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https://github.com/acamarata/pray-calc.git
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6f165b79ad
- Add @typescript-eslint/parser and @typescript-eslint/eslint-plugin as direct devDependencies (were only transitive, not linked in node_modules) - Add files pattern and parserOptions.project to eslint.config.mjs so ESLint finds and type-checks src/**/*.ts files correctly - Run prettier --write to fix formatting across all src files
47 lines
1.5 KiB
TypeScript
47 lines
1.5 KiB
TypeScript
/**
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* Asr prayer time calculation.
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*
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* Asr begins when the shadow of an object equals (Shafi'i/Maliki/Hanbali)
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* or twice (Hanafi) the object's length plus its shadow at solar noon.
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* This is a pure spherical trigonometry problem once solar declination
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* and solar noon are known.
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*/
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import { DEG } from "./constants.js";
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/**
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* Compute Asr time as fractional hours.
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*
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* @param solarNoon - Solar noon in fractional hours (from getSpa)
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* @param latitude - Observer latitude in degrees
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* @param declination - Solar declination in degrees (from solarEphemeris)
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* @param hanafi - true for Hanafi (shadow factor 2), false for Shafi'i (factor 1)
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* @returns Fractional hours, or NaN if the sun never reaches the required altitude
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*/
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export function getAsr(
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solarNoon: number,
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latitude: number,
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declination: number,
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hanafi = false,
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): number {
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const phi = latitude * DEG;
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const delta = declination * DEG;
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const shadowFactor = hanafi ? 2 : 1;
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// Required solar altitude:
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// tan(A) = 1 / (shadowFactor + tan(|φ − δ|))
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const X = Math.abs(phi - delta);
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const tanA = 1 / (shadowFactor + Math.tan(X));
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const sinA = tanA / Math.sqrt(1 + tanA * tanA); // sin(atan(tanA))
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// Solve the hour-angle equation:
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// cos(H0) = (sin(A) − sin(φ)sin(δ)) / (cos(φ)cos(δ))
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const cosH0 = (sinA - Math.sin(phi) * Math.sin(delta)) / (Math.cos(phi) * Math.cos(delta));
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if (cosH0 < -1 || cosH0 > 1) return NaN; // sun never reaches A
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// H0 in hours (15°/hr)
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const H0h = Math.acos(cosH0) / DEG / 15;
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return solarNoon + H0h;
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}
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