0.8.0

@0xf3cd/celestial

Astronomical calculations and Gregorian / Chinese Lunar date conversion from CelestialCalendar, as an ESM package that bundles its own WebAssembly module.

中文

This guide describes version 0.9.0. The install command below gets the latest version on npm; an older installed version may lack some APIs shown here, so check its version before using an example.

Install

Install from npm in your application directory:

npm install @0xf3cd/celestial

For each release, the matching GitHub Release also provides celestial-wasm.zip, which contains a tarball byte-identical to the npm publication and its SHA-256 checksum file.

Node 22 or newer is supported. Browser consumers are tested on Chrome and Firefox. Importing the package performs no I/O; call init() once and await it before using the synchronous calculation APIs.

Each JavaScript block below is a standalone ES module for example.mjs in your application directory; run it with node example.mjs from that directory.

Examples

Chart Snapshot at a Fixed Instant

chart.snapshot() is new in 0.8.0.

import { init, chart } from "@0xf3cd/celestial";

await init();
const snapshot = chart.snapshot(
  { year: 2026, month: 1, day: 1, fraction: 0.5 },
  {
    scale: "utc",
    location: { latitudeDeg: 51.5, longitudeDeg: 0 },
    system: "placidus",
    model: "default",
  },
);
console.log(snapshot.times); // { jdUt1, jdeTt }
console.log(snapshot.bodies[2]); // Mercury: position, distance and longitude rate
console.log(snapshot.houses.cuspsDeg); // twelve house cusps in house order

civil is a Gregorian date with a day fraction in [0, 1); a JavaScript Date is not converted automatically. scale is required and is "utc" or "ut1"; input before 1972-01-01 must select scale: "ut1" explicitly. Civil years are limited to 1885–2099. Latitude is north-positive degrees in (-90, 90); longitude is east-positive degrees in [-180, 180]. system is required and is "equal", "wholeSign", or "placidus". Omitting model, or setting it as an own property to undefined or "default", selects Algo5; "algo1" through "algo5" select a model explicitly. Input fields must be own properties: an inherited model throws instead of silently falling back to the default model. options accepts only the four keys scale, location, system, and model; a misspelled or extra key throws TypeError.

The result contains times: { jdUt1, jdeTt } and fourteen bodies in a fixed order, whose target values are sun, moon, mercury, venus, mars, jupiter, saturn, uranus, neptune, pluto, meanAscending, meanDescending, trueAscending, and trueDescending. Each member has longitudeDeg, latitudeDeg, distanceAu, and longitudeRateDegPerTtDay. All members use the true ecliptic and equinox of date: the Sun, Moon, and planets give apparent geocentric coordinates; the nodes give mean / true node longitudes with zero latitude and null for distanceAu. All distances are in AU, including the Moon's; the standalone moon.apparentGeocentricCoordinate() gives the Moon's distance in kilometres. longitudeRateDegPerTtDay is the instantaneous longitude rate in degrees per TT day: a negative value means the longitude is decreasing at that instant (retrograde motion). houses contains ascendantDeg, midheavenDeg, descendantDeg, imumCoeliDeg, and cuspsDeg, with every angle in [0, 360) degrees. The result is made of independent plain objects and arrays; later calls or WASM memory growth do not change an earlier result.

The core checks the converted JDE(TT) against the shared range [2409543.875, 2488069.125); Algo4 also requires JDE < 2464328.5. UT1 derived from UTC is modelled, not observed DUT1; see Time scales. Ascendant directions that degenerate at round-off scale, and Placidus polar cases, fail. Type or record-shape errors throw TypeError; invalid dates, day fractions, locations, or option strings throw RangeError; calling before initialization, allocation failure, native calculation failure, or a malformed native success record throws CelestialError, with operation set to "chart.snapshot". recorded is true only when the message came from the native error channel. A failure never returns a partial snapshot.

Today's Lunar Date

Use /date to choose the fixed UTC+8 civil date, then convert it with the selected algorithm; "today" here does not depend on the host time zone. Destructure only the year, month, and day: the full civil record contains fraction, which a Gregorian date input must not carry.

import * as celestial from "@0xf3cd/celestial";
import { dateToCivilAtOffset } from "@0xf3cd/celestial/date";

await celestial.init();
const { year, month, day } = dateToCivilAtOffset(new Date(), 480);
const gregorian = { year, month, day };
const lunar = celestial.lunar.fromGregorian("algo3", gregorian);
console.log(gregorian, lunar);

The date bridge does not convert clock UTC to UT1 or TT. For the lunar date basis and algorithm year domains, see Lunar Algorithms.

Next Jieqi and Remaining Days

The supplied cutoff is 2026-12-31 00:00 UT1. No Date timestamp is treated as UT1.

import * as celestial from "@0xf3cd/celestial";

await celestial.init();
const cutoffUt1 = { year: 2026, month: 12, day: 31, fraction: 0 };
const cutoffJd = celestial.time.ut1ToJd(cutoffUt1);
const upcoming = [2026, 2027]
  .flatMap((year) => Object.values(celestial.Jieqi).map((jieqi) => {
    const event = celestial.jieqi.moment(year, jieqi);
    return { ...event, jdUt1: celestial.time.ut1ToJd(event.momentUt1) };
  }))
  .filter((event) => event.jdUt1 > cutoffJd)
  .sort((a, b) => a.jdUt1 - b.jdUt1)[0];
if (upcoming === undefined) throw new Error("No later Jieqi in the queried years.");
console.log(celestial.jieqi.name(upcoming.jieqi));
console.log("UT1:", upcoming.momentUt1);
console.log("Remaining UT1 days:", upcoming.jdUt1 - cutoffJd);

This enumerates all terms in both years and orders them by moment, not enum value. Remaining days are fractional UT1 days. momentUt1 is UT1, not UTC or a UTC+8 wall clock. This is a fixed contemporary query across one year boundary, not a search algorithm for the entire API domain; the query-year domain and failure conditions of jieqi.moment() are listed under API Contract.

Moon Illumination and a Jieqi Moment

2448724.5 is a JDE on TT. The Jieqi result's momentUt1 is a UT1 civil moment, not a UTC+8 display time.

import * as celestial from "@0xf3cd/celestial";

await celestial.init();

const moon = celestial.moon.illumination(2448724.5);
const lichun = celestial.jieqi.moment(2026, celestial.Jieqi.LICHUN);
console.log(moon.fraction, lichun.momentUt1, celestial.jieqi.name(lichun.jieqi));

Gregorian / Lunar Round Trip

import * as celestial from "@0xf3cd/celestial";

await celestial.init();
const result = celestial.lunar.fromGregorian("algo3", { year: 2026, month: 8, day: 15 });
console.log(result);
console.log(celestial.lunar.toGregorian("algo3", result));

Initialization and Browser Deployment

Concurrent and repeated init() calls share one initialization promise, resolving to undefined. A failed load rejects with the loader's original error, and a later explicit call may retry. Before initialization completes, namespace methods throw CelestialError before argument validation, with operation set to the public method name and recorded === false, even for zero-count requests.

For browser deployment, make sure the package's .wasm asset is reachable at the URL your build output points to, and have the static server send it with Content-Type: application/wasm.

API Contract

The root entry provides the config, time, sun, moon, jieqi, lunar, and chart namespaces, and the package ships TypeScript declarations. The package uses only the jieqi / lunar names, with no solarTerms / lunarCalendar aliases. Model, Moon phase, and logging options are string-literal unions such as "full", "algo3", and "debug". Jieqi is a frozen object of 24 named constants, from Jieqi.LICHUN = 0 to Jieqi.DAHAN = 23, spelled as in the Python package. The TypeScript type Jieqi is the matching 0 | 1 | ... | 23 union; a plain number variable must be narrowed before it is passed to jieqi.moment() or jieqi.name(). jieqi.name() returns the Chinese name, for example "立春" for Jieqi.LICHUN.

Keep time scales and units distinct:

• Operation names for JD inputs and outputs distinguish UT1 from JDE (TT).
• jieqi.moment() returns { jieqi, momentUt1 }. Its civil moment is UT1, not UTC or a UTC+8 wall clock; the same instant shown in UTC+8 can fall on a different date. Establish the time-scale conversion before displaying it as UTC or a fixed-offset time. The returned UT1 year can differ from the requested year.
• sun.apparentSolarTime() takes a UTC civil moment and an east-positive geographic longitude in [-180, 180] degrees.
• Angle results are in degrees. Sun distance is in AU. Moon distance from moon.apparentGeocentricCoordinate() is in kilometres; chart.snapshot() gives every distance, the Moon's included, in AU.
• time.deltaT() returns seconds.
• The equation of time is in degrees of hour angle; multiply by 240 for seconds of time.
• sun.apparentGeocentricCoordinate(jde) and moon.apparentGeocentricCoordinate(jde) each return one coordinate record for the given TT-based JDE.

jieqi.moment(year, index) accepts a Gregorian year in [401, 32766] and a Jieqi index in [0, 23]. Even with in-range input, it throws CelestialError when the native calculation cannot produce a unique moment. sun.longitudeCrossings(year, longitudeDeg), moon.phaseMoments(year, phase), and moon.newMoonsInYear(year) accept Gregorian years in [1, 32766].

time.localApparentSiderealTime(jdUt1, longitudeDeg) accepts a finite JD on UT1 whose Gregorian year is in [401, 32766], and a finite, east-positive geographic longitude in [-180, 180] degrees. It returns degrees in [0, 360). The native boundary checks the year range, so a JD outside it throws CelestialError, not the RangeError used by the JavaScript finiteness and longitude checks.

sun.longitudeCrossings(year, longitudeDeg) accepts a finite apparent geocentric solar longitude in [0, 360) degrees. It returns an array of TT-based JDEs, or [] when the Sun does not reach that longitude in the year.

After initialization, wrong shapes and types throw TypeError, and JavaScript range checks throw RangeError. Native failures throw CelestialError, whose operation is the public method name and whose recorded says whether the message came from the native error channel. A legitimate absence remains null or [].

moon.newMoonsAfter(jde, count) accepts count in [0, 4096]; zero returns []. The cap keeps the WASM output buffer within 32 KiB.

Date Records

The three input kinds below are mutually exclusive, both in TypeScript and at runtime:

Civil moments and Gregorian dates need an integer year in [1, 32767] and a month and day that form a valid Gregorian date; a lunar year and date must exist in the selected algorithm. fraction is a finite day fraction in [0, 1). The runtime allows unrelated extra properties, but rejects an excluded field even when it is inherited or set to undefined. With TypeScript's exactOptionalPropertyTypes enabled, type checking also rejects an explicit undefined on the optional never fields.

CivilDateTimeResult adds an integer hour and minute and a possibly fractional second to CivilDateTime. time.jdeToUt1(), sun.apparentSolarTime(), and JieqiMoment.momentUt1 use this output. These fields are derived from the day fraction and are not rounded to milliseconds; second counts seconds, not milliseconds. A result can still be passed as a civil-moment input: functions read fraction, not the derived clock fields. The operation name, not this shared record type, says which time scale applies.

Record inputs are recognized by shape: explicit own fields and excluded tags decide the kind, not the constructor, and Date methods or timestamps are never read. A plain Date lacks the required fields; use the date bridge below to turn a timestamp into civil fields. A civil-moment result or a lunar date cannot be passed directly to lunar.fromGregorian(); choose the date basis first, then build a GregorianDate explicitly.

Fixed-Offset Date Bridge

@0xf3cd/celestial/date is a pure calendar bridge with no runtime dependencies. It does not import the root entry or load WASM, and needs no init(). Offsets are minutes east of UTC; 480 selects fixed UTC+8.

import { civilAtOffsetToDate, dateToCivilAtOffset } from "@0xf3cd/celestial/date";

const date = new Date("2024-02-09T16:00:00.123Z");
const eastEight = dateToCivilAtOffset(date, 480);
const restored = civilAtOffsetToDate(eastEight, 480);
console.log(eastEight);
console.log(restored.toISOString(), restored.getTime() === date.getTime());

Here 2024-02-09T16:00:00.123Z becomes the UTC+8 civil date 2024-02-10, 00:00:00.123. The same-offset round trip preserves the Date millisecond timestamp, so the last line ends with true.

offsetMinutesEast is a safe integer in [-1439, 1439], in minutes, positive east of UTC. Conversion uses epoch arithmetic and UTC fields, never the host's local time zone, Intl, locale-dependent parsing, IANA zones, or DST rules.

Converting a Date to a civil moment keeps the host value's millisecond resolution; the fractional second carries no extra sub-millisecond information. Converting a civil moment to a Date rounds the non-negative local time of day to the nearest millisecond, rounds an exact half millisecond toward the later civil moment, and handles carry into the next day explicitly. For any representable Date whose civil year at the chosen offset is in [1, 32767], a Date -> civil -> Date round trip at the same offset preserves getTime() exactly.

The year bounds constrain the local civil record, not necessarily the UTC value that carries it. Both the input local year and the local year after rounding carry must be in [1, 32767]; rounding the end of local year 32767 into 32768 is rejected. At the offset edges, the returned UTC Date can have year 0 or 32768 and still converts back with the same offset. The UTC-only functions apply the year domain to the UTC civil fields.

Wrong types, missing civil fields, mixed date kinds, and non-integer date or offset fields throw TypeError. An invalid Date, an invalid Gregorian date, non-finite or unsafe numbers, an out-of-range day fraction or offset, and a local year outside the domain throw RangeError.

The bridge converts no time scales and does not handle leap seconds; it provides neither Date to JDE nor UTC to UT1 conversion. Do not pass momentUt1 to civilUtcToDate() as though it were UTC: UT1 is not UTC, and the library does not model observed DUT1. Time scales explains how far apart the two can be.

Lunar Algorithms

Gregorian inputs and outputs are calendar-date labels on the selected algorithm's basis, not instants. Choose the source and compatibility role you need, not an assumed accuracy ranking:

Algo2 assigns dates by rendering TT moments in the library's modelled UTC (time scales) and then applying a fixed eight-hour eastward offset. Algo1 preserves HKO's published labels rather than imposing this rule on their history; algo3 inherits the basis of each source slice. The 2500 ceiling is the computed algorithm's enforced civil-date domain, not a physical limit of VSOP87D or ELP2000-82B.

lunar.supportedYearRange(algorithm) queries the native algorithm's inclusive range of lunar years, not Gregorian January-to-December bounds. lunar.yearInfo(algorithm, year) returns the Gregorian firstDay, the traditional leapMonth number or null, and monthLengths in calendar order, with a leap month directly after its ordinary namesake. lunar.toGregorian() takes a traditional month number in [1, 12] and an explicit isLeap boolean, not a positional month index. Each call to yearInfo() and toGregorian() first queries the native range once, then validates the lunar year; the JavaScript package keeps no separate range table or cache.

All four lunar methods require await init() first; a call before initialization completes throws CelestialError with recorded === false, before argument validation. After initialization:

• TypeError means a wrong input type, a missing own field, an excluded date-kind tag, a non-integer date field, or a non-boolean isLeap.
• RangeError means an unknown algorithm, a non-finite or unsafe number, an invalid Gregorian date (including a year outside [1, 32767]), a lunar year outside the native range, or a lunar month/day outside [1, 12] / [1, 30].
• CelestialError means a native failure, including a Gregorian date label outside the selected algorithm's coverage or a lunar leap-month/day combination that does not exist. A failed native range query also uses this class, with operation naming the caller, such as "lunar.yearInfo", rather than the internal query.

For example, lunar.fromGregorian("algo1", { year: 1900, month: 1, day: 1 }) passes Gregorian validation, but algo1 cannot represent that date, so it throws CelestialError; lunar.yearInfo("algo1", 1900) throws RangeError instead.

Delta T Models

time.deltaT(year, model) returns TT minus UT1 in seconds for a finite decimal Gregorian year. The default is the current project model; the named older models allow historical comparison:

"Frozen" describes model maintenance, not dead code: algo2 still supplies the pre-2005 values used by the current default. A hard domain is an input contract, not an accuracy claim. Fitted residuals and test tolerances are not statistical error guarantees, and an unbounded model does not promise useful accuracy at arbitrary years.

License

Project-authored package material is licensed under MIT.

Bundled third-party components retain their own terms in THIRD_PARTY_NOTICES.txt.