<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Lunar Orbit & Moon Phases</title>
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
      -webkit-tap-highlight-color: transparent;
    }

    body {
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      background: #f8fafc;
      color: #1e2945;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: flex-start;
      width: 100%;
      min-height: 100%;
      padding: 8px;
      user-select: none;
    }

    .diagram-wrap {
      position: relative;
      width: 100%;
      max-width: 460px;
      aspect-ratio: 500 / 440;
    }

    svg {
      width: 100%;
      height: 100%;
      display: block;
      overflow: visible;
    }

    .phase-node {
      cursor: pointer;
      outline: none;
    }

    .phase-hitbox {
      fill: transparent;
    }

    .node-highlight {
      fill: none;
      stroke: #22b7ff;
      stroke-width: 2.5;
      opacity: 0;
      transition: opacity 0.2s ease;
    }

    .phase-node.active .node-highlight {
      opacity: 1;
    }

    .phase-node:hover .node-highlight {
      opacity: 0.6;
    }

    .phase-node.active:hover .node-highlight {
      opacity: 1;
    }

    /* Bottom Info Card */
    .info-card {
      width: 100%;
      max-width: 460px;
      background: #ffffff;
      border: 1px solid #e6e6e6;
      border-radius: 12px;
      padding: 8px 12px;
      margin-top: 6px;
      display: flex;
      flex-direction: column;
      gap: 6px;
    }

    .card-top {
      display: flex;
      align-items: center;
      justify-content: space-between;
      gap: 6px;
    }

    .phase-title-group {
      display: flex;
      align-items: center;
      gap: 6px;
    }

    .phase-num {
      background: #22b7ff;
      color: #ffffff;
      font-size: 11px;
      font-weight: 700;
      padding: 2px 6px;
      border-radius: 99px;
      line-height: 1.2;
    }

    .phase-name {
      font-size: 14px;
      font-weight: 700;
      color: #0f172a;
    }

    .controls {
      display: flex;
      align-items: center;
      gap: 4px;
    }

    .ctrl-btn {
      background: #f1f5f9;
      border: 1px solid #cbd5e1;
      color: #334155;
      border-radius: 6px;
      padding: 3px 8px;
      font-size: 12px;
      font-weight: 600;
      cursor: pointer;
      display: inline-flex;
      align-items: center;
      justify-content: center;
      transition: background 0.15s;
    }

    .ctrl-btn:hover {
      background: #e2e8f0;
    }

    .ctrl-btn.active-play {
      background: #22b7ff;
      border-color: #0284c7;
      color: #ffffff;
    }

    .card-views {
      display: grid;
      grid-template-columns: 1fr 1fr;
      gap: 8px;
      background: #f8fafc;
      border: 1px solid #edf2f7;
      border-radius: 8px;
      padding: 5px 8px;
    }

    .view-col {
      display: flex;
      align-items: center;
      gap: 8px;
    }

    .view-svg {
      width: 30px;
      height: 30px;
      flex-shrink: 0;
    }

    .view-meta {
      display: flex;
      flex-direction: column;
      min-width: 0;
    }

    .view-label {
      font-size: 10px;
      font-weight: 700;
      text-transform: uppercase;
      letter-spacing: 0.5px;
      color: #64748b;
    }

    .view-desc {
      font-size: 12px;
      font-weight: 600;
      color: #1e2945;
      line-height: 1.2;
      white-space: nowrap;
      overflow: hidden;
      text-overflow: ellipsis;
    }

    /* Step Selector Pills */
    .pill-nav {
      display: flex;
      justify-content: space-between;
      gap: 3px;
      width: 100%;
    }

    .pill-btn {
      flex: 1;
      border: 1px solid #e2e8f0;
      background: #ffffff;
      color: #64748b;
      font-size: 11px;
      font-weight: 700;
      padding: 3px 0;
      border-radius: 4px;
      cursor: pointer;
      text-align: center;
      transition: all 0.15s ease;
    }

    .pill-btn:hover {
      border-color: #94a3b8;
      color: #1e2945;
    }

    .pill-btn.active {
      background: #22b7ff;
      border-color: #22b7ff;
      color: #ffffff;
    }

    @media (max-width: 380px) {
      .phase-name {
        font-size: 13px;
      }
      .view-desc {
        font-size: 11px;
      }
      .pill-btn {
        font-size: 10px;
        padding: 2px 0;
      }
    }
  </style>
</head>
<body>

  <div class="diagram-wrap">
    <svg id="orbitSvg" viewBox="0 0 500 440">
      <defs>
        
        <radialGradient id="earthGrad" cx="35%" cy="35%" r="65%">
          <stop offset="0%" stop-color="#93c5fd"/>
          <stop offset="40%" stop-color="#3b82f6"/>
          <stop offset="85%" stop-color="#1d4ed8"/>
          <stop offset="100%" stop-color="#1e3a8a"/>
        </radialGradient>

        
        <marker id="orbitArrow" viewBox="0 0 6 6" refX="3" refY="3" markerWidth="4" markerHeight="4" orient="auto">
          <path d="M 1 1 L 5 3 L 1 5 Z" fill="#64748b"/>
        </marker>
        <marker id="sunRayArrow" viewBox="0 0 8 8" refX="2" refY="4" markerWidth="5" markerHeight="5" orient="auto">
          <path d="M 7 1.5 L 1 4 L 7 6.5 Z" fill="#eab308"/>
        </marker>
      </defs>

      
      <rect x="0" y="0" width="500" height="440" rx="14" fill="#0b1329"/>

      
      <g id="sunlightGroup">
        <line x1="490" y1="90" x2="415" y2="90" stroke="#eab308" stroke-width="1.5" stroke-dasharray="3,3" marker-end="url(#sunRayArrow)"/>
        <line x1="490" y1="155" x2="415" y2="155" stroke="#eab308" stroke-width="2" marker-end="url(#sunRayArrow)"/>
        <line x1="490" y1="220" x2="415" y2="220" stroke="#eab308" stroke-width="2.5" marker-end="url(#sunRayArrow)"/>
        <line x1="490" y1="285" x2="415" y2="285" stroke="#eab308" stroke-width="2" marker-end="url(#sunRayArrow)"/>
        <line x1="490" y1="350" x2="415" y2="350" stroke="#eab308" stroke-width="1.5" stroke-dasharray="3,3" marker-end="url(#sunRayArrow)"/>
        
        
        <rect x="424" y="204" width="68" height="32" rx="6" fill="#ca8a04" opacity="0.95"/>
        <text x="458" y="224" fill="#ffffff" font-size="11" font-weight="700" text-anchor="middle" letter-spacing="0.5">SUNLIGHT</text>
      </g>

      
      <circle cx="220" cy="220" r="125" fill="none" stroke="#334155" stroke-width="1.5" stroke-dasharray="4,4"/>

      
      <path d="M 342 195 A 125 125 0 0 0 255 98" fill="none" stroke="#64748b" stroke-width="1.5" marker-end="url(#orbitArrow)"/>
      <path d="M 185 98 A 125 125 0 0 0 98 195" fill="none" stroke="#64748b" stroke-width="1.5" marker-end="url(#orbitArrow)"/>
      <path d="M 98 245 A 125 125 0 0 0 185 342" fill="none" stroke="#64748b" stroke-width="1.5" marker-end="url(#orbitArrow)"/>
      <path d="M 255 342 A 125 125 0 0 0 342 245" fill="none" stroke="#64748b" stroke-width="1.5" marker-end="url(#orbitArrow)"/>

      
      <g id="earthGroup" transform="translate(220, 220)">
        <circle cx="0" cy="0" r="34" fill="#38bdf8" opacity="0.15"/>
        <circle cx="0" cy="0" r="28" fill="url(#earthGrad)"/>
        
        <path d="M -10 -16 Q -5 -21 5 -18 Q 16 -13 13 -2 Q 9 10 -3 7 Q -15 5 -10 -16 Z" fill="#22c55e" opacity="0.85"/>
        <path d="M -19 3 Q -9 1 -5 15 Q -12 22 -20 13 Z" fill="#22c55e" opacity="0.85"/>
        <path d="M 7 5 Q 17 3 20 14 Q 13 22 5 19 Z" fill="#22c55e" opacity="0.85"/>
        
        <path d="M 0 -28 A 28 28 0 0 0 0 28 Z" fill="#0b1329" opacity="0.65"/>
        <text x="0" y="3" fill="#ffffff" font-size="10" font-weight="700" text-anchor="middle" filter="drop-shadow(0 1px 2px rgba(0,0,0,0.8))">EARTH</text>
        <text x="0" y="40" fill="#94a3b8" font-size="10" font-weight="600" text-anchor="middle">North Pole</text>
      </g>

      
      <line id="sightline" x1="220" y1="220" x2="345" y2="220" stroke="#22b7ff" stroke-width="1.5" stroke-dasharray="3,3" opacity="0.7"/>

      
      <g id="moonNodes"></g>
    </svg>
  </div>

  
  <div class="info-card">
    <div class="card-top">
      <div class="phase-title-group">
        <span id="cardNum" class="phase-num">1</span>
        <span id="cardName" class="phase-name">New Moon</span>
      </div>
      <div class="controls">
        <button id="prevBtn" class="ctrl-btn" title="Previous Phase">◀</button>
        <button id="playBtn" class="ctrl-btn" title="Auto Play Orbit">▶ Play</button>
        <button id="nextBtn" class="ctrl-btn" title="Next Phase">▶</button>
      </div>
    </div>

    <div class="card-views">
      
      <div class="view-col">
        <svg class="view-svg" viewBox="0 0 36 36">
          <circle cx="18" cy="18" r="15" fill="#1e293b" stroke="#475569" stroke-width="1"/>
          
          <path d="M 18 3 A 15 15 0 0 1 18 33 Z" fill="#fef08a"/>
        </svg>
        <div class="view-meta">
          <span class="view-label">In Space</span>
          <span id="spaceDesc" class="view-desc">Sunlit on right half</span>
        </div>
      </div>

      
      <div class="view-col">
        <svg id="earthViewSvg" class="view-svg" viewBox="0 0 36 36"></svg>
        <div class="view-meta">
          <span class="view-label">From Earth</span>
          <span id="earthDesc" class="view-desc">Completely dark</span>
        </div>
      </div>
    </div>

    
    <div class="pill-nav" id="pillNav"></div>
  </div>

  <script>
    const CX = 220;
    const CY = 220;
    const R = 125;

    // Phase definitions (8 phases, CCW from Sun-facing side)
    const phases = [
      {
        id: 0,
        name: "New Moon",
        short: "1. New",
        x: CX + R, // 345
        y: CY,     // 220
        earthSvg: '<circle cx="18" cy="18" r="15" fill="#1e293b" stroke="#475569" stroke-width="1"/>',
        spaceDesc: "Sunlit on right half",
        earthDesc: "Completely dark disk"
      },
      {
        id: 1,
        name: "Waxing Crescent",
        short: "2. Wx.Cr",
        x: Math.round(CX + R * Math.cos(-Math.PI / 4)), // 308
        y: Math.round(CY + R * Math.sin(-Math.PI / 4)), // 132
        earthSvg: `
          <circle cx="18" cy="18" r="15" fill="#1e293b" stroke="#475569" stroke-width="1"/>
          <path d="M 18 3 A 15 15 0 0 1 18 33 A 8 15 0 0 0 18 3 Z" fill="#fef08a"/>
        `,
        spaceDesc: "Sunlit on right half",
        earthDesc: "Thin sliver on right"
      },
      {
        id: 2,
        name: "First Quarter",
        short: "3. 1st Qtr",
        x: CX,     // 220
        y: CY - R, // 95
        earthSvg: `
          <circle cx="18" cy="18" r="15" fill="#1e293b" stroke="#475569" stroke-width="1"/>
          <path d="M 18 3 A 15 15 0 0 1 18 33 Z" fill="#fef08a"/>
        `,
        spaceDesc: "Sunlit on right half",
        earthDesc: "Half disk lit on right"
      },
      {
        id: 3,
        name: "Waxing Gibbous",
        short: "4. Wx.Gb",
        x: Math.round(CX - R * Math.cos(Math.PI / 4)),  // 132
        y: Math.round(CY - R * Math.sin(Math.PI / 4)),  // 132
        earthSvg: `
          <circle cx="18" cy="18" r="15" fill="#1e293b" stroke="#475569" stroke-width="1"/>
          <path d="M 18 3 A 15 15 0 0 1 18 33 A 8 15 0 0 1 18 3 Z" fill="#fef08a"/>
        `,
        spaceDesc: "Sunlit on right half",
        earthDesc: "> Half lit on right"
      },
      {
        id: 4,
        name: "Full Moon",
        short: "5. Full",
        x: CX - R, // 95
        y: CY,     // 220
        earthSvg: '<circle cx="18" cy="18" r="15" fill="#fef08a" stroke="#ca8a04" stroke-width="1"/>',
        spaceDesc: "Sunlit on right half",
        earthDesc: "Fully illuminated disk"
      },
      {
        id: 5,
        name: "Waning Gibbous",
        short: "6. Wn.Gb",
        x: Math.round(CX - R * Math.cos(Math.PI / 4)),  // 132
        y: Math.round(CY + R * Math.sin(Math.PI / 4)),  // 308
        earthSvg: `
          <circle cx="18" cy="18" r="15" fill="#1e293b" stroke="#475569" stroke-width="1"/>
          <path d="M 18 3 A 15 15 0 0 0 18 33 A 8 15 0 0 0 18 3 Z" fill="#fef08a"/>
        `,
        spaceDesc: "Sunlit on right half",
        earthDesc: "> Half lit on left"
      },
      {
        id: 6,
        name: "Third Quarter",
        short: "7. 3rd Qtr",
        x: CX,     // 220
        y: CY + R, // 345
        earthSvg: `
          <circle cx="18" cy="18" r="15" fill="#1e293b" stroke="#475569" stroke-width="1"/>
          <path d="M 18 3 A 15 15 0 0 0 18 33 Z" fill="#fef08a"/>
        `,
        spaceDesc: "Sunlit on right half",
        earthDesc: "Half disk lit on left"
      },
      {
        id: 7,
        name: "Waning Crescent",
        short: "8. Wn.Cr",
        x: Math.round(CX + R * Math.cos(Math.PI / 4)),  // 308
        y: Math.round(CY + R * Math.sin(Math.PI / 4)),  // 308
        earthSvg: `
          <circle cx="18" cy="18" r="15" fill="#1e293b" stroke="#475569" stroke-width="1"/>
          <path d="M 18 3 A 15 15 0 0 0 18 33 A 8 15 0 0 1 18 3 Z" fill="#fef08a"/>
        `,
        spaceDesc: "Sunlit on right half",
        earthDesc: "Thin sliver on left"
      }
    ];

    let currentPhase = 0;
    let autoPlayTimer = null;

    const moonNodesGroup = document.getElementById('moonNodes');
    const pillNav = document.getElementById('pillNav');
    const sightline = document.getElementById('sightline');

    // Build SVG Moon nodes
    phases.forEach((p, idx) => {
      const g = document.createElementNS("http://www.w3.org/2000/svg", "g");
      g.setAttribute("class", "phase-node" + (idx === 0 ? " active" : ""));
      g.setAttribute("id", `node-${idx}`);
      g.setAttribute("transform", `translate(${p.x}, ${p.y})`);

      // 1. Selection indicator ring
      const ring = document.createElementNS("http://www.w3.org/2000/svg", "circle");
      ring.setAttribute("cx", "0");
      ring.setAttribute("cy", "0");
      ring.setAttribute("r", "20");
      ring.setAttribute("class", "node-highlight");
      g.appendChild(ring);

      // 2. Space Moon Sphere (R = 12.5). Right half is ALWAYS sunlit in space
      const darkHalf = document.createElementNS("http://www.w3.org/2000/svg", "circle");
      darkHalf.setAttribute("cx", "0");
      darkHalf.setAttribute("cy", "0");
      darkHalf.setAttribute("r", "12.5");
      darkHalf.setAttribute("fill", "#1e293b");
      darkHalf.setAttribute("stroke", "#475569");
      darkHalf.setAttribute("stroke-width", "1");
      g.appendChild(darkHalf);

      const litHalf = document.createElementNS("http://www.w3.org/2000/svg", "path");
      litHalf.setAttribute("d", "M 0 -12.5 A 12.5 12.5 0 0 1 0 12.5 Z");
      litHalf.setAttribute("fill", "#fef08a");
      g.appendChild(litHalf);

      // 3. Mini phase view disk bubble attached facing outward
      const dx = p.x - CX;
      const dy = p.y - CY;
      const dist = Math.hypot(dx, dy);
      const bx = (dx / dist) * 24;
      const by = (dy / dist) * 24;

      const badgeGroup = document.createElementNS("http://www.w3.org/2000/svg", "g");
      badgeGroup.setAttribute("transform", `translate(${bx}, ${by})`);

      const badgeBg = document.createElementNS("http://www.w3.org/2000/svg", "circle");
      badgeBg.setAttribute("cx", "0");
      badgeBg.setAttribute("cy", "0");
      badgeBg.setAttribute("r", "9");
      badgeBg.setAttribute("fill", "#0f172a");
      badgeBg.setAttribute("stroke", "#22b7ff");
      badgeBg.setAttribute("stroke-width", "1.2");
      badgeGroup.appendChild(badgeBg);

      // Small Earth View shape inside bubble
      const earthMini = document.createElementNS("http://www.w3.org/2000/svg", "g");
      earthMini.setAttribute("transform", "scale(0.45) translate(-18, -18)");
      earthMini.innerHTML = p.earthSvg;
      badgeGroup.appendChild(earthMini);

      g.appendChild(badgeGroup);

      // 4. Hit area for effortless click/tap
      const hit = document.createElementNS("http://www.w3.org/2000/svg", "circle");
      hit.setAttribute("cx", "0");
      hit.setAttribute("cy", "0");
      hit.setAttribute("r", "26");
      hit.setAttribute("class", "phase-hitbox");
      g.appendChild(hit);

      // Interactions
      g.addEventListener("click", () => selectPhase(idx, true));
      moonNodesGroup.appendChild(g);

      // Create Nav Pill Button
      const btn = document.createElement("button");
      btn.className = "pill-btn" + (idx === 0 ? " active" : "");
      btn.id = `pill-${idx}`;
      btn.textContent = (idx + 1);
      btn.title = p.name;
      btn.addEventListener("click", () => selectPhase(idx, true));
      pillNav.appendChild(btn);
    });

    function updateCard(idx) {
      const p = phases[idx];
      document.getElementById('cardNum').textContent = (idx + 1);
      document.getElementById('cardName').textContent = p.name;
      document.getElementById('earthDesc').textContent = p.earthDesc;
      document.getElementById('spaceDesc').textContent = p.spaceDesc;
      document.getElementById('earthViewSvg').innerHTML = p.earthSvg;

      // Update active SVG node
      phases.forEach((_, i) => {
        const node = document.getElementById(`node-${i}`);
        const pill = document.getElementById(`pill-${i}`);
        if (i === idx) {
          node.classList.add("active");
          pill.classList.add("active");
        } else {
          node.classList.remove("active");
          pill.classList.remove("active");
        }
      });

      // Update sightline
      sightline.setAttribute("x2", p.x);
      sightline.setAttribute("y2", p.y);
    }

    function selectPhase(idx, userAction = false) {
      if (userAction && autoPlayTimer) {
        stopAutoPlay();
      }
      currentPhase = idx;
      updateCard(currentPhase);
    }

    function nextPhase() {
      selectPhase((currentPhase + 1) % 8);
    }

    function prevPhase() {
      selectPhase((currentPhase + 7) % 8);
    }

    function toggleAutoPlay() {
      if (autoPlayTimer) {
        stopAutoPlay();
      } else {
        startAutoPlay();
      }
    }

    function startAutoPlay() {
      const playBtn = document.getElementById('playBtn');
      playBtn.textContent = "⏸ Pause";
      playBtn.classList.add("active-play");
      autoPlayTimer = setInterval(() => {
        nextPhase();
      }, 3000);
    }

    function stopAutoPlay() {
      if (autoPlayTimer) {
        clearInterval(autoPlayTimer);
        autoPlayTimer = null;
      }
      const playBtn = document.getElementById('playBtn');
      playBtn.textContent = "▶ Play";
      playBtn.classList.remove("active-play");
    }

    // Attach button listeners
    document.getElementById('prevBtn').addEventListener('click', () => {
      stopAutoPlay();
      prevPhase();
    });

    document.getElementById('nextBtn').addEventListener('click', () => {
      stopAutoPlay();
      nextPhase();
    });

    document.getElementById('playBtn').addEventListener('click', toggleAutoPlay);

    // Initial render
    updateCard(0);
  </script>
</body>
</html>

Modeling the Lunar Orbit and Monthly Moon Phases

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <title>Sidereal vs Synodic Month</title>
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    :root {
      --primary: #22b7ff;
      --primary-dark: #009be3;
      --text: #1e2945;
      --muted: #64748b;
      --bg: #f8fafc;
      --earth: #3b82f6;
      --sun: #f59e0b;
      --moon: #94a3b8;
    }
    body {
      font-family: system-ui, -apple-system, sans-serif;
      background: #ffffff;
      color: var(--text);
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      min-height: 100%;
      padding: 12px;
      user-select: none;
    }
    .stepper {
      display: flex;
      gap: 6px;
      width: 100%;
      max-width: 520px;
      justify-content: center;
      margin-bottom: 8px;
    }
    .step-btn {
      flex: 1;
      padding: 8px 6px;
      font-size: 13px;
      font-weight: 600;
      border: 1px solid #e2e8f0;
      border-radius: 8px;
      background: #ffffff;
      color: var(--muted);
      cursor: pointer;
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 4px;
      transition: all 0.2s ease;
    }
    .step-btn.active {
      background: #e0f2fe;
      border-color: var(--primary);
      color: #0369a1;
      box-shadow: 0 0 0 2px rgba(34, 183, 255, 0.2);
    }
    .play-btn {
      width: 36px;
      padding: 8px 0;
      font-size: 14px;
      flex: 0 0 36px;
    }
    .canvas-wrap {
      width: 100%;
      max-width: 520px;
      position: relative;
      background: var(--bg);
      border-radius: 12px;
      border: 1px solid #e2e8f0;
      overflow: hidden;
    }
    svg {
      width: 100%;
      height: auto;
      display: block;
    }
    .status-card {
      width: 100%;
      max-width: 520px;
      margin-top: 8px;
      padding: 8px 12px;
      background: #ffffff;
      border: 1px solid #e2e8f0;
      border-radius: 8px;
      text-align: center;
      font-size: 13px;
      font-weight: 600;
      color: var(--text);
      min-height: 38px;
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 8px;
    }
    .badge {
      display: inline-block;
      padding: 2px 8px;
      border-radius: 6px;
      font-size: 12px;
      font-weight: 700;
      background: #e0f2fe;
      color: #0284c7;
    }
    .star-ref {
      stroke: #94a3b8;
      stroke-dasharray: 4 3;
      stroke-width: 1.5;
    }
    .sun-ray {
      stroke: #f59e0b;
      stroke-width: 1.5;
      stroke-dasharray: 3 3;
    }
    .orbit-arc {
      fill: none;
      stroke: #cbd5e1;
      stroke-width: 1.5;
      stroke-dasharray: 4 4;
    }
    .anim-smooth {
      transition: all 0.6s cubic-bezier(0.4, 0, 0.2, 1);
    }
  </style>
</head>
<body>

  
  <div class="stepper">
    <button class="step-btn play-btn" id="playToggle" title="Auto play">⏸</button>
    <button class="step-btn active" data-step="0">Day 0</button>
    <button class="step-btn" data-step="1">Day 27.3</button>
    <button class="step-btn" data-step="2">Day 29.5</button>
  </div>

  
  <div class="canvas-wrap">
    <svg viewBox="0 0 500 340" aria-label="Sidereal vs Synodic Month Lunar Orbit">
      <defs>
        <radialGradient id="sunGlow" cx="50%" cy="50%" r="50%">
          <stop offset="0%" stop-color="#fef08a" />
          <stop offset="60%" stop-color="#f59e0b" />
          <stop offset="100%" stop-color="#f59e0b" stop-opacity="0.2" />
        </radialGradient>
        <marker id="arrow" viewBox="0 0 10 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
          <path d="M 0 1.5 L 8 5 L 0 8.5 z" fill="#64748b" />
        </marker>
        <marker id="arrowSun" viewBox="0 0 10 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
          <path d="M 0 1.5 L 8 5 L 0 8.5 z" fill="#f59e0b" />
        </marker>
        <marker id="arrowPrimary" viewBox="0 0 10 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
          <path d="M 0 1.5 L 8 5 L 0 8.5 z" fill="#22b7ff" />
        </marker>
      </defs>

      
      <g opacity="0.85">
        <line x1="80" y1="55" x2="80" y2="18" class="star-ref" marker-end="url(#arrow)" />
        <line x1="250" y1="55" x2="250" y2="18" class="star-ref" marker-end="url(#arrow)" />
        <line x1="420" y1="55" x2="420" y2="18" class="star-ref" marker-end="url(#arrow)" />
        <text x="250" y="14" text-anchor="middle" font-size="12" font-weight="600" fill="#64748b">✦ Distant Stars Reference Direction ✦</text>
      </g>

      
      <path d="M 120 120 Q 250 105 400 135" class="orbit-arc" />
      <text x="260" y="96" text-anchor="middle" font-size="11" fill="#94a3b8" font-style="italic">Earth orbit around Sun →</text>

      
      <g transform="translate(250, 305)">
        <circle r="42" fill="url(#sunGlow)" />
        <circle r="22" fill="#f59e0b" />
        <text x="0" y="5" text-anchor="middle" font-size="13" font-weight="700" fill="#ffffff">Sun</text>
      </g>

      
      
      <g id="ghost0" opacity="0.35" style="display: none;">
        <circle cx="150" cy="115" r="16" fill="none" stroke="#3b82f6" stroke-width="1.5" stroke-dasharray="3 3" />
        <line x1="150" y1="115" x2="150" y2="157" stroke="#94a3b8" stroke-width="1.5" stroke-dasharray="2 2" />
        <circle cx="150" cy="157" r="7" fill="none" stroke="#64748b" stroke-width="1.5" />
      </g>
      
      <g id="ghost27" opacity="0.35" style="display: none;">
        <circle cx="345" cy="120" r="16" fill="none" stroke="#3b82f6" stroke-width="1.5" stroke-dasharray="3 3" />
        <line x1="345" y1="120" x2="345" y2="162" stroke="#94a3b8" stroke-width="1.5" stroke-dasharray="2 2" />
        <circle cx="345" cy="162" r="7" fill="none" stroke="#64748b" stroke-width="1.5" />
      </g>

      
      
      <line id="sunEarthRay" x1="250" y1="305" x2="150" y2="115" class="sun-ray anim-smooth" />

      
      <line id="starVector" x1="150" y1="115" x2="150" y2="50" class="star-ref anim-smooth" marker-end="url(#arrow)" />

      
      <g id="earthSystem" class="anim-smooth" transform="translate(150, 115)">
        
        <circle r="42" fill="none" stroke="#e2e8f0" stroke-width="1.5" />

        
        <circle r="16" fill="#3b82f6" />
        <circle r="16" fill="none" stroke="#2563eb" stroke-width="2" />
        <text y="4" text-anchor="middle" font-size="11" font-weight="700" fill="#ffffff">Earth</text>

        
        <g id="moonGroup" class="anim-smooth" transform="rotate(180)">
          
          <line x1="0" y1="0" x2="0" y2="42" stroke="#22b7ff" stroke-width="2" id="moonArm" />
          
          <circle cx="0" cy="42" r="8" fill="#e2e8f0" stroke="#64748b" stroke-width="1.5" id="moonCircle" />
        </g>
      </g>

      
      <g id="alignmentPill" class="anim-smooth" transform="translate(150, 185)" opacity="1">
        <rect x="-42" y="-10" width="84" height="20" rx="10" fill="#22b7ff" fill-opacity="0.15" stroke="#22b7ff" stroke-width="1" />
        <text y="4" text-anchor="middle" font-size="10" font-weight="700" fill="#0284c7" id="pillText">New Moon 🌑</text>
      </g>
    </svg>
  </div>

  
  <div class="status-card" id="statusCard">
    <span class="badge" id="stepBadge">Day 0</span>
    <span id="stepDesc">Start: Sun, Moon, Earth aligned (New Moon)</span>
  </div>

  <script>
    const steps = [
      {
        badge: 'Day 0: Start',
        desc: 'Sun, Moon, and Earth align in a straight line (New Moon)',
        earthX: 150,
        earthY: 115,
        moonRot: 152, // points straight towards Sun (250, 305) from (150, 115): dx=100, dy=190 -> angle ~ 152 deg
        pillText: 'New Moon 🌑',
        pillVisible: true,
        pillX: 150,
        pillY: 185,
        showGhost0: false,
        showGhost27: false,
        armColor: '#22b7ff'
      },
      {
        badge: 'Day 27.3: 360° Complete',
        desc: 'Full 360° turn to stars, but NOT aligned with Sun',
        earthX: 345,
        earthY: 120,
        moonRot: 152, // exactly same 360° direction relative to distant stars
        pillText: 'Not Aligned ✕',
        pillVisible: false,
        pillX: 345,
        pillY: 190,
        showGhost0: true,
        showGhost27: false,
        armColor: '#94a3b8'
      },
      {
        badge: 'Day 29.5: Synodic Restored',
        desc: 'Realigned between Earth & Sun (+2.2 days)',
        earthX: 375,
        earthY: 125,
        moonRot: 215, // Realigned with Sun (250, 305) from (375, 125): dx=-125, dy=180 -> angle ~ 215 deg
        pillText: 'New Moon 🌑',
        pillVisible: true,
        pillX: 375,
        pillY: 195,
        showGhost0: true,
        showGhost27: true,
        armColor: '#22b7ff'
      }
    ];

    let currentStep = 0;
    let isPlaying = true;
    let timer = null;

    const earthSystem = document.getElementById('earthSystem');
    const moonGroup = document.getElementById('moonGroup');
    const moonArm = document.getElementById('moonArm');
    const starVector = document.getElementById('starVector');
    const sunEarthRay = document.getElementById('sunEarthRay');
    const alignmentPill = document.getElementById('alignmentPill');
    const pillText = document.getElementById('pillText');
    const ghost0 = document.getElementById('ghost0');
    const ghost27 = document.getElementById('ghost27');
    const stepBadge = document.getElementById('stepBadge');
    const stepDesc = document.getElementById('stepDesc');
    const playToggle = document.getElementById('playToggle');
    const stepBtns = document.querySelectorAll('.step-btn[data-step]');

    function renderStep(idx) {
      currentStep = idx;
      const s = steps[idx];

      // Update control buttons
      stepBtns.forEach((btn, i) => {
        btn.classList.toggle('active', i === idx);
      });

      // Update Earth & Moon system
      earthSystem.setAttribute('transform', `translate(${s.earthX}, ${s.earthY})`);
      moonGroup.setAttribute('transform', `rotate(${s.moonRot})`);
      moonArm.setAttribute('stroke', s.armColor);

      // Star reference vector at Earth
      starVector.setAttribute('x1', s.earthX);
      starVector.setAttribute('y1', s.earthY);
      starVector.setAttribute('x2', s.earthX);
      starVector.setAttribute('y2', s.earthY - 60);

      // Sun-Earth alignment ray
      sunEarthRay.setAttribute('x2', s.earthX);
      sunEarthRay.setAttribute('y2', s.earthY);

      // Ghosts
      ghost0.style.display = s.showGhost0 ? 'inline' : 'none';
      ghost27.style.display = s.showGhost27 ? 'inline' : 'none';

      // Alignment Pill
      alignmentPill.setAttribute('transform', `translate(${s.pillX}, ${s.pillY})`);
      pillText.textContent = s.pillText;
      alignmentPill.setAttribute('opacity', s.pillVisible ? '1' : '0');

      // Status text
      stepBadge.textContent = s.badge;
      stepDesc.textContent = s.desc;
    }

    function nextStep() {
      const next = (currentStep + 1) % steps.length;
      renderStep(next);
    }

    function startAuto() {
      stopAuto();
      isPlaying = true;
      playToggle.textContent = '⏸';
      timer = setInterval(nextStep, 3200);
    }

    function stopAuto() {
      isPlaying = false;
      playToggle.textContent = '▶';
      if (timer) clearInterval(timer);
    }

    // Step button interactions
    stepBtns.forEach((btn) => {
      btn.addEventListener('click', () => {
        const stepIdx = parseInt(btn.getAttribute('data-step'), 10);
        renderStep(stepIdx);
        stopAuto();
      });
    });

    // Play/Pause button
    playToggle.addEventListener('click', () => {
      if (isPlaying) {
        stopAuto();
      } else {
        nextStep();
        startAuto();
      }
    });

    // Initialize
    renderStep(0);
    startAuto();
  </script>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="1:1">
  <title>Lunar Synchronous Rotation</title>
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      width: 100%;
      height: 100%;
      background: #f8f9fa;
      font-family: system-ui, -apple-system, sans-serif;
      color: #1e2945;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      padding: 10px;
      user-select: none;
      -webkit-user-select: none;
    }

    /* Controls Header */
    .controls {
      display: flex;
      flex-wrap: wrap;
      gap: 6px;
      justify-content: center;
      width: 100%;
      max-width: 420px;
      z-index: 10;
    }

    .mode-btn {
      flex: 1;
      min-width: 140px;
      padding: 7px 10px;
      font-size: 13px;
      font-weight: 600;
      border: 1px solid #cbd5e1;
      border-radius: 8px;
      background: #ffffff;
      color: #64748b;
      cursor: pointer;
      text-align: center;
      transition: background 0.2s, color 0.2s, box-shadow 0.2s;
    }
    .mode-btn.active {
      background: #22b7ff;
      border-color: #0284c7;
      color: #ffffff;
      box-shadow: 0 2px 6px rgba(34, 183, 255, 0.35);
    }

    /* Diagram Area */
    .diagram-container {
      position: relative;
      width: 100%;
      max-width: 390px;
      aspect-ratio: 1 / 1;
      margin: auto;
    }
    svg {
      width: 100%;
      height: 100%;
      overflow: visible;
    }

    /* Bottom Control Bar */
    .bottom-bar {
      display: flex;
      align-items: center;
      justify-content: space-between;
      gap: 8px;
      width: 100%;
      max-width: 400px;
      background: #ffffff;
      border: 1px solid #e2e8f0;
      border-radius: 10px;
      padding: 6px 12px;
      box-shadow: 0 1px 3px rgba(0,0,0,0.05);
    }

    .view-card {
      display: flex;
      align-items: center;
      gap: 8px;
    }
    .view-label {
      font-size: 12px;
      font-weight: 600;
      color: #1e2945;
      line-height: 1.1;
    }
    .view-canvas-wrap {
      width: 32px;
      height: 32px;
      border-radius: 50%;
      background: #0f172a;
      display: flex;
      align-items: center;
      justify-content: center;
      position: relative;
      border: 1.5px solid #94a3b8;
    }

    .playback {
      display: flex;
      align-items: center;
      gap: 6px;
    }
    .btn {
      border: none;
      background: #f1f5f9;
      color: #1e2945;
      font-size: 13px;
      font-weight: 600;
      padding: 6px 10px;
      border-radius: 6px;
      cursor: pointer;
      display: inline-flex;
      align-items: center;
      justify-content: center;
    }
    .btn:hover { background: #e2e8f0; }

    .step-btn {
      width: 26px;
      height: 26px;
      border-radius: 50%;
      border: 1px solid #cbd5e1;
      background: #ffffff;
      font-size: 11px;
      font-weight: bold;
      color: #475569;
      cursor: pointer;
      display: inline-flex;
      align-items: center;
      justify-content: center;
    }
    .step-btn.active {
      background: #1e2945;
      color: #ffffff;
      border-color: #1e2945;
    }
  </style>
</head>
<body>

  
  <div class="controls">
    <button id="btnSync" class="mode-btn active">Synchronous (Real)</button>
    <button id="btnNoRot" class="mode-btn">No Rotation (Hypothetical)</button>
  </div>

  
  <div class="diagram-container">
    <svg viewBox="0 0 400 400">
      <defs>
        
        <linearGradient id="sunRayGrad" x1="1" y1="0" x2="0" y2="0">
          <stop offset="0%" stop-color="#f59e0b" stop-opacity="0.8"/>
          <stop offset="100%" stop-color="#f59e0b" stop-opacity="0"/>
        </linearGradient>

        
        <clipPath id="moonClip">
          <circle cx="0" cy="0" r="15" />
        </clipPath>
      </defs>

      
      <g id="sunlight" transform="translate(390, 0)">
        <line x1="-30" y1="80" x2="-2" y2="80" stroke="#f59e0b" stroke-width="2" stroke-dasharray="3,3" />
        <line x1="-45" y1="140" x2="-2" y2="140" stroke="#f59e0b" stroke-width="2" />
        <line x1="-50" y1="200" x2="-2" y2="200" stroke="#f59e0b" stroke-width="2.5" />
        <line x1="-45" y1="260" x2="-2" y2="260" stroke="#f59e0b" stroke-width="2" />
        <line x1="-30" y1="320" x2="-2" y2="320" stroke="#f59e0b" stroke-width="2" stroke-dasharray="3,3" />
        <text x="-48" y="218" font-size="11" font-weight="bold" fill="#d97706">Sunlight ☀️</text>
      </g>

      
      <circle cx="200" cy="200" r="125" fill="none" stroke="#cbd5e1" stroke-width="1.5" stroke-dasharray="5,4" />

      
      <path d="M 196 75 L 204 75 L 200 70 Z" fill="#94a3b8" />
      <path d="M 75 196 L 75 204 L 70 200 Z" fill="#94a3b8" />
      <path d="M 204 325 L 196 325 L 200 330 Z" fill="#94a3b8" />
      <path d="M 325 204 L 325 196 L 330 200 Z" fill="#94a3b8" />

      
      <g transform="translate(200, 200)">
        <circle cx="0" cy="0" r="28" fill="#0284c7" stroke="#38bdf8" stroke-width="2"/>
        
        <circle cx="-6" cy="-6" r="14" fill="#22c55e" opacity="0.85" clip-path="url(#earthClip)"/>
        <ellipse cx="10" cy="8" rx="8" ry="12" fill="#22c55e" opacity="0.85"/>
        <text x="0" y="4" text-anchor="middle" font-size="11" font-weight="bold" fill="#ffffff">Earth</text>
      </g>

      
      <g id="ghostMarkers" opacity="0.35">
        <circle cx="325" cy="200" r="15" fill="none" stroke="#64748b" stroke-width="1" />
        <circle cx="200" cy="75" r="15" fill="none" stroke="#64748b" stroke-width="1" />
        <circle cx="75" cy="200" r="15" fill="none" stroke="#64748b" stroke-width="1" />
        <circle cx="200" cy="325" r="15" fill="none" stroke="#64748b" stroke-width="1" />
      </g>

      
      <line id="sightline" x1="200" y1="200" x2="325" y2="200" stroke="#ef4444" stroke-width="1.5" stroke-dasharray="2,3" opacity="0.6"/>

      
      <g id="moonOrbitGroup" transform="translate(325, 200)">
        
        
        <g clip-path="url(#moonClip)">
          
          <rect x="-15" y="-15" width="30" height="30" fill="#f1f5f9" />
          
          <rect x="-15" y="-15" width="15" height="30" fill="#334155" />
          
          <line x1="0" y1="-15" x2="0" y2="15" stroke="#94a3b8" stroke-width="1" />
        </g>
        <circle cx="0" cy="0" r="15" fill="none" stroke="#64748b" stroke-width="1.2" />

        
        <g id="moonSurfaceRotator">
          
          <line x1="0" y1="0" x2="-14" y2="0" stroke="#ef4444" stroke-width="2.5" stroke-linecap="round" />
          <circle cx="-14" cy="0" r="3.5" fill="#ef4444" stroke="#ffffff" stroke-width="1" />
        </g>
      </g>
    </svg>
  </div>

  
  <div class="bottom-bar">
    
    <div class="view-card">
      <div class="view-canvas-wrap">
        <svg viewBox="-16 -16 32 32" width="28" height="28">
          <circle cx="0" cy="0" r="13" fill="#cbd5e1" />
          <circle id="earthViewMarker" cx="0" cy="0" r="4.5" fill="#ef4444" stroke="#ffffff" stroke-width="1" />
          <line id="earthViewHiddenLine" x1="-8" y1="-8" x2="8" y2="8" stroke="#64748b" stroke-width="2" opacity="0"/>
        </svg>
      </div>
      <div class="view-label">
        Earth View:<br>
        <span id="viewStatus" style="color: #ef4444; font-size: 11px;">Near Side Visible</span>
      </div>
    </div>

    
    <div class="playback">
      <button id="playBtn" class="btn" title="Play/Pause">⏸</button>
      <button class="step-btn active" data-step="0">1</button>
      <button class="step-btn" data-step="1">2</button>
      <button class="step-btn" data-step="2">3</button>
      <button class="step-btn" data-step="3">4</button>
    </div>
  </div>

  <script>
    // State
    let isSynchronous = true;
    let isPlaying = true;
    let angleDeg = 0; // 0 = right (New Moon position), CCW
    const ORBIT_RADIUS = 125;
    const CENTER_X = 200;
    const CENTER_Y = 200;

    // Elements
    const moonOrbitGroup = document.getElementById('moonOrbitGroup');
    const moonSurfaceRotator = document.getElementById('moonSurfaceRotator');
    const sightline = document.getElementById('sightline');
    const earthViewMarker = document.getElementById('earthViewMarker');
    const earthViewHiddenLine = document.getElementById('earthViewHiddenLine');
    const viewStatus = document.getElementById('viewStatus');
    const btnSync = document.getElementById('btnSync');
    const btnNoRot = document.getElementById('btnNoRot');
    const playBtn = document.getElementById('playBtn');
    const stepBtns = document.querySelectorAll('.step-btn');

    function updateVisuals() {
      const rad = (angleDeg * Math.PI) / 180;
      // Moon orbital position (CCW: 0 = (325, 200), 90 = (200, 75), 180 = (75, 200), 270 = (200, 325))
      const mx = CENTER_X + ORBIT_RADIUS * Math.cos(-rad);
      const my = CENTER_Y + ORBIT_RADIUS * Math.sin(-rad);

      moonOrbitGroup.setAttribute('transform', `translate(${mx}, ${my})`);
      sightline.setAttribute('x2', mx);
      sightline.setAttribute('y2', my);

      // Surface rotation
      // In Synchronous mode, near-side marker rotates with orbit angle so it always points to Earth (angleDeg relative to orbit)
      // In No-Rotation mode, marker stays locked in absolute space pointing strictly left (angle 0)
      let markerAngle;
      if (isSynchronous) {
        markerAngle = -angleDeg; // Rotates CCW exactly in sync
      } else {
        markerAngle = 0; // Fixed direction in space (facing 9:00 / West)
      }
      moonSurfaceRotator.setAttribute('transform', `rotate(${markerAngle})`);

      // Calculate what Earth sees:
      // Earth-to-moon vector angle in space is -angleDeg (from Moon to Earth it is -angleDeg + 180).
      // The marker's physical pointing angle in space is (180 + markerAngle).
      // Angle difference between marker and direction toward Earth:
      const markerDir = (180 + markerAngle) % 360;
      const towardEarthDir = (-angleDeg + 180) % 360;
      let diff = Math.abs((markerDir - towardEarthDir + 540) % 360 - 180);

      // Diff 0 = marker faces Earth directly; Diff 180 = marker is on far side
      if (diff < 85) {
        // Visible on near side
        const offset = Math.sin((markerDir - towardEarthDir) * Math.PI / 180) * 10;
        earthViewMarker.setAttribute('cx', offset.toFixed(1));
        earthViewMarker.setAttribute('opacity', '1');
        earthViewHiddenLine.setAttribute('opacity', '0');
        viewStatus.textContent = "Near Side (Red)";
        viewStatus.style.color = "#ef4444";
      } else {
        // Far side / hidden from Earth
        earthViewMarker.setAttribute('opacity', '0.15');
        earthViewHiddenLine.setAttribute('opacity', '0.6');
        viewStatus.textContent = "Far Side (Hidden)";
        viewStatus.style.color = "#64748b";
      }

      // Update active step indicator (0°, 90°, 180°, 270°)
      const currentQuarter = Math.round(angleDeg / 90) % 4;
      stepBtns.forEach((btn, idx) => {
        btn.classList.toggle('active', idx === currentQuarter);
      });
    }

    // Animation Loop
    let lastTime = performance.now();
    function animate(time) {
      const dt = (time - lastTime) / 1000;
      lastTime = time;

      if (isPlaying) {
        // 360° in 8 seconds (calm pedagogical pace)
        angleDeg = (angleDeg + (360 / 8) * dt) % 360;
        updateVisuals();
      }

      requestAnimationFrame(animate);
    }
    requestAnimationFrame(animate);

    // Event Handlers
    btnSync.addEventListener('click', () => {
      isSynchronous = true;
      btnSync.classList.add('active');
      btnNoRot.classList.remove('active');
      updateVisuals();
    });

    btnNoRot.addEventListener('click', () => {
      isSynchronous = false;
      btnNoRot.classList.add('active');
      btnSync.classList.remove('active');
      updateVisuals();
    });

    playBtn.addEventListener('click', () => {
      isPlaying = !isPlaying;
      playBtn.textContent = isPlaying ? '⏸' : '▶';
    });

    stepBtns.forEach((btn) => {
      btn.addEventListener('click', () => {
        const step = parseInt(btn.dataset.step, 10);
        angleDeg = step * 90;
        isPlaying = false;
        playBtn.textContent = '▶';
        updateVisuals();
      });
    });

    updateVisuals();
  </script>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      font-family: system-ui, -apple-system, sans-serif;
      background: #f8f9fa;
      color: #1e2945;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      min-height: 100%;
      padding: 12px;
      user-select: none;
    }

    .toolbar {
      display: flex;
      align-items: center;
      gap: 12px;
      margin-bottom: 4px;
      z-index: 10;
    }

    .btn {
      background: #ffffff;
      border: 1px solid #cbd5e1;
      border-radius: 20px;
      padding: 6px 14px;
      font-size: 14px;
      font-weight: 600;
      color: #1e2945;
      cursor: pointer;
      display: flex;
      align-items: center;
      gap: 6px;
      box-shadow: 0 1px 3px rgba(0,0,0,0.06);
    }
    .btn:active {
      background: #f1f5f9;
    }

    .legend {
      display: flex;
      gap: 16px;
      font-size: 13px;
      font-weight: 600;
    }
    .legend-item {
      display: flex;
      align-items: center;
      gap: 6px;
    }
    .legend-swatch {
      width: 12px;
      height: 12px;
      border-radius: 50%;
      border: 1px solid rgba(0,0,0,0.15);
    }

    svg {
      width: 100%;
      max-width: 540px;
      height: auto;
      overflow: visible;
    }

    .sun-ray {
      stroke: #f59e0b;
      stroke-width: 2.5;
      stroke-dasharray: 6 4;
      animation: rayPulse 2s linear infinite;
    }
    @keyframes rayPulse {
      to { stroke-dashoffset: -20; }
    }

    .orbit-path {
      fill: none;
      stroke: #cbd5e1;
      stroke-width: 1.5;
      stroke-dasharray: 4 4;
    }

    .label-text {
      font-size: 13px;
      font-weight: 700;
      fill: #1e2945;
    }
    .sun-text {
      font-size: 14px;
      font-weight: 800;
      fill: #d97706;
      letter-spacing: 0.5px;
    }

    .arrow-head {
      fill: #94a3b8;
    }
  </style>
</head>
<body>

  <div class="toolbar">
    <button class="btn" id="orbitBtn">
      <span id="playIcon">▶</span> Orbit
    </button>
    <div class="legend">
      <div class="legend-item">
        <span class="legend-swatch" style="background:#fef08a; border-color:#eab308;"></span>
        <span>Lit side</span>
      </div>
      <div class="legend-item">
        <span class="legend-swatch" style="background:#334155;"></span>
        <span>Dark side</span>
      </div>
    </div>
  </div>

  <svg viewBox="0 0 540 360">
    <defs>
      
      <marker id="rayArrow" viewBox="0 0 10 10" refX="6" refY="5" markerWidth="6" markerHeight="6" orient="auto">
        <path d="M 10 0 L 0 5 L 10 10 z" fill="#f59e0b"/>
      </marker>
      
      <marker id="orbitArrow" viewBox="0 0 10 10" refX="5" refY="5" markerWidth="5" markerHeight="5" orient="auto">
        <path d="M 0 1 L 8 5 L 0 9 z" fill="#64748b"/>
      </marker>

      
      <g id="moonGraphic">
        
        <path d="M 0 -15 A 15 15 0 0 0 0 15 Z" fill="#334155" stroke="#1e293b" stroke-width="0.5"/>
        
        <path d="M 0 -15 A 15 15 0 0 1 0 15 Z" fill="#fef08a" stroke="#eab308" stroke-width="0.5"/>
        
        <line x1="0" y1="-15" x2="0" y2="15" stroke="#94a3b8" stroke-width="0.75"/>
      </g>
    </defs>

    
    <g>
      <line x1="510" y1="70" x2="380" y2="70" class="sun-ray" marker-end="url(#rayArrow)"/>
      <line x1="510" y1="125" x2="380" y2="125" class="sun-ray" marker-end="url(#rayArrow)"/>
      <line x1="510" y1="180" x2="380" y2="180" class="sun-ray" marker-end="url(#rayArrow)"/>
      <line x1="510" y1="235" x2="380" y2="235" class="sun-ray" marker-end="url(#rayArrow)"/>
      <line x1="510" y1="290" x2="380" y2="290" class="sun-ray" marker-end="url(#rayArrow)"/>

      
      <text x="460" y="325" text-anchor="middle" class="sun-text">SUNLIGHT</text>
    </g>

    
    <circle cx="210" cy="180" r="110" class="orbit-path"/>

    
    
    <path d="M 220 70 A 110 110 0 0 0 195 72" fill="none" stroke="#64748b" stroke-width="2" marker-end="url(#orbitArrow)"/>
    
    <path d="M 100 170 A 110 110 0 0 0 102 195" fill="none" stroke="#64748b" stroke-width="2" marker-end="url(#orbitArrow)"/>
    
    <path d="M 200 290 A 110 110 0 0 0 225 288" fill="none" stroke="#64748b" stroke-width="2" marker-end="url(#orbitArrow)"/>
    
    <path d="M 320 190 A 110 110 0 0 0 318 165" fill="none" stroke="#64748b" stroke-width="2" marker-end="url(#orbitArrow)"/>

    
    <g transform="translate(210, 180)">
      
      <path d="M 0 -26 A 26 26 0 0 0 0 26 Z" fill="#0f172a"/>
      
      <path d="M 0 -26 A 26 26 0 0 1 0 26 Z" fill="#38bdf8"/>
      
      <path d="M 6 -14 Q 16 -10 12 2 Q 4 6 8 16 Q 2 12 1 -2 Z" fill="#4ade80" opacity="0.9"/>
      
      <circle cx="0" cy="0" r="26" fill="none" stroke="#0284c7" stroke-width="1.5"/>
      <line x1="0" y1="-26" x2="0" y2="26" stroke="#0369a1" stroke-width="1"/>
      <text x="0" y="38" text-anchor="middle" class="label-text">Earth</text>
    </g>

    
    
    <g id="moonTop" transform="translate(210, 70)">
      <use href="#moonGraphic"/>
    </g>

    
    <g id="moonLeft" transform="translate(100, 180)">
      <use href="#moonGraphic"/>
    </g>

    
    <g id="moonBottom" transform="translate(210, 290)">
      <use href="#moonGraphic"/>
    </g>

    
    <g id="moonRight" transform="translate(320, 180)">
      <use href="#moonGraphic"/>
    </g>

    
    <g id="activeMoon" transform="translate(320, 180)" style="display:none;">
      <circle cx="0" cy="0" r="18" fill="none" stroke="#38bdf8" stroke-width="2" stroke-dasharray="3 3"/>
      <use href="#moonGraphic"/>
    </g>
  </svg>

  <script>
    const orbitBtn = document.getElementById('orbitBtn');
    const playIcon = document.getElementById('playIcon');
    const activeMoon = document.getElementById('activeMoon');
    const cardinalMoons = [
      document.getElementById('moonTop'),
      document.getElementById('moonLeft'),
      document.getElementById('moonBottom'),
      document.getElementById('moonRight')
    ];

    let isOrbiting = false;
    let angle = 0; // Starts at 0 rad (right position: x = 210 + 110, y = 180)
    let animId = null;

    const centerX = 210;
    const centerY = 180;
    const radius = 110;

    function setOpacity(val) {
      cardinalMoons.forEach(m => m.style.opacity = val);
    }

    function animate() {
      // Counter-clockwise: decrement angle in SVG coordinate system (where +y is down)
      angle -= 0.015;
      if (angle < -Math.PI * 2) angle += Math.PI * 2;

      const x = centerX + radius * Math.cos(angle);
      const y = centerY + radius * Math.sin(angle);

      activeMoon.setAttribute('transform', `translate(${x.toFixed(2)}, ${y.toFixed(2)})`);

      if (isOrbiting) {
        animId = requestAnimationFrame(animate);
      }
    }

    orbitBtn.addEventListener('click', () => {
      isOrbiting = !isOrbiting;
      if (isOrbiting) {
        playIcon.textContent = '⏸';
        activeMoon.style.display = 'block';
        setOpacity('0.35');
        animate();
      } else {
        playIcon.textContent = '▶';
        cancelAnimationFrame(animId);
        activeMoon.style.display = 'none';
        setOpacity('1');
      }
    });
  </script>
</body>
</html>

OGS.7.ESS.6

Aligned learning resources

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