nrel_spa

NREL Solar Position Algorithm for Dart and Flutter. Calculates solar zenith, azimuth, sunrise, sunset, and solar noon for any location and time. Pure Dart, zero dependencies.

Based on Reda & Andreas (2004), NREL/TP-560-34302. Accurate to ±0.0003 degrees.

Installation

dependencies:
  nrel_spa: ^1.0.0

Quick Start

import 'package:nrel_spa/nrel_spa.dart';

void main() {
  final result = getSpa(
    DateTime.utc(2024, 3, 15, 17, 0, 0),
    40.7128,   // latitude (NYC)
    -74.0060,  // longitude
    -5.0,      // UTC offset (EST)
  );

  print('Zenith:     ${result.zenith.toStringAsFixed(4)}°');
  print('Azimuth:    ${result.azimuth.toStringAsFixed(4)}°');
  print('Sunrise:    ${result.sunrise.toStringAsFixed(4)} h');
  print('Solar Noon: ${result.solarNoon.toStringAsFixed(4)} h');
  print('Sunset:     ${result.sunset.toStringAsFixed(4)} h');
}

Custom Zenith Angles

Calculate rise/set times for any solar depression angle (twilight, prayer times, etc.):

final result = getSpa(
  DateTime.utc(2024, 3, 15, 12, 0, 0),
  40.7128, -74.0060, -5.0,
  customAngles: [96.0, 102.0, 108.0], // civil, nautical, astronomical
);

for (final angle in result.angles) {
  print('Rise: ${angle.sunrise}, Set: ${angle.sunset}');
}

API

getSpa(date, latitude, longitude, timezone, {...})

Parameter	Type	Default	Description
date	DateTime	required	UTC date and time
latitude	double	required	Degrees (-90 to 90)
longitude	double	required	Degrees (-180 to 180)
timezone	double	required	Hours from UTC
elevation	double	0	Meters above sea level
pressure	double	1013	Atmospheric pressure (mbar)
temperature	double	15	Temperature (Celsius)
deltaT	double	67	TT - UT1 (seconds)
customAngles	List<double>	[]	Zenith angles for rise/set

Returns SpaResult with zenith, azimuth, sunrise, solarNoon, sunset, and angles.

Architecture

A direct port of the NREL Solar Position Algorithm (Reda & Andreas 2004) to Dart. The implementation follows the original algorithm's 142 intermediate calculations exactly. Custom zenith angles are computed in a single pass alongside the primary solar angles using the same ephemeris state.

Compatibility

Dart SDK 3.7.0+. Works in Flutter, Dart CLI, and server-side Dart. Zero dependencies.

Related

• pray_calc_dart - Islamic prayer times (uses nrel_spa)
• nrel-spa - JavaScript/TypeScript version (npm)

Acknowledgments

Reda, I. and Andreas, A. (2004). Solar Position Algorithm for Solar Radiation Applications. NREL/TP-560-34302. DOI: 10.2172/15003974

License

MIT. See LICENSE for NREL third-party notice.