🌙 moon-phase-js

A lightweight JavaScript utility to calculate the moon phase for any date, including phase name, emoji, and illumination percentage.

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📦 Installation

npm install @tortaruga/moon-phase-js

🚀 Usage

import { getPhase, getEmoji, getIllumination } from "@tortaruga/moon-phase-js";

const date = new Date("2026-04-01");

console.log(getPhase(date));       // "Full Moon"
console.log(getEmoji(date));       // "🌕"
console.log(getIllumination(date)); // 0.985077512880078

Note: getIllumination returns a value from 0 to 1. Multiply by 100 for a percentage.

🧠 API

getPhase(date)

Returns the moon phase name for a given date.

• Parameters: date (Date object)
• Returns: string — phase name
• Example:

getPhase(new Date("2026-04-01")); // "Full Moon"
getPhase(new Date("17 April 2026")); // "New Moon"

getEmoji(date)

Returns an emoji representing the moon phase.

• Parameters: date (Date object)
• Returns: string — emoji
• Example:

getEmoji(new Date('1 march 2026')); // "🌔"

getIllumination(date)

Returns the fraction of the moon illuminated.

• Parameters: date (Date object)
• Returns: number — 0–1
• Example:

getIllumination(new Date("2026-04-01")); // 0.985077512880078

• Tip: Use toFixed() to round to a preferred number of decimal places:

getIllumination(new Date("2026-04-01")).toFixed(2) // 0.99 

🌕 Moon Phases Reference

Phase Name	Emoji	Approx. Illumination
New Moon	🌑	0%
Waxing Crescent	🌒	1–49%
First Quarter	🌓	50%
Waxing Gibbous	🌔	51–99%
Full Moon	🌕	100%
Waning Gibbous	🌖	51–99%
Last Quarter	🌗	50%
Waning Crescent	🌘	1–49%

⚠️ Notes

The moon phase calculation is based only on the age of the moon, not precise astronomical positions. This means that near phase transitions, the result may be slightly off compared to astronomical data.

🎯 Features

• 🌙 Moon phase calculation from date
• 😀 Emoji support
• 📊 Illumination fraction
• ⚡ Lightweight and dependency-free

🧮 How it works

This library calculates moon phases based on the age of the moon (days since the last new moon).

1. Convert the given date to a Julian date.
2. Calculate the moon’s age in days using a standard lunar cycle (synodic month ≈ 29.53 days).
3. Determine the phase by comparing the age to thresholds for each phase:

Moon Phase	Moon age
New Moon	< 1.84566173161
Waxing Crescent	< 5.53698519483
First Quarter	< 9.22830865805
Waxing Gibbous	< 12.91963212127
Full Moon	< 16.61095558449
Waning Gibbous	< 20.30227904771
Last Quarter	< 23.99360251093
Waning Crescent	< 29.530589

4. Compute illumination fraction with the formula:

   illumination = (1 - cos(phaseAngle)) / 2 where phaseAngle = 2π * moonAge / 29.53

(For more details on the calculation method see docs/logic.md.)

📄 License

MIT