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Moon Phase Calculator

Estimates the Moon's phase name, illuminated fraction and age (days since new moon) for a calendar date from the mean synodic month of 29.530588853 days counted from the new moon of 2000-01-06 18:14 UTC, plus the dates of the next new and full moons.

When to use

You need the approximate lunar phase for a date, e.g. for night photography, fishing or tide context, gardening calendars or historical dates.

Do not use when: You need phase times to the hour, eclipse predictions or moonrise/moonset (use an ephemeris such as USNO or JPL Horizons); for daylight times use sunrise-sunset.

Formula

age = (JD(date 12:00 UTC) − 2451550.2597) mod 29.530588853; fraction = age / 29.530588853; illumination = (1 − cos(2π · fraction)) / 2; phase index = floor(((fraction + 1/16) mod 1) × 8)

Uses the mean lunation only; the true phase can lead or lag the mean by up to about 14 hours because of the Moon's elliptical orbit, so phase names and dates are accurate to about ±1 day and illumination to roughly ±10 percentage points near the quarters. Each named phase spans one eighth of the cycle (±1/16 around the exact event).

Inputs

Parameter	Type	Unit	Required	Description
`date`	date		default today	Calendar date (YYYY-MM-DD); the phase is evaluated at 12:00 UTC of that day. Defaults to the current UTC date.

Outputs

Output	Type	Unit	Description
`phase_name`	string		One of the eight phases: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, Waning Crescent.
`illumination_percent`	number	%	Fraction of the disc that is lit, from the phase angle: (1 − cos(2π · fraction)) / 2.
`age_days`	number	days	Days since the last (mean) new moon at 12:00 UTC on the date.
`phase_fraction`	number		Position in the lunation from 0 (new) through 0.5 (full) to 1 (next new).
`is_waxing`	boolean		true while illumination increases (new to full).
`next_new_moon_date`	date		Date (UTC) of the next mean new moon, ±1 day.
`next_full_moon_date`	date		Date (UTC) of the next mean full moon, ±1 day.
`days_until_next_full_moon`	number	days	From 12:00 UTC on the date to the next mean full moon.

Example

2026-08-12 (new moon and total solar eclipse): {"date":"2026-08-12"} → {"phase_name":"New Moon","is_waxing":false,"next_full_moon_date":"2026-08-28"}

2026-03-03 (full moon and total lunar eclipse): {"date":"2026-03-03"} → {"phase_name":"Full Moon","illumination_percent":99.8,"age_days":14.36,"is_waxing":true}

GET https://tttkmbb.com/api/v1/calculate/moon-phase?date=2026-08-12

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/moon-phase (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/moon-phase · Markdown: https://tttkmbb.com/time/moon-phase.md · JSON definition: https://tttkmbb.com/time/moon-phase.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="moon-phase"
OpenAPI operationId: calculate_moon_phase
Freshness: daily. Authentication: none. Rate limit: fair use (see rate limits).

Sources

USNO Astronomical Applications – Phases of the Moon (government)
Lunar phase (Wikipedia) (reference)
Lunar month – synodic month 29.530588 days (Wikipedia) (reference)

FAQ

How accurate is the phase?

Within about one day: the mean synodic month ignores the orbital perturbations that shift real new and full moons by up to ±14 hours. USNO's Phases of the Moon table gives exact times.

Is the illumination the same everywhere on Earth?

Yes; the illuminated fraction depends only on the Sun–Earth–Moon geometry. Only the orientation of the lit side differs between hemispheres (the waxing side appears on the right in the north and on the left in the south).

Related calculators

Sunrise and Sunset Calculator — Sunrise and sunset for the same date and place.
Julian Date Calculator — Julian Date used to count lunations.
Days Until Calculator — Countdown to the next full moon date.