{
  "address": "6baaaf7a65c3ef5efad5dc1728a050e7",
  "category": "other",
  "question": "What is the approximate moon phase on 2000-01-07 (00:00 UTC), using the mean synodic month from the 2000-01-06 18:14 UTC reference new moon?",
  "answer": "New Moon, age 0.24 days",
  "askedBy": {
    "kind": "guest",
    "display": "method deck",
    "stamped": false,
    "id": null
  },
  "answeredBy": "method",
  "madeAt": "2026-09-30T22:39:30.722Z",
  "validUntil": null,
  "validityClass": "forever",
  "sources": [
    {
      "label": "Astronomical Algorithms, 2nd ed., Chapter 49 (Phases of the Moon), p. 349",
      "url": null
    }
  ],
  "supersedes": null,
  "supersededBy": null,
  "isSeed": false,
  "seedSource": null,
  "seedAttribution": null,
  "duplicateOfQid": null,
  "publishedAt": null,
  "unpublishedAt": null,
  "voteCount": 0,
  "queuedForSubstrate": false,
  "answerMethod": "method",
  "unsourced": false,
  "source": {
    "kind": "edition_page",
    "value": "Meeus, Jean. Astronomical Algorithms, 2nd ed. Willmann-Bell, 1998.",
    "retrieved_utc": "2026-09-30T19:40:00Z"
  },
  "qsha256": "0241be2fcb3ec8720059178a3496413046db81e6125d27d43d4bf3d2dfae4449",
  "runOne": null,
  "cardContractVersion": "card_contract/1",
  "deck_id": "method-deck",
  "deck_title": "Method Deck",
  "method": {
    "name": "Moon phase from a mean synodic month",
    "version": "1",
    "source_url_or_file": "methods/moon-phase-by-synodic-month.cjs",
    "sha256_of_method_file": "a60d9de6cceabb46571be2a6d470c8bfcf15d791d2bb60161280f5a58cf2a118"
  },
  "receipt": {
    "inSubstrate": false,
    "local": true,
    "modelCallsAtMint": 0,
    "tokensAtMint": 0
  },
  "method_deck": {
    "inputs": [
      {
        "name": "date",
        "type": "string",
        "unit": "ISO date (YYYY-MM-DD, taken as 00:00 UTC)",
        "required": true,
        "description": "Calendar date to evaluate, interpreted at 00:00 UTC."
      }
    ],
    "outputs": [
      {
        "name": "phase",
        "unit": "phase name",
        "description": "One of the eight named moon phases."
      },
      {
        "name": "age",
        "unit": "days",
        "description": "Days since the most recent mean new moon, 0 to about 29.53."
      }
    ],
    "steps": [
      "Take a cited reference new moon: 2000-01-06 18:14 UTC (Meeus, Astronomical Algorithms, 2nd ed., chapter 49).",
      "Take the mean synodic month: 29.530588853 days (Meeus, same chapter).",
      "Compute the number of days between the reference new moon and the target date at 00:00 UTC.",
      "Reduce that count modulo the synodic month, wrapping negative results into 0..synodic month, to get the moon's age in days.",
      "Divide the synodic month into 8 named phases at boundaries of 1, 3, 5, 7, 9, 11, 13, and 15 sixteenths of the synodic month, and report the phase whose range contains the age.",
      "Round the reported age to 2 decimal places.",
      "This is a mean-motion approximation, not a full ephemeris; the true phase time can differ from this estimate by up to about half a day because the Moon's actual orbital speed varies."
    ],
    "constants": [
      {
        "name": "synodic_month_days",
        "value": 29.530588853,
        "unit": "days",
        "source": 0
      },
      {
        "name": "reference_new_moon_utc",
        "value": "2000-01-06T18:14:00Z",
        "unit": "UTC datetime",
        "source": 0
      }
    ],
    "examples": [
      {
        "name": "worked example",
        "inputs": {
          "date": "2000-01-07"
        },
        "expected": {
          "answer": "New Moon, age 0.24 days"
        },
        "check": "By hand: 2000-01-07 00:00 UTC is 5h46m (0.2403 days) after the cited 2000-01-06 18:14 UTC reference new moon, so the age is that same small value and the phase is New Moon by definition of the reference epoch."
      },
      {
        "name": "test vector: nine days after reference",
        "inputs": {
          "date": "2000-01-16"
        },
        "expected": {
          "answer": "Waxing Gibbous, age 9.24 days"
        },
        "check": "By hand: 2000-01-16 00:00 UTC is 9 days 5h46m (9.2403 days) after the reference new moon. The First Quarter band is 5*29.530588853/16=9.228 up to 7*29.530588853/16=12.920 days, but the boundary comparison in steps uses a strict less-than at each edge, so 9.2403 (just past the 9.228 edge) falls in the Waxing Gibbous band, which spans from that same 9.228-day edge up to the next one."
      }
    ],
    "disclaimer": null,
    "deck_category": "astronomy"
  },
  "status": "minted",
  "kind": "method",
  "provenanceClass": "own_question",
  "provenance": {
    "schema": "card_provenance/2.0.0",
    "chain": [
      {
        "source": {
          "kind": "edition_page",
          "value": "Meeus, Jean. Astronomical Algorithms, 2nd ed. Willmann-Bell, 1998.",
          "retrieved_utc": "2026-09-30T19:40:00Z"
        },
        "sha256": "180d6c4f0c2310f7556e8a8379fd319b330f211d771e0b03825b39b646696a04",
        "supersedes": null,
        "added_utc": "2026-09-30T22:39:30.722Z",
        "added_by": "method:Moon phase from a mean synodic month",
        "why": "the method that computed this answer",
        "check": null,
        "status_label": "pending",
        "signature": null
      }
    ]
  }
}