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- Add @typescript-eslint/parser and @typescript-eslint/eslint-plugin as direct devDependencies (were only peer deps of @acamarata/eslint-config, causing ERR_MODULE_NOT_FOUND in CI) - Add files: ['**/*.ts'] to eslint.config.mjs so ESLint 10 flat config picks up TypeScript files; add parserOptions.project for type-aware rules - Run prettier --write src/ to fix formatting after config changes - Add coverage/ to .gitignore and untrack previously committed coverage files |
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nrel-spa
Pure JavaScript implementation of the NREL Solar Position Algorithm (SPA). Computes solar zenith angle, azimuth, sunrise, sunset, and solar noon for any location and date. Zero dependencies, synchronous. Validated to produce identical results to the original NREL C reference implementation.
Installation
npm install nrel-spa
Quick Start
import { getSpa, calcSpa } from 'nrel-spa';
const date = new Date('2025-06-21T00:00:00Z');
// Raw fractional hours
const raw = getSpa(date, 40.7128, -74.006, -4); // New York, EDT
console.log(raw.sunrise); // 5.417
console.log(raw.solarNoon); // 12.965
console.log(raw.sunset); // 20.509
// Formatted HH:MM:SS strings
const fmt = calcSpa(date, 40.7128, -74.006, -4);
console.log(fmt.sunrise); // "05:25:03"
console.log(fmt.solarNoon); // "12:57:56"
console.log(fmt.sunset); // "20:30:35"
CommonJS:
const { getSpa } = require('nrel-spa');
Pass a zenith angles array as the sixth argument to getSpa/calcSpa for civil (96°), nautical (102°), or astronomical (108°) twilight times.
TypeScript
import { getSpa, calcSpa, formatTime, SPA_ZA_RTS } from 'nrel-spa';
import type { SpaOptions, SpaResult, SpaFunctionCode } from 'nrel-spa';
Documentation
Full API reference, algorithm notes, and twilight calculation guide: GitHub Wiki
Related
- solar-spa: WASM build of the same algorithm, async, for high-throughput batch work
- pray-calc: Islamic prayer times built on nrel-spa
Acknowledgments
The core algorithm is a JavaScript port of the NREL SPA by Ibrahim Reda and Afshin Andreas:
Reda, I., Andreas, A. (2004). "Solar Position Algorithm for Solar Radiation Applications." Solar Energy, 76(5), 577-589.
License
MIT (TypeScript wrapper and build tooling). The core algorithm in lib/spa.js is a port of NREL's SPA C source, subject to its own terms. See LICENSE.