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    "title": "Fizeau: timing light with a rotating toothed wheel",
    "summary": "Fizeau measures light propagation over a terrestrial round-trip path by observing when a rotating toothed wheel blocks or transmits the returning beam.\n\nLight returns so quickly that an ordinary clock cannot resolve its journey. Fizeau uses regularly spaced gaps and teeth as a rapid mechanical timing device. A beam leaves through a gap, reflects at a distant station and returns; depending on wheel speed, the return meets a tooth or another gap. The succession of disappearances and reappearances links a measured rotation rate to light travel time. This is an early physical measurement with explicit apparatus conditions, rather than the later definition of an exact constant.",
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    "area": "Optics and physical measurement",
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      "scope": "Fizeau measures light propagation over a terrestrial round-trip path by observing when a rotating toothed wheel blocks or transmits the returning beam. The target is the 1849 toothed-wheel experiment in air, not the 1851 moving-water interference experiment or a universal invariant-speed theorem. Light travels to a distant reflector and back through a rotating wheel; eclipse and reappearance depend on its speed and round-trip delay. The paper reports approximately 8633 m separation, 720 teeth, a first eclipse near 12.6 turns per second and a mean from 28 observations in historical units. These are author observations, not a modern exact constant or reconstructed uncertainty budget. Fizeau explicitly promises a fuller memoir after studying the conditions more completely. The speed is a round-trip, path-dependent measurement in air, not a separately synchronized one-way measurement. Formula glyphs and pictures were not certified, and no experiment was replayed. The appended page 132 corrects commission attribution, not the measured speed. Neighboring journal material is outside the target.",
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        "description": "The target is the 1849 toothed-wheel experiment in air, not the 1851 moving-water interference experiment or a universal invariant-speed theorem. Light travels to a distant reflector and back through a rotating wheel; eclipse and reappearance depend on its speed and round-trip delay. The paper reports approximately 8633 m separation, 720 teeth, a first eclipse near 12.6 turns per second and a mean from 28 observations in historical units. These are author observations, not a modern exact constant or reconstructed uncertainty budget. Fizeau explicitly promises a fuller memoir after studying the conditions more completely. The speed is a round-trip, path-dependent measurement in air, not a separately synchronized one-way measurement. Formula glyphs and pictures were not certified, and no experiment was replayed. The appended page 132 corrects commission attribution, not the measured speed. Neighboring journal material is outside the target.",
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          "url": "https://www.academie-sciences.fr/pdf/dossiers/Fizeau/Fizeau_pdf/CR1849_p90.pdf"
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        {
          "citation": "Académie des sciences Fizeau bibliography:1849light-speed versus1851moving-water work",
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      "statement": "# Fizeau: timing light with a rotating toothed wheel\n\nFizeau measures light propagation over a terrestrial round-trip path by observing when a rotating toothed wheel blocks or transmits the returning beam.\n\nLight returns so quickly that an ordinary clock cannot resolve its journey. Fizeau uses regularly spaced gaps and teeth as a rapid mechanical timing device. A beam leaves through a gap, reflects at a distant station and returns; depending on wheel speed, the return meets a tooth or another gap. The succession of disappearances and reappearances links a measured rotation rate to light travel time. This is an early physical measurement with explicit apparatus conditions, rather than the later definition of an exact constant.\n\n## Scientific scope\n\nThe target is the 1849 toothed-wheel experiment in air, not the 1851 moving-water interference experiment or a universal invariant-speed theorem. Light travels to a distant reflector and back through a rotating wheel; eclipse and reappearance depend on its speed and round-trip delay. The paper reports approximately 8633 m separation, 720 teeth, a first eclipse near 12.6 turns per second and a mean from 28 observations in historical units. These are author observations, not a modern exact constant or reconstructed uncertainty budget. Fizeau explicitly promises a fuller memoir after studying the conditions more completely. The speed is a round-trip, path-dependent measurement in air, not a separately synchronized one-way measurement. Formula glyphs and pictures were not certified, and no experiment was replayed. The appended page 132 corrects commission attribution, not the measured speed. Neighboring journal material is outside the target.\n\n## People and systems\n\nHippolyte Fizeau\n\n\n\n## Original sources\n\n- [H. Fizeau, Sur une expérience relative à la vitesse de propagation de la lumière (1849), Académie original facsimile](https://www.academie-sciences.fr/pdf/dossiers/Fizeau/Fizeau_pdf/CR1849_p90.pdf)\n- [Académie des sciences Fizeau bibliography:1849light-speed versus1851moving-water work](https://www.academie-sciences.fr/pdf/dossiers/Fizeau/Fizeau_pubd1.htm)\n",
      "summary": "Fizeau measures light propagation over a terrestrial round-trip path by observing when a rotating toothed wheel blocks or transmits the returning beam.\n\nLight returns so quickly that an ordinary clock cannot resolve its journey. Fizeau uses regularly spaced gaps and teeth as a rapid mechanical timing device. A beam leaves through a gap, reflects at a distant station and returns; depending on wheel speed, the return meets a tooth or another gap. The succession of disappearances and reappearances links a measured rotation rate to light travel time. This is an early physical measurement with explicit apparatus conditions, rather than the later definition of an exact constant.",
      "title": "Fizeau: timing light with a rotating toothed wheel"
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    "id": "fizeau-toothed-wheel-light-speed-1849",
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    "title": "Fizeau: timing light with a rotating toothed wheel",
    "context": "Optics and physical measurement",
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    "what_changed": "Fizeau measures light propagation over a terrestrial round-trip path by observing when a rotating toothed wheel blocks or transmits the returning beam.",
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    "evidence_summary": "Source-linked, scoped account of the original work. The published scope states the scientific assumptions and remaining verification limits. WRS has not independently certified the full proof, experimental result or system benchmark.",
    "references": [
      {
        "title": "H. Fizeau, Sur une expérience relative à la vitesse de propagation de la lumière (1849), Académie original facsimile",
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