{
  "format": "codexwork-learning-notebook",
  "version": 1,
  "id": "moon-in-plain-sight",
  "title": "The Moon in plain sight",
  "intro": "Why can half a Moon be called a quarter? Try six short questions, explain your reasoning, then compare it with the notes. This is self-practice, not an exam or an automatic grade.",
  "sources": [
    {
      "id": "light",
      "title": "N1 · A lit sphere, a changing view",
      "text": "The Moon reflects sunlight. Normally, half its sphere is sunlit; from Earth we see different amounts of that half as the Moon travels around us. New Moon puts the mostly unlit face toward us. Full Moon puts the lit face toward us. Ordinary phases are a change of viewing geometry, not Earth’s shadow. A lunar eclipse is the separate exception.",
      "url": "https://science.nasa.gov/moon/moon-phases/"
    },
    {
      "id": "sequence",
      "title": "N2 · Names describe both shape and direction",
      "text": "Waxing means the visible lit fraction is increasing; waning means decreasing. First quarter is a half-lit disk heading toward full; last quarter is a half-lit disk heading toward new. “Quarter” names progress through the cycle, not the lit fraction. Gibbous means more than half the disk is lit. The cycle averages about 29.5 days. One half-lit view alone does not tell you whether it is waxing or waning.",
      "url": "https://aa.usno.navy.mil/faq/moon_phases"
    },
    {
      "id": "eclipses",
      "title": "N3 · Phase is necessary, alignment is another condition",
      "text": "A lunar eclipse needs full Moon and a path through Earth’s shadow. A solar eclipse needs new Moon and an alignment that places the Moon in front of the Sun for an observer. Neither happens every month: the Moon’s orbit is tilted relative to Earth’s orbit around the Sun, so it usually misses the required alignment.",
      "url": "https://science.nasa.gov/moon/eclipses/"
    }
  ],
  "cards": [
    {
      "id": "quarter",
      "kind": "distinguish",
      "prompt": "A friend says: “It is first quarter, so only a quarter of the Moon’s visible disk is bright.” What would you correct?",
      "answer": "At first quarter, half the visible disk is bright. “Quarter” refers to the phase cycle.",
      "explanation": "Separate two quantities: progress through the cycle and the fraction of the disk that you see illuminated. The name does not mean a quarter-lit disk.",
      "sourceIds": [
        "sequence"
      ]
    },
    {
      "id": "shadow",
      "kind": "explain",
      "prompt": "An ordinary crescent appears on a clear evening. Someone says Earth is casting a shadow over most of the Moon. Give a better explanation.",
      "answer": "We see only a small part of the Moon’s sunlit half. Earth’s shadow is not making this ordinary crescent.",
      "explanation": "Keep the lit half and your viewing direction separate. Moving around Earth changes how much of that half faces us; the Moon is not losing most of its daylight.",
      "sourceIds": [
        "light"
      ]
    },
    {
      "id": "tomorrow",
      "kind": "apply",
      "prompt": "Over several evenings the visible bright part grows from a thin crescent toward a half disk. After the half disk, what shape comes next if that trend continues? Why?",
      "answer": "Waxing gibbous: more than half the disk will look lit, on the way toward full.",
      "explanation": "The change over time tells you waxing, not just the shape in one picture. Gibbous means more than half but not completely lit.",
      "sourceIds": [
        "sequence"
      ]
    },
    {
      "id": "half-clue",
      "kind": "apply",
      "prompt": "You are given one unlabeled picture of a half-lit Moon, with no orientation or earlier observations. Can you conclude it is first quarter? What extra clue would help?",
      "answer": "No. It could be first or last quarter. Earlier or later observations showing whether the lit fraction grows or shrinks would help.",
      "explanation": "The same fraction appears in two parts of the cycle. Do not supply missing time information from the shape alone.",
      "sourceIds": [
        "sequence"
      ]
    },
    {
      "id": "monthly-eclipse",
      "kind": "explain",
      "prompt": "If a full Moon is on the opposite side of Earth from the Sun, why is every full Moon not a lunar eclipse?",
      "answer": "The Moon usually passes above or below the alignment needed to enter Earth’s shadow.",
      "explanation": "A flat drawing hides the orbit’s tilt. Full Moon supplies the phase; passing through the shadow requires the additional alignment.",
      "sourceIds": [
        "eclipses"
      ]
    },
    {
      "id": "two-claims",
      "kind": "distinguish",
      "prompt": "Which claim is supported: “Every new Moon produces a solar eclipse” or “A solar eclipse requires new Moon”? Explain the difference.",
      "answer": "A solar eclipse requires new Moon, but new Moon alone does not guarantee an eclipse.",
      "explanation": "A necessary condition is not a sufficient one. The Moon must also line up with the Sun for the observer.",
      "sourceIds": [
        "eclipses"
      ]
    }
  ]
}