Astronomia

Astronomical algorithms in Dart, ported from Jean Meeus's Astronomical Algorithms (2nd Ed.) via the Go meeus library and the JS astronomia library.

Features

56 modules covering positional astronomy, celestial mechanics, and calendar computations:

Category	Modules
Time & Calendar	Julian Day, Delta T, sidereal time, equation of time, Easter
Coordinates	Ecliptic/equatorial/horizontal/galactic transforms, precession, nutation, parallax, refraction, aberration
Sun	Solar position (low-acc + VSOP87), solstices & equinoxes, sunrise/sunset, solar disk ephemeris
Moon	Lunar position, phases, illumination, nodes, apsis, max declination, moonrise/moonset, physical libration, selenographic coords
Planets	VSOP87 heliocentric positions (all 8 planets), geocentric positions with light-time correction, Kepler solvers, orbital elements, conjunctions, oppositions, elongations, Pluto
Orbits	Elliptic, parabolic, and near-parabolic motion, velocity, orbit length
Planet Details	Illumination & magnitudes, Jupiter physical ephemeris, Galilean moons, Saturn ring geometry, Saturn moons, Mars physical ephemeris
Rise/Set	Meeus ch. 15 refined algorithm with 3-day interpolation for any body
Geodesy	Earth ellipsoid, geodetic distance, parallax constants
Stellar	Magnitude arithmetic, binary star orbits, angular separation
Misc	Eclipse prediction, semidiameters, sundials, smallest circle, collinearity

Getting started

dependencies:
  astronomia: ^0.3.1

import 'package:astronomia/astronomia.dart';

Usage

The barrel import gives you foundations (julian, coordinates, nutation, etc.):

import 'package:astronomia/astronomia.dart';

void main() {
  // Julian Day for J2000.0
  final jd = calendarGregorianToJD(2000, 1, 1.5);
  print('J2000.0 = JD $jd'); // 2451545.0

  // Date of Easter 2025
  final e = gregorian(2025);
  print('Easter 2025: April ${e.day}'); // April 20
}

For specialized modules, import them directly — many share common names like position, radius, eccentricity, so prefixed imports keep things clear:

import 'package:astronomia/astronomia.dart';
import 'package:astronomia/solar.dart' as solar;
import 'package:astronomia/moonposition.dart' as moon;
import 'package:astronomia/moonphase.dart' as phase;
import 'package:astronomia/rise.dart' as rise;
import 'package:astronomia/globe.dart' as globe;

void main() {
  final jd = calendarGregorianToJD(2000, 1, 1.5);

  // Solar ecliptic longitude
  final sunLon = solar.apparentLongitude(j2000Century(jd));

  // Moon position
  final pos = moon.position(jd);
  print('Moon: lon=${toDeg(pos.lon)}°, lat=${toDeg(pos.lat)}°');

  // Next new moon
  final newMoonJDE = phase.newMoon(2025.5);

  // Earth surface distance (km)
  final km = globe.distance(lat1, lon1, lat2, lon2);
}

Moonrise / Moonset

import 'package:astronomia/astronomia.dart';
import 'package:astronomia/rise.dart' as rise;
import 'package:astronomia/moonposition.dart' as moon;
import 'package:astronomia/sidereal.dart' as sid;
import 'package:astronomia/coord.dart' as coord;
import 'dart:math' as math;

void main() {
  // London, 2025 March 8 at midnight UT
  final jd = calendarGregorianToJD(2025, 3, 8.0);
  final lat = toRad(51.5);
  final lon = toRad(0.0); // Greenwich, positive west
  final th0 = sid.apparent0UT(jd);

  final result = rise.moonTimes(jd, lat, lon, 69, th0, (jde) {
    final pos = moon.position(jde);
    final eps = toRad(23.44);
    final eq = coord.eclToEq(pos.lon, pos.lat, math.sin(eps), math.cos(eps));
    return (ra: eq.ra, dec: eq.dec, parallax: moon.parallax(pos.delta));
  });

  if (result != null) {
    print('Moonrise:  ${(result.rise / 3600).toStringAsFixed(1)}h UT');
    print('Transit:   ${(result.transit / 3600).toStringAsFixed(1)}h UT');
    print('Moonset:   ${(result.set / 3600).toStringAsFixed(1)}h UT');
  }
}

Planet positions

import 'package:astronomia/planetposition.dart';
import 'package:astronomia/elliptic.dart' as elliptic;
import 'package:astronomia/astronomia.dart';

void main() {
  final earth = Planet(planetEarth);
  final mars = Planet(planetMars);
  final jde = 2451545.0; // J2000

  // Heliocentric ecliptic (VSOP87)
  final pos = mars.position2000(jde);
  print('Mars: L=${toDeg(pos.lon)}°, R=${pos.range} AU');

  // Geocentric equatorial (observed)
  final eq = elliptic.position(mars, earth, jde);
  print('Mars: RA=${toDeg(eq.ra)}°, Dec=${toDeg(eq.dec)}°');
}

Conventions

All angles are in radians (double). Use toRad() / toDeg() to convert.
Time is represented as Julian Day numbers (double).
Multi-value returns use Dart records: ({double lon, double lat, double delta}).
The J2000.0 epoch constant is j2000 = 2451545.0.

Status

All 56 modules fully implemented (1 stub: Jewish/Moslem calendars). 216 tests passing, validated against examples from Meeus's book.

References

Meeus, Jean. Astronomical Algorithms. 2nd ed. Richmond: Willmann-Bell, 1998.
soniakeys/meeus (Go)
commenthol/astronomia (JS)

License

MIT