{
  "id": "moon-phase",
  "version": "bca1af72f8ef",
  "status": "published",
  "name": "Moon Phase Calculator",
  "question": "What is the moon phase on my date?",
  "summary": "Shows the Moon’s phase, illuminated percent and age on any date and time from 1900 to 2149, with the dates and times of the next new moon, first quarter, full moon and last quarter.",
  "category": "physics",
  "url": "https://www.acalculator.org/physics/moon-phase-calculator",
  "markdown": "https://www.acalculator.org/physics/moon-phase-calculator.md",
  "kind": "function",
  "method": "Phase times: Meeus ch. 49 (mean phase plus periodic and planetary terms), converted from TD to UT with NASA’s ΔT polynomials. Illuminated fraction k = (1 + cos i) ÷ 2 with the phase angle i from Meeus eq. 48.4.",
  "assumptions": [
    "Times are to the nearest minute; Meeus gives phase times to within about a minute of the full theory for 1900 to 2100.",
    "The phase name is that of a new moon, quarter or full moon that falls on your local date; otherwise crescent (under 50% lit) or gibbous (50% or more), waxing or waning.",
    "Illumination is for the Earth’s centre and the time you type."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "date": {
        "title": "Date",
        "description": "The local date to look at.",
        "type": "string",
        "format": "date"
      },
      "time": {
        "title": "Local time",
        "description": "The local time of day; the phase changes through the day.",
        "type": "string",
        "pattern": "^\\d{2}:\\d{2}(:\\d{2})?$"
      },
      "utc": {
        "title": "UTC offset (hours)",
        "description": "Your time zone’s offset from UTC on that date: −5 for New York in winter, −4 in summer, 1 for Paris in winter.",
        "type": "number",
        "minimum": -12,
        "maximum": 14
      }
    }
  },
  "outputs": {
    "phase": {
      "label": "Moon phase",
      "description": "The phase name on that local date.",
      "format": "text"
    },
    "illuminated": {
      "label": "Illuminated",
      "description": "The share of the Moon’s disk lit by the Sun, as seen from Earth.",
      "format": "percent"
    },
    "age": {
      "label": "Moon age (days)",
      "description": "Days since the last new moon.",
      "format": "number"
    },
    "trend": {
      "label": "Waxing or waning",
      "description": "Waxing from new to full, waning from full to new.",
      "format": "text"
    },
    "lastNew": {
      "label": "Last new moon",
      "description": "Local date and time of the last new moon.",
      "format": "text"
    },
    "nextFirst": {
      "label": "Next first quarter",
      "description": "Local date and time of the next first quarter.",
      "format": "text"
    },
    "nextFull": {
      "label": "Next full moon",
      "description": "Local date and time of the next full moon.",
      "format": "text"
    },
    "nextLast": {
      "label": "Next last quarter",
      "description": "Local date and time of the next last quarter.",
      "format": "text"
    },
    "nextNew": {
      "label": "Next new moon",
      "description": "Local date and time of the next new moon.",
      "format": "text"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "date": "2026-10-02",
      "time": "12:00",
      "utc": 0
    },
    "outputs": {
      "phase": "Waning gibbous",
      "illuminated": 62.303663038783995,
      "age": 21.35636534402147,
      "trend": "Waning",
      "lastNew": "2026-09-11 03:27",
      "nextFirst": "2026-10-18 16:13",
      "nextFull": "2026-10-26 04:12",
      "nextLast": "2026-10-03 13:25",
      "nextNew": "2026-10-10 15:50"
    },
    "text": "Moon phase on October 2, 2026: Waning gibbous, 62.3% lit. Next full moon: 2026-10-26 04:12."
  },
  "examples": [
    {
      "given": {
        "date": "2026-06-29",
        "time": "23:56",
        "utc": 0
      },
      "expect": {
        "phase": "Full moon",
        "lastNew": "2026-06-15 02:54",
        "nextLast": "2026-07-07 19:29",
        "nextNew": "2026-07-14 09:43"
      },
      "source": "Meeus (1998), Astronomical Algorithms, 2nd ed., ch. 49; U.S. Naval Observatory, Phases of the Moon 2026, the check (new moon June 15 02:54, last quarter July 7 19:29, new moon July 14 09:43 UT). https://aa.usno.navy.mil/data/MoonPhases"
    },
    {
      "given": {
        "date": "2026-10-01",
        "time": "12:00",
        "utc": 0
      },
      "expect": {
        "phase": "Waning gibbous",
        "trend": "Waning",
        "lastNew": "2026-09-11 03:27",
        "nextLast": "2026-10-03 13:25",
        "nextNew": "2026-10-10 15:50",
        "nextFull": "2026-10-26 04:12"
      },
      "source": "Meeus (1998), Astronomical Algorithms, 2nd ed., ch. 49; U.S. Naval Observatory, Phases of the Moon 2026, the check (new moon Sept 11 03:27, last quarter Oct 3 13:25, new moon Oct 10 15:50, full moon Oct 26 04:12 UT). https://aa.usno.navy.mil/data/MoonPhases"
    },
    {
      "given": {
        "date": "2026-10-10",
        "time": "09:00",
        "utc": -5
      },
      "expect": {
        "phase": "New moon",
        "lastNew": "2026-09-10 22:27",
        "nextNew": "2026-10-10 10:50",
        "nextFull": "2026-10-25 23:12"
      },
      "source": "Meeus (1998), Astronomical Algorithms, 2nd ed., ch. 49; U.S. Naval Observatory, Phases of the Moon 2026, the check (new moon Sept 11 03:27, new moon Oct 10 15:50, full moon Oct 26 04:12 UT). https://aa.usno.navy.mil/data/MoonPhases"
    }
  ],
  "sources": [
    "Jean Meeus, Astronomical Algorithms, 2nd ed. (Willmann-Bell, 1998), chapter 47 (D, M, M′), chapter 48 (illuminated fraction, eq. 48.4) and chapter 49 (phases of the Moon; examples 49.a and 49.b).",
    "NASA Goddard Space Flight Center (F. Espenak), Polynomial Expressions for Delta T (ΔT), 1900 to 2150. https://eclipse.gsfc.nasa.gov/SEhelp/deltatpoly2004.html (retrieved 2026-10-01)",
    "U.S. Naval Observatory, Astronomical Applications Department, Phases of the Moon (UT), the check for 2026. https://aa.usno.navy.mil/data/MoonPhases (retrieved 2026-10-01)"
  ],
  "related": [
    "days-until",
    "date"
  ],
  "changelog": []
}