moon_sighting

Lunar crescent visibility for Dart and Flutter. Computes moon phase, topocentric position, illumination, and Yallop/Odeh crescent visibility criteria using Meeus algorithms. Zero dependencies.

Installation

dependencies:
  moon_sighting: ^1.0.0

Quick Start

import 'package:moon_sighting/moon_sighting.dart';

// Current moon phase
final phase = getMoonPhase();
print('${phase.phaseName} ${phase.phaseSymbol}');
print('Illumination: ${phase.illumination.toStringAsFixed(1)}%');
print('Age: ${phase.age.toStringAsFixed(1)} hours');

// Moon position for an observer
final pos = getMoonPosition(DateTime.now(), 51.5074, -0.1278, elevation: 10);
print('Azimuth: ${pos.azimuth.toStringAsFixed(1)}');
print('Altitude: ${pos.altitude.toStringAsFixed(1)}');
print('Distance: ${pos.distance.toStringAsFixed(0)} km');

// Crescent visibility estimate (pass a post-sunset time)
final vis = getMoonVisibilityEstimate(
  DateTime.utc(2025, 3, 31, 18, 30),
  21.4225, 39.8262, // Mecca
);
print('Zone: ${vis.zone.label}'); // A through D
print('Visible naked eye: ${vis.isVisibleNakedEye}');

API

getMoonPhase(DateTime? date)

Returns MoonPhaseResult with:

Field	Type	Description
phase	MoonPhaseName	Enum: newMoon, waxingCrescent, firstQuarter, etc.
phaseName	String	Human-readable name
phaseSymbol	String	Moon emoji
illumination	double	Percent illuminated (0-100)
age	double	Hours since last new moon
elongationDeg	double	Moon-Sun elongation in degrees
isWaxing	bool	True when illumination is increasing
nextNewMoon	DateTime	Next new moon (UTC)
nextFullMoon	DateTime	Next full moon (UTC)
prevNewMoon	DateTime	Previous new moon (UTC)

getMoonPosition(DateTime? date, double lat, double lon, {double elevation = 0})

Returns MoonPosition with:

Field	Type	Description
azimuth	double	Degrees from North, clockwise
altitude	double	Degrees above horizon (refraction applied)
distance	double	Earth-Moon distance in km
parallacticAngle	double	Parallactic angle in radians

getMoonIllumination(DateTime? date)

Returns MoonIlluminationResult with:

Field	Type	Description
fraction	double	Illuminated fraction (0-1)
phase	double	Phase cycle (0=new, 0.25=Q1, 0.5=full, 0.75=Q3)
angle	double	Bright limb position angle (radians)
isWaxing	bool	True when waxing

getMoonVisibilityEstimate(DateTime? date, double lat, double lon, {double elevation = 0})

Returns MoonVisibilityEstimate with:

Field	Type	Description
v	double	Odeh V parameter
zone	OdehZone	Visibility zone (a through d)
description	String	Zone description
isVisibleNakedEye	bool	True for zone A
isVisibleWithOpticalAid	bool	True for zones A and B
arcl	double	Sun-Moon elongation (degrees)
arcv	double	Arc of vision (degrees)
w	double	Crescent width (arc minutes)
moonAboveHorizon	bool	Moon above horizon at given time

getMoon(DateTime? date, double lat, double lon, {double elevation = 0})

Returns MoonSnapshot combining all four results: phase, position, illumination, visibility.

Visibility Criteria

The Odeh criterion (2006) classifies crescent visibility into four zones:

Zone	V threshold	Meaning
A	V >= 5.65	Visible with naked eye
B	V >= 2.00	Visible with optical aid, may be seen naked eye
C	V >= -0.96	Visible with optical aid only
D	V < -0.96	Not visible even with optical aid

The Yallop q-test (1997) uses six categories A through F with different threshold semantics. Both criteria use the same underlying polynomial for the minimum arc of vision as a function of crescent width.

Accuracy

All positions use Meeus (1998) approximations:

• Sun: < 0.01 deg ecliptic longitude error
• Moon: < 0.3 deg longitude, < 0.2 deg latitude
• Distance: ~300 km error (~0.08%)
• New/full moon times: within ~2 hours

The GCRS-to-topocentric conversion uses a simplified Earth rotation (ERA only, no nutation/precession), adding ~1 deg systematic error. This is acceptable for phase displays, illumination calculations, and approximate visibility estimates.

For high-precision crescent sighting reports with DE442S ephemeris accuracy, see the TypeScript moon-sighting package.

Lite Mode vs Full Mode

This Dart package implements lite mode only. It covers moon phase, position, illumination, and quick visibility estimates using analytical Meeus algorithms with no external data files.

The full-mode features (DE442S JPL ephemeris, sub-arcsecond accuracy, rise/set event finding, best-time optimization, full sighting reports) are available in the TypeScript package.

Compatibility

• Dart SDK >= 3.7.0
• Works in Flutter, Dart CLI, and server-side Dart
• Zero runtime dependencies

Related Packages

• moon-sighting (TypeScript) - Full-accuracy lunar crescent visibility with DE442S ephemeris
• nrel-spa (TypeScript) - Pure JS NREL Solar Position Algorithm
• pray-calc (JavaScript) - Islamic prayer time calculation
• luxon-hijri (TypeScript) - Hijri/Gregorian calendar conversion

License

MIT