hemisphere and named seasonal convention mapped to a seasonal label, or no applicable label Prevents universal assignment of spring or autumn and distinguishes astronomical seasons from other seasonal systems.
Where this came from
wikidata · CC0 1.0
Also registered as vr.tr.equinox
Drafted structure
Bundle to layer to finding to question, as the second pass will find it: 5 bundles · 9 layers · 16 findings · 27 questions.
Equinox identity Establishes which solar crossing is being represented and separates the astronomical event from related names and observances.
An agent cannot identify an equinox reliably from a season name or calendar date alone.
Crossing and occurrence
Identifies the event through its astronomical criterion, crossing direction and occurrence year.
Identified solar crossing
Record the intended crossing and year, including whether the reference denotes one occurrence or the recurring event class.
- Does this reference identify a particular year's equinox or equinoxes as a recurring class? definition
- Is the intended crossing northward or southward, and what evidence resolves ambiguous labels such as 'spring equinox'? definition
Neighbouring event boundaries
Separates equinox identity from solstice, equilux and cultural-event identity.
Equinox reference boundary
Record whether a cited date or label actually refers to the astronomical crossing, a local daylight condition or an associated observance.
- Does the cited date denote the solar crossing, local equal day and night, or a celebration linked to the equinox? boundary
- Which related records should be linked while preserving their separate identities? boundary
Event time and evidence Makes the adopted equinox instant traceable, comparable and appropriately precise.
Equinox-based decisions depend on a defensible instant and on understanding differences between published estimates.
Timing convention
Captures the conventions required to interpret a calculated or published equinox time.
Interpretable event instant
Record the event time together with its time scale, astronomical criterion and stated precision.
- What equinox instant is supplied, in which time scale, and with what stated precision or uncertainty? measurement
- Which reference frame and treatment of solar position define the crossing in this source? definition
Timing support and disagreement
Assesses the provenance of event times and whether apparent discrepancies affect the intended use.
Supported timing selection
Retain the consulted source and explain selection among candidate times without silently discarding disagreement.
- Which consulted publication or reproducible calculation supports each candidate equinox time? provenance
- After accounting for time scales and rounding, what discrepancy remains and does it exceed the task's timing tolerance? measurement
- Which estimate may be adopted for this task, and what unresolved limitation must accompany it? action
Local and seasonal meaning Relates one astronomical instant to local calendars, seasonal conventions and daylight expectations.
The equinox is shared as an astronomical event, while its date labels and practical meaning depend on context.
Calendar and hemisphere
Derives local date and seasonal language from explicit contextual choices.
Contextual event label
Record how the occurrence is expressed for a particular time zone, calendar, hemisphere and seasonal convention.
- What local date and time correspond to the adopted instant in the requested time zone and calendar? measurement
- Which hemisphere and seasonal convention justify a spring or autumn label, or make either label unsuitable? boundary
Daylight interpretation
Controls conclusions about local daylight that may be drawn from the equinox.
Local daylight claim
Distinguish the astronomical crossing from claims about measured day length, referring local daylight calculations to their own evidence.
- Is the requested claim about the equinox instant or about equal daylight and darkness at a specified location? boundary
- What location, horizon, atmospheric assumptions and sunrise/sunset definitions support any associated daylight estimate? measurement
- Should the agent attach a separate equilux or daylight record before answering the local claim? action
Equinox-based decisions Determines how the identified and supported event may be used in schedules, observations and seasonal rules.
Knowing an equinox time does not by itself establish a valid reminder, observing opportunity or seasonal trigger.
Status and scheduling
Relates the event instant to the current time and an explicit scheduling policy.
Event-relative action window
Record whether an action follows the astronomical instant, its local calendar date or a deliberate offset.
- Relative to the current time and adopted timing tolerance, is this equinox upcoming, being reached or elapsed? measurement
- Should the action occur at the instant, on its local date, or at a specified offset, and which time zone governs the schedule? action
Observation and seasonal triggers
Checks whether the proposed use follows from the equinox or depends on additional conditions.
Supported equinox use
Record the intended use and the external conditions needed before the event can justify an observation or seasonal action.
- Does the proposed seasonal trigger explicitly use an astronomical boundary, or does it require weather, ecological or cultural evidence? boundary
- For an observing activity, what observable phenomenon is intended and which local conditions must be checked separately? action
- What timing discrepancy or missing contextual information would require deferring or revising the proposed action? action
Evidence and external alignment What the world already says about this thing, gathered so the model can be checked against it.
A model that cannot be lined up against existing standards, identifiers and practice cannot be adopted by anyone who already uses them.
Reported evidence
Findings from the breadth pass, kept separate from the structural claims.
Kinds and varieties
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- March equinox (Sun at ecliptic longitude 0°; northern spring / southern autumn)
- September equinox (Sun at ecliptic longitude 180°; northern autumn / southern spring)
- True (apparent) equinox of date, including nutation
- Mean equinox, with nutation averaged out and used as a reference-frame origin
- Equinox as a celestial point (First Point of Aries or First Point of Libra)
- Catalogued reference equinox of an epoch (B1950.0, J2000.0, and similar)
- Civil equinox date in a named time zone, as opposed to the TT/UTC instant
- Which of these kinds and varieties hold for the sense of equinox this model covers, and on what evidence? provenance
Identifiers and schemes
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- solar ecliptic longitude - 0° (March equinox) or 180° (September equinox) - Geocentric apparent longitude of the Sun; the operational astronomical definition.
- reference-frame epoch - mean equinox and equator of JYYYY.y (e.g. J2000.0) - Used to name equatorial coordinate systems; J2000.0 is 2000 January 1.5 TT.
- East Asian solar term - 春分 (chūnfēn / shunbun) and 秋分 (qiūfēn / shūbun) - The fourth and sixteenth of the twenty-four solar terms, defined at the same longitudes.
- Which of these identifiers and schemes hold for the sense of equinox this model covers, and on what evidence? provenance
Standards and regulation
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- IAU 2006 Resolution B1 (P03 precession and definition of the ecliptic) - International Astronomical Union
- IAU 2000 Resolution B1.6 (precession-nutation model) - International Astronomical Union
- IAU 2009 System of Astronomical Constants - International Astronomical Union
- Astronomical Almanac conventions for equinox and solstice instants - US Naval Observatory and HM Nautical Almanac Office
- Gregorian calendar reform (keeping the March equinox near 21 March for the Easter computus) - papal bull Inter gravissimas (1582), implemented in civil calendars
- ISO 8601 - representation of the equinox as a dated instant, not a definition of the event
- Which of these standards and regulation hold for the sense of equinox this model covers, and on what evidence? provenance
Real-world use
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Almanacs and ephemerides publish the TT/UTC instant of each equinox as a seasonal and calendrical marker.
- The Iranian (Solar Hijri) calendar and Nowruz, and the Baháʼí calendar's Naw-Rúz, are tied to the March equinox instant.
- Western computus uses an ecclesiastical approximation of the March equinox (taken as 21 March) to fix Easter.
- Astrometry and spacecraft navigation convert between equinox-of-date equatorial coordinates and a fixed frame such as ICRS/J2000.
- Japan gazettes 春分の日 and 秋分の日 as national holidays on the civil dates of the two equinoxes.
- Agriculture, daylight models, and public 'first day of spring/autumn' reporting treat the equinox as a seasonal hinge, even when they quote only the calendar date.
- Which of these real-world use hold for the sense of equinox this model covers, and on what evidence? provenance
Typical measurements
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Gregorian UTC date of the March equinox - 19-21 March - calendar date
- Gregorian UTC date of the September equinox - 21-24 September - calendar date
- Interval between successive March equinoxes (tropical year) - 365.2422 - mean solar day
- Apparent geocentric solar declination at the instant - 0 (by definition of the event) - degree
- Civil day length at the equator on the equinox date - about 12.1-12.14 (longer than 12 h because of refraction and the Sun's semidiameter) - hour
- General precession in longitude - about 50.29 - arcsecond per tropical year
- Mean obliquity of the ecliptic (J2000.0) - 23.4393 - degree
- Which of these typical measurements hold for the sense of equinox this model covers, and on what evidence? provenance
Failure modes and hazards
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Treating the equinox as the day of equal day and night: atmospheric refraction and the Sun's apparent radius make daylight longer than night on the equinox date at most latitudes.
- Using a local calendar date as if it were the physical instant: the same UTC moment can fall on different dates in Tokyo and Honolulu.
- Reducing star or spacecraft coordinates with the wrong equinox (mean versus true, B1950 versus J2000, or equinox-based RA versus ICRS) produces systematic pointing errors.
- Assuming the First Point of Aries still lies in the constellation Aries; precession has moved it into Pisces.
- Seasonal and holiday planning that follows 'vernal/autumnal' labels without hemisphere or time-zone checks.
- Which of these failure modes and hazards hold for the sense of equinox this model covers, and on what evidence? provenance
Regional variation
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- IAU and many almanacs prefer March/September equinox over vernal/autumnal because the seasons reverse by hemisphere.
- In southern-hemisphere English, 'spring equinox' often means September.
- Iran and neighbouring solar-Hijri users define the year from the March equinox instant (Nowruz), not from a fixed civil date.
- East Asian calendars treat 春分 and 秋分 as solar terms at exact solar longitudes 0° and 180°.
- Japan makes both equinox civil dates national holidays, with the year's dates announced by government gazette.
- Which of these regional variation hold for the sense of equinox this model covers, and on what evidence? provenance
Neighbouring kinds and how to tell them apart
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- solstice - A solstice is the instant when the Sun's apparent ecliptic longitude is 90° or 270° (maximum or minimum declination); an equinox is 0° or 180° (declination zero).
- equilux - Equilux is the date when civil day and night are actually equal at a given latitude; it is typically a few days from the equinox except near the equator.
- ICRS / celestial intermediate origin - ICRS right ascension is not origin-defined by the equinox; equinox-based RA of date and ICRS RA of a source differ by the accumulated precession-nutation and frame bias.
- perihelion and aphelion - Those are extrema of Earth-Sun distance and do not coincide with the equinoxes; they currently fall in early January and early July.
- cross-quarter day - Cross-quarter days (e.g. Imbolc, Beltane in some calendrical systems) are midpoints between an equinox and a solstice, not the equator-crossing instants.
- Chevrolet Equinox (and other product names) - A homonym: a motor-vehicle model and various software/brands; they share no astronomical or calendrical definition.
- Which of these neighbouring kinds and how to tell them apart hold for the sense of equinox this model covers, and on what evidence? provenance
Sources
- Equinox - Civil dates, seasonal naming, the equal-day-and-night misconception, and the distinction from equilux.
- Explanatory Supplement to the Astronomical Almanac, 3rd edition - Specialist definition of the equinox as a solar-longitude event, mean versus true equinox, and the equinox as a celestial-reference origin.
- Astronomical Algorithms, 2nd edition - Computation of equinox instants from solar longitude, tropical-year spacing, and practical date ranges.
- IAU 2006 Resolution B1: Adoption of the P03 Precession Theory and Definition of the Ecliptic - Current IAU treatment of precession, the ecliptic, and the moving equinox as a reference-frame concept.
What the second pass must settle
- Does registry entry vr.tr.equinox intend Earth's astronomical equinox, a broader planetary event class, or another meaning that explains its ACT / ACT.ACT placement?
- Does an existing Vercy world model already own equinox identity or seasonal astronomical events, requiring a link rather than a separate publication?
- Which astronomical definition and reference conventions should govern the model when consulted sources use different equinox calculation criteria?
- Which timing sources, historical range and future prediction range can be supported, and what precision is defensible across those ranges?
- Which seasonal naming conventions and equatorial cases must the model support without imposing a spring/autumn interpretation?