Moon Phase Calendar

Generate a moon phase calendar for any date range. Give it a start date and a number of days, and it returns one row per day with the phase name, an emoji, illumination percent, moon age, and a flag for the exact day of each new moon, first quarter, full moon and last quarter.

Everything is computed offline from astronomical formulas. There is no API key, no account, no rate limit, and nothing to break. Point it at 1868 or 2140 and it still works.

What you get

Each day in the range produces one dataset record:

Field	Meaning
date	The UTC day, YYYY-MM-DD.
phaseName	One of the eight phases, from New Moon to Waning Crescent.
phaseEmoji	The matching moon emoji, oriented for your hemisphere.
illuminationPercent	Share of the disc lit by the Sun, 0 to 100.
moonAgeDays	Days elapsed since the last new moon.
phaseAngleDeg	Moon minus Sun ecliptic longitude, 0 to 360.
isWaxing	true while the Moon is filling, false while it wanes.
isPrincipalPhase	true on the exact day a principal phase occurs.
principalPhaseName	The principal phase name on that day, otherwise empty.

Input

• Start date: first day of the calendar in UTC (YYYY-MM-DD). Leave it empty to start today.
• Number of days: how many days to compute, 1 to 366. Defaults to 30.
• Hemisphere: Northern or Southern. This only flips the emoji, since the lit side looks mirrored below the equator. The numbers are the same everywhere.

Example output

{
  "date": "2026-03-03",
  "phaseName": "Full Moon",
  "phaseEmoji": "🌕",
  "illuminationPercent": 100.0,
  "moonAgeDays": 14.8,
  "phaseAngleDeg": 180.0,
  "isWaxing": false,
  "isPrincipalPhase": true,
  "principalPhaseName": "Full Moon"
}

How it works

The actor computes the geocentric ecliptic longitude of the Sun and the Moon with truncated series from Jean Meeus, Astronomical Algorithms. The phase follows from their elongation: illumination is (1 - cos elongation) / 2, and moon age scales with the mean synodic month of 29.53 days. Principal phases are pinned to the day their exact instant falls within, in UTC.

Accuracy is well under a degree, so phase names, illumination and the dates of new and full moons line up with published lunar calendars.

Uses

• Fill a calendar or almanac app with lunar data.
• Plan night photography, camping, fishing or stargazing around dark skies and full moons.
• Feed moon phases into gardening, tide or wildlife models.
• Add a lunar column to an astrology or wellness product.

Notes

Times are UTC. A principal phase that happens just after midnight UTC can land on the previous calendar day in your local zone, which is normal for any lunar calendar. Rise and set times depend on your exact location and are out of scope here; this actor is about the phase itself, which is effectively the same for everyone on Earth at a given moment.