Apparent geocentric positions for the sun, moon, planets, Chiron, Sirius and the lunar nodes, from Moshier's ephemeris.

Pure JavaScript, no dependencies, no data files. The calculation engine is pluggable if you want Swiss Ephemeris accuracy instead.

Installation

npm install ephemeris

Usage

app.js

const ephemeris = require('ephemeris');
const dateObj = new Date('2015-08-10T17:09:01.000+00:00');

// parameters: ephemeris.getAllPlanets(dateObj, longitude, latitude, height);
const all = ephemeris.getAllPlanets(dateObj, 10.0014, 53.5653, 0);

// parameters: ephemeris.getPlanet(name, dateObj, longitude, latitude, height);
const venus = ephemeris.getPlanet('venus', dateObj, 10.0014, 53.5653, 0);

// the names getPlanet accepts
ephemeris.getBodyNames();
// [ 'sun', 'mercury', 'venus', 'moon', 'mars', 'jupiter', 'saturn',
//   'uranus', 'neptune', 'pluto', 'chiron', 'sirius',
//   'rahu', 'ketu', 'trueRahu', 'trueKetu' ]

Lunar nodes: Rahu and Ketu

The two points where the moon's orbit crosses the ecliptic are included as bodies, so they come back from getAllPlanets like anything else.

const nodes = ephemeris.getAllPlanets(dateObj, 10.0014, 53.5653, 0).observed;

nodes.rahu.apparentLongitudeDd;      // 342.16922, mean ascending node
nodes.ketu.apparentLongitudeDd;      // 162.16922, always rahu + 180
nodes.trueRahu.apparentLongitudeDd;  // 340.95728, osculating
nodes.trueKetu.apparentLongitudeDd;  // 160.95728

| | rahu and ketu | trueRahu and trueKetu | | --- | --- | --- | | from | the moon's mean elements | the orbital plane the moon is on right now | | motion | steady -0.052954 degrees a day, always retrograde | wanders, and moves forwards about a quarter of the time | | differ from the mean by | - | up to 1.9 degrees | | usually wanted by | Vedic work | much Western practice, matching Swiss Ephemeris SE_TRUE_NODE |

A node is a direction, not an object, so geocentricDistanceKm is always -1 and there is no rise, set or altitude for it.

These are tropical longitudes, of date, exactly like every other longitude here. Sidereal work needs an ayanamsa subtracted, and this library does not provide one, so rahu is not the sidereal Rahu until you subtract your chosen ayanamsa yourself.

The date is read in UTC. Passing null as the date uses the current time. getPlanet throws if it is given a name that is not in getBodyNames().

What each observed body reports

| Field | Meaning | | --- | --- | | name | the body's key, same as the key it is filed under | | raw | the full internal state for the body, including position.altaz | | apparentLongitudeDms360 | apparent ecliptic longitude, degrees/minutes/seconds over the whole circle | | apparentLongitudeDms30 | the same longitude reduced into its 30 degree sign | | apparentLongitudeDd | apparent ecliptic longitude in decimal degrees | | apparentLongitudeDdPerDay | apparent motion in longitude, decimal degrees per day; negative when retrograde | | geocentricDistanceKm | distance from the earth, where the body reports one | | is_retrograde | true when apparent longitude is decreasing; undefined where retrograde motion cannot happen | | available | false when the current backend cannot produce this body; every value field is then null | | unavailableReason | why, when available is false | | backend | which backend produced, or failed to produce, this body |

Apparent motion is measured between one hour before the requested instant and the instant itself, so a station lands within about an hour of the real one.

is_retrograde is only reported for bodies that can be retrograde, which means the planets and the asteroids. The sun and the moon are never overtaken by the earth, so they cannot appear to move backwards and they report undefined rather than a permanent false; their apparentLongitudeDdPerDay is still given, and is always positive. A star reports undefined for both, since it does not move against the ecliptic.

Rise and set times are the analytic approximation from transit.js. They agree with a bisection on the library's own elevation to within a few seconds, at every latitude tested up to 78 degrees north. A body that never rises or sets on a given day simply has no rise and set fields.

TypeScript

Types ship with the package, hand written in index.d.ts, so there is nothing to install from DefinitelyTyped.

import ephemeris = require('ephemeris');

const all = ephemeris.getAllPlanets(new Date(), 10.0014, 53.5653, 0);
const lon: number = all.observed.venus.apparentLongitudeDd;

// getPlanet narrows the result to the body you asked for, so
// one.observed.moon type checks and one.observed.venus does not
const one = ephemeris.getPlanet('moon', null, 0, 0, 0);

BodyName, ObservedBody, Observer, DateReport and Result are all exported for use in your own signatures. Note that is_retrograde and apparentLongitudeDdPerDay are typed as possibly undefined, because stars do not have them.

Sample output

{
  date: {
    gregorianTerrestrial: '10.8.2015 17:9:1',
    gregorianTerrestrialRaw: {
      day: 10,
      month: 8,
      year: 2015,
      hours: 17,
      minutes: 9,
      seconds: 1,
      julianDate: 2457244.5,
      julianTime: 0.7145949074074074,
      julian: 2457245.2145949076,
      j2000: 2015.6063370154895,
      b1950: 2015.6065478300004,
      j1900: 2015.6063370154895,
      delta: 68.01714323102905,
      terrestrial: 2457245.2145949076,
      universal: 2457245.2138076723,
      universalDate: [Object],
      universalDateString: '10.8.2015 17:7:52.983'
    },
    gregorianUniversal: '10.8.2015 17:7:52.983',
    gregorianDelta: '00:00:68.01714323102905',
    julianTerrestrial: 2457245.2145949076,
    julianUniversal: 2457245.2138076723,
    julianDelta: 0.0007872354540628362
  },
  observer: {
    name: 'earth',
    longitudeGeodetic: 10.0014,
    longitudeGeocentric: 10.0014,
    latitudeGeodetic: 53.5653,
    latitudeGeocentric: 53.38123133267187,
    heightGeodetic: 0,
    heightGeocentric: 6364.336264644461
  },
  observed: {
    sun: {
      name: 'sun',
      raw: [Object],
      apparentLongitudeDms30: '17°45\'39"',
      apparentLongitudeDms360: '137°45\'39"',
      apparentLongitudeDd: 137.7610585591896,
      apparentLongitudeDdPerDay: 0.9593885486374347,
      geocentricDistanceKm: -1,
      is_retrograde: false
    },
    venus: {
      name: 'venus',
      raw: [Object],
      apparentLongitudeDms30: '25°43\'16"',
      apparentLongitudeDms360: '145°43\'16"',
      apparentLongitudeDd: 145.7212923824036,
      apparentLongitudeDdPerDay: -0.5678333325058702,
      geocentricDistanceKm: -1,
      is_retrograde: true
    },
    // ... mercury, moon, mars, jupiter, saturn, uranus, neptune, pluto,
    //     chiron and sirius follow in the same shape
    sirius: {
      name: 'sirius',
      raw: [Object],
      apparentLongitudeDms30: '11°27\'17"',
      apparentLongitudeDms360: '101°27\'17"',
      apparentLongitudeDd: 1.7707222368344282,
      apparentLongitudeDdPerDay: undefined,
      geocentricDistanceKm: 7777,
      is_retrograde: undefined
    }
  }
}

Backends

The calculation engine is pluggable. The built in one is Moshier's analytic theory, the same engine this package has always used: no dependencies, no data files, works in a browser. Two adapters ship alongside it for engines you install yourself.

const ephemeris = require('ephemeris');

ephemeris.backend();    // 'moshier'
ephemeris.backends();   // every backend and whether it is usable here

| backend | engine to install | licence of that engine | bodies | notes | | --- | --- | --- | --- | --- | | moshier | none, built in | GPL-3.0, this package | all 16 | the default. Pure JS, ~142 kB, browser safe | | swisseph | sweph | AGPL-3.0 or commercial | all 16, plus asteroids and stars with data files | ~6 MB, prebuilt binaries, no compiler needed | | astronomy-engine | astronomy-engine | MIT | sun, moon, planets | ~1.8 MB, pure JS, no Chiron, no stars, no nodes |

Neither engine is a dependency of this package, so a plain npm install ephemeris still pulls nothing else. Installing one is your decision, and with Swiss Ephemeris it is a licensing decision: AGPL-3.0 reaches hosted services, which GPL-3.0 does not, so a closed source product behind an API typically needs Astrodienst's commercial licence. That is precisely why it is not installed for you.

Switching

// after: npm install sweph
ephemeris.use(ephemeris.swisseph({ ephePath: './ephe' }));

// after: npm install astronomy-engine
ephemeris.use(ephemeris.astronomyEngine());

ephemeris.use('moshier');   // back to the built in one

Switching throws if the engine is missing, rather than quietly falling back to a different one and leaving you to wonder why the numbers moved:

Backend "swisseph" is not usable: the sweph package is not installed:
Cannot find module 'sweph'. Try: npm install sweph

Getting the Swiss Ephemeris data files

Without data files, Swiss Ephemeris runs in its own Moshier mode, which is the same theory the built in backend already uses, so it buys nothing: no Chiron, no fixed stars, and the same numbers you had. The data files are what make it worth installing.

sweph bundles none, which is deliberate and is why it is 6 MB rather than 30. There is a command to fetch them:

npx ephemeris-fetch-ephe --dir ./ephe

  fetching       sepl_18.se1 ... 473 kB
  fetching       semo_18.se1 ... 1274 kB
  fetching       seas_18.se1 ... 218 kB
  fetching       sefstars.txt ... 133 kB

Ready in /your/project/ephe

2.1 MB in total, about a second. Run it again and it leaves the files alone; corrupt one and it replaces it. --only sepl_18.se1,semo_18.se1 takes a subset, and --help lists what is available.

Downloads are pinned to a specific upstream commit and verified against a recorded SHA-256, so you get the same bytes every time and a substituted or truncated file is refused rather than written. The same thing is available programmatically:

await ephemeris.fetchEphemeris({ ephePath: './ephe' });

If you would rather be asked than run a command, prompt does that at setup time:

await ephemeris.fetchEphemeris({ ephePath: './ephe', prompt: true });
// Download 4 Swiss Ephemeris data file(s), 2.0 MB, from https://raw.github...?
// They are licensed AGPL-3.0 or commercially by Astrodienst. [y/N]

It only asks when something is actually missing, and off a terminal it refuses rather than hanging or assuming yes, so it cannot wedge a build. Hosts with their own interface can answer for it by passing confirm.

Nothing downloads on install, or during a calculation. There is no postinstall hook, and the calculation path never touches the network. Two reasons, and they are practical rather than principled. getAllPlanets is synchronous, so asking mid-calculation would mean blocking the event loop or making every signature async. And there is no terminal to ask on in CI, in a server, or in a spawned process, so the honest answer there is to fail with a message, which is what happens. Setup is where a question can be answered, so that is where the question is.

This package also dropped a postinstall script back in 2.1.0, and the data files carry Swiss Ephemeris licensing, so fetching them should be something you chose.

When a body needs a file you do not have, the reason says so and names the command:

SwissEph file 'seas_18.se1' not found in PATH '.'.
Run: npx ephemeris-fetch-ephe --dir <dir>, then pass that directory as ephePath.

Then:

ephemeris.use(ephemeris.swisseph({ ephePath: './ephe' }));

The _18 files cover 1800 to 2400. Swap the suffix for other six century spans, _12 for 1200 to 1800 and so on, or take sepl_*.se1 wholesale for the full range. Only add what your date range actually needs.

Numbered minor planets beyond Chiron each need their own file, named after the number: asking for {asteroid: 433} without one reports SwissEph file 'se00433s.se1' not found, which tells you exactly what to fetch.

The alternative swisseph package bundles data instead, but costs 30 MB and 12 seconds, and 9.5 MB of that is seasnam.txt, an asteroid name list no calculation reads. sweph plus the four files above is the smaller and faster route, and it needs no compiler.

When a body is not available

Every backend returns the whole catalogue, so the shape of the result never depends on which engine is in use. A body the backend cannot produce comes back with the same keys and every value null:

ephemeris.use(ephemeris.astronomyEngine());
ephemeris.getAllPlanets(new Date(), 0, 0, 0).observed.chiron;
// {
//   name: 'chiron',
//   available: false,
//   unavailableReason: 'the astronomy-engine backend does not provide chiron',
//   backend: 'astronomy-engine',
//   raw: null,
//   apparentLongitudeDms30: null,
//   apparentLongitudeDms360: null,
//   apparentLongitudeDd: null,
//   apparentLongitudeDdPerDay: null,
//   geocentricDistanceKm: null,
//   is_retrograde: null
// }

Check available before reading a longitude. The reason is often actionable: with Swiss Ephemeris installed but no data files, asking for Chiron says SwissEph file 'seas_18.se1' not found, which tells you exactly what to fetch.

Asking for things the catalogue does not name

getBody takes a body specification instead of a name, for numbered minor planets, fixed stars, or an engine's own identifiers. It accepts one or a list, and returns the same envelope as getAllPlanets.

ephemeris.use(ephemeris.swisseph({ ephePath: './ephe' }));

ephemeris.getBody({ asteroid: 433 }, date, lon, lat, 0);      // Eros
ephemeris.getBody({ star: 'Aldebaran' }, date, lon, lat, 0);
ephemeris.getBody({ id: 12 }, date, lon, lat, 0);             // SE_MEAN_APOG, Lilith
ephemeris.getBody(['mars', { asteroid: 433 }], date, lon, lat, 0);

Results are keyed by a readable rendering of the specification, so {asteroid: 433} lands under asteroid:433 and {star: 'Aldebaran'} under star:Aldebaran. Anything the backend cannot resolve gets the unavailable template, so a mixed request never fails as a whole.

Writing your own

A backend is a plain object. src/backends/registry.js documents the contract in full; the short version:

ephemeris.registerBackend({
  name: 'mine',
  available: true,
  bodies: ['sun', 'moon'],
  supports (spec) { return this.bodies.indexOf(spec.name) >= 0 },
  calc (date, spec) {
    // date.julian is terrestrial time, date.universal is UT
    return { longitude: 123.456, motion: 0.98, distance: -1, raw: null };
  }
});
ephemeris.use('mine');

Core owns the envelope: the observer, the date arithmetic, the degrees-minutes-seconds strings and the retrograde rules. A backend supplies longitude, motion and distance and nothing else, which is what keeps every engine's output directly comparable. Return null or {error: 'why'} for a body you cannot do, and core fills in the template.

Contributing

Setting the project up, the lint and test commands, and how a release is cut are in CONTRIBUTING.md.

Changelog

Full history is in CHANGELOG.md, and each release on GitHub carries its own entry.

3.x requires Node 22 or newer, and reports is_retrograde only for bodies that can actually be retrograde. Read the 3.0.0 entry before upgrading from 2.x.

Credits

Algorithms by Steve Moshier, ported to JavaScript by mivion and packaged for node by xerik, whose ephemeris-moshier is no longer published.

License

GPLv3