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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Why We See Only One Side of the Moon</title>
    <!-- Fonts for header and body -->
    <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;700&family=Space+Grotesk:wght@700&display=swap" rel="stylesheet">
    <script src="https://cdn.tailwindcss.com"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.9.0/p5.min.js"></script>
    <style>
        html, body, .font-sans {
            font-family: 'Inter', system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Arial, "Noto Sans", sans-serif;
        }
        body {
            margin: 0;
            padding: 0;
            overflow: hidden;
        }
        #animation-container {
            position: relative;
            width: 100vw;
            height: 100vh;
            overflow: hidden;
            background-color: #000010;
        }
        #animation-canvas {
            display: block;
            position: absolute;
            top: 0;
            left: 0;
            z-index: 1;
        }
        .header-astro {
            font-family: 'Space Grotesk', 'Inter', Arial, sans-serif;
            font-weight: 700;
            letter-spacing: -0.01em;
        }
        .control-panel {
            position: absolute;
            bottom: 2rem;
            left: 2rem;
            z-index: 10;
            background: rgba(0, 0, 20, 0.7);
            backdrop-filter: blur(10px);
            border-radius: 1rem;
            padding: 1rem;
            box-shadow: 0 0 20px rgba(186, 215, 255, 0.2);
            display: flex;
            flex-direction: column;
            gap: 0.5rem;
        }
        .explanation-panel {
            position: absolute;
            top: 2rem;
            right: 2rem;
            z-index: 10;
            background: rgba(0, 0, 20, 0.7);
            backdrop-filter: blur(10px);
            border-radius: 1rem;
            padding: 1.5rem;
            box-shadow: 0 0 20px rgba(186, 215, 255, 0.2);
            color: white;
            width: 220px;
            font-size: 1rem;
            line-height: 1.6;
            transition: opacity 0.3s ease-in-out;
            opacity: 0;
        }
        .control-btn {
            padding: 0.5rem 1rem;
            font-size: 0.92rem;
            font-weight: 600;
            color: white;
            background-color: rgba(255, 255, 255, 0.1);
            border: 1px solid rgba(255, 255, 255, 0.2);
            border-radius: 0.5rem;
            cursor: pointer;
            transition: background-color 0.2s ease;
            text-align: center;
            font-family: 'Inter', system-ui, sans-serif;
        }
        .control-btn:hover {
            background-color: rgba(255, 255, 255, 0.2);
        }
        .control-btn.active {
            background-color: #4f46e5;
            border-color: #4f46e5;
        }
        /* Disclaimer at bottom, single line, smaller, no glow */
        .disclaimer-line {
            position: fixed;
            left: 50%;
            bottom: 1.1rem;
            transform: translateX(-50%);
            z-index: 40;
            font-size: 0.89rem;
            color: #b6bed7;
            font-family: 'Inter', system-ui, sans-serif;
            text-align: center;
            padding: 0 0.5rem;
            border-radius: 0.4rem;
            pointer-events: none;
            white-space: nowrap;
            background: transparent;
            opacity: 0.88;
        }
    </style>
</head>
<body class="bg-black text-white overflow-hidden font-sans">
    <div id="animation-container" class="relative">
        <h1 class="header-astro absolute text-center top-8 left-1/2 -translate-x-1/2 text-3xl md:text-4xl font-bold text-white z-20 w-11/12" style="text-shadow: 0 0 15px rgba(186, 215, 255, 0.7);">
            Why We See Only One Side of the Moon
        </h1>
        <!-- Canvas will be inserted here by p5.js -->
    </div>
    <div id="explanation-panel" class="explanation-panel"></div>
    <div class="control-panel">
        <button id="btn-no-rotation" class="control-btn">No Rotation</button>
        <button id="btn-fast-rotation" class="control-btn">Fast Rotation</button>
        <button id="btn-locked-rotation" class="control-btn active">Tidally Locked</button>
    </div>
    <div class="disclaimer-line">
        Disclaimer: Earth’s rotation is hidden to keep the focus on the Moon’s motion.
    </div>
    <script>
        document.addEventListener('DOMContentLoaded', () => {
            const sketch = (p) => {
                let earth;
                let moon;
                let stars = [];
                let rotationMode = 'locked';
                let starCount = 800;

                const btnNoRotation = document.getElementById('btn-no-rotation');
                const btnFastRotation = document.getElementById('btn-fast-rotation');
                const btnLockedRotation = document.getElementById('btn-locked-rotation');
                const explanationPanel = document.getElementById('explanation-panel');

                const explanations = {
                    none: '<strong>No Rotation:</strong> The Moon does not spin on its own axis. The red line always points in the same direction, so we see all sides of the Moon as it orbits.',
                    fast: '<strong>Fast Rotation:</strong> The Moon spins on its own axis very quickly, much faster than it orbits the Earth. We can easily see all of its sides.',
                    locked: '<strong>Tidally Locked:</strong> The Moon rotates on its axis at the same speed it orbits the Earth. This is why the red line (and the same face) always points toward the Earth!'
                };

                function updateActiveButton() {
                    document.querySelectorAll('.control-btn').forEach(btn => btn.classList.remove('active'));
                    explanationPanel.style.opacity = '1';
                    if (rotationMode === 'none') {
                        btnNoRotation.classList.add('active');
                        explanationPanel.innerHTML = explanations.none;
                    } else if (rotationMode === 'fast') {
                        btnFastRotation.classList.add('active');
                        explanationPanel.innerHTML = explanations.fast;
                    } else if (rotationMode === 'locked') {
                        btnLockedRotation.classList.add('active');
                        explanationPanel.innerHTML = explanations.locked;
                    }
                }

                btnNoRotation.addEventListener('click', () => {
                    rotationMode = 'none';
                    updateActiveButton();
                });
                btnFastRotation.addEventListener('click', () => {
                    rotationMode = 'fast';
                    updateActiveButton();
                });
                btnLockedRotation.addEventListener('click', () => {
                    rotationMode = 'locked';
                    updateActiveButton();
                });

                function fillStars(n) {
                    stars = [];
                    for (let i = 0; i < n; i++) {
                        stars.push(p.createVector(p.random(p.width), p.random(p.height)));
                    }
                }

                p.setup = () => {
                    let canvas = p.createCanvas(p.windowWidth, p.windowHeight);
                    canvas.parent('animation-container');
                    canvas.id('animation-canvas');
                    p.colorMode(p.HSB, 360, 100, 100, 100);
                    p.angleMode(p.DEGREES);
                    earth = new Earth(p.width / 2, p.height / 2);
                    moon = new Moon(earth.pos.x, earth.pos.y, p.min(p.width, p.height) * 0.4);

                    starCount = Math.floor((p.width * p.height) / 1800);
                    fillStars(starCount);

                    updateActiveButton();
                };

                p.draw = () => {
                    p.background(245, 90, 10);
                    drawStars();
                    moon.update();
                    earth.update();
                    earth.display(moon.pos);
                    moon.display();
                };

                function drawStars() {
                    p.noStroke();
                    for (const star of stars) {
                        star.x -= 0.02;
                        if (star.x < 0) {
                            star.x = p.width;
                        }
                        let alpha = p.random(50, 100);
                        let size = p.random(1, 3);
                        if (p.random() > 0.99) {
                            alpha = 100;
                            size = p.random(2.5, 4);
                        }
                        p.fill(0, 0, 100, alpha);
                        p.ellipse(star.x, star.y, size, size);
                    }
                }

                p.windowResized = () => {
                    p.resizeCanvas(p.windowWidth, p.windowHeight);
                    if (earth) earth.pos.set(p.width / 2, p.height / 2);
                    if (moon) {
                        moon.center.set(p.width / 2, p.height / 2);
                        moon.radius = p.min(p.width, p.height) * 0.35;
                    }
                    starCount = Math.floor((p.width * p.height) / 1800);
                    fillStars(starCount);
                };

                class Earth {
                    constructor(x, y) {
                        this.pos = p.createVector(x, y);
                        this.bodySize = p.min(p.width, p.height) * 0.25;
                        this.eyes = [
                            { offset: p.createVector(-this.bodySize * 0.27, -this.bodySize * 0.13), size: this.bodySize * 0.25 },
                            { offset: p.createVector(this.bodySize * 0.27, -this.bodySize * 0.13), size: this.bodySize * 0.25 }
                        ];
                    }
                    update() {
                        this.bodySize = p.min(p.width, p.height) * 0.25;
                        this.eyes[0].offset.set(-this.bodySize * 0.27, -this.bodySize * 0.13);
                        this.eyes[1].offset.set(this.bodySize * 0.27, -this.bodySize * 0.13);
                        this.eyes[0].size = this.bodySize * 0.25;
                        this.eyes[1].size = this.bodySize * 0.25;
                    }
                    display(target) {
                        p.push();
                        p.translate(this.pos.x, this.pos.y);
                        p.drawingContext.shadowBlur = 60;
                        p.drawingContext.shadowColor = p.color(200, 80, 100, 50);
                        p.fill(210, 80, 80);
                        p.noStroke();
                        p.ellipse(0, 0, this.bodySize, this.bodySize);

                        p.fill(120, 60, 65);
                        p.noStroke();
                        p.beginShape();
                        p.curveVertex(this.bodySize * -0.15, this.bodySize * -0.4);
                        p.curveVertex(this.bodySize * -0.1, this.bodySize * -0.4);
                        p.curveVertex(this.bodySize * -0.2, this.bodySize * -0.2);
                        p.curveVertex(this.bodySize * -0.15, 0.05);
                        p.curveVertex(this.bodySize * -0.3, this.bodySize * 0.3);
                        p.curveVertex(this.bodySize * -0.2, this.bodySize * 0.45);
                        p.curveVertex(this.bodySize * 0.05, this.bodySize * 0.4);
                        p.curveVertex(this.bodySize * 0.1, 0.1);
                        p.curveVertex(this.bodySize * -0.1, this.bodySize * -0.4);
                        p.curveVertex(this.bodySize * -0.1, this.bodySize * -0.4);
                        p.endShape(p.CLOSE);

                        p.beginShape();
                        p.curveVertex(this.bodySize * 0.4, this.bodySize * -0.3);
                        p.curveVertex(this.bodySize * 0.45, this.bodySize * -0.2);
                        p.curveVertex(this.bodySize * 0.35, 0);
                        p.curveVertex(this.bodySize * 0.4, this.bodySize * 0.25);
                        p.curveVertex(this.bodySize * 0.2, this.bodySize * 0.4);
                        p.curveVertex(this.bodySize * 0.15, this.bodySize * 0.2);
                        p.curveVertex(this.bodySize * 0.25, 0);
                        p.endShape(p.CLOSE);

                        p.fill(200, 5, 100, 90);
                        p.noStroke();
                        p.arc(0, 0, this.bodySize, this.bodySize, 250, 290, p.CHORD);
                        p.arc(0, 0, this.bodySize, this.bodySize, 70, 110, p.CHORD);

                        p.drawingContext.shadowBlur = 0;

                        this.eyes.forEach(eye => this.drawEye(eye.offset.x, eye.offset.y, eye.size, target));

                        p.noFill();
                        p.stroke(210, 20, 20, 80);
                        p.strokeWeight(4);
                        p.arc(0, this.bodySize * 0.15, this.bodySize * 0.4, this.bodySize * 0.27, 0, 180);

                        p.pop();
                    }
                    drawEye(x, y, size, target) {
                        let eyePos = p.createVector(this.pos.x + x, this.pos.y + y);
                        let angle = p.atan2(target.y - eyePos.y, target.x - eyePos.x);
                        let dist = p5.Vector.dist(target, eyePos);
                        let pupilOffset = p.min(dist, size / 2 - (size / 1.8 / 2));
                        p.push();
                        p.translate(x, y);
                        p.stroke(200, 10, 20, 80);
                        p.strokeWeight(2);
                        p.fill(200, 5, 100);
                        p.ellipse(0, 0, size, size);
                        p.rotate(angle);
                        p.noStroke();
                        p.fill(0);
                        p.ellipse(pupilOffset, 0, size / 1.8, size / 1.8);
                        p.pop();
                    }
                }
                class Moon {
                    constructor(centerX, centerY, radius) {
                        this.center = p.createVector(centerX, centerY);
                        this.radius = radius;
                        this.size = p.min(p.width, p.height) * 0.08;
                        this.orbitalAngle = 0;
                        this.selfRotationAngle = 0;
                        this.speed = 0.5;
                        this.pos = p.createVector();
                    }
                    update() {
                        this.size = p.min(p.width, p.height) * 0.08;
                        this.orbitalAngle += this.speed;
                        this.selfRotationAngle += 4;
                        this.pos.x = this.center.x + this.radius * p.cos(this.orbitalAngle);
                        this.pos.y = this.center.y + this.radius * p.sin(this.orbitalAngle);
                    }
                    display() {
                        p.noFill();
                        p.stroke(100, 0, 100, 10);
                        p.strokeWeight(1);
                        p.ellipse(this.center.x, this.center.y, this.radius * 2, this.radius * 2);

                        p.push();
                        p.translate(this.pos.x, this.pos.y);

                        if (rotationMode === 'locked') {
                            let angleToEarth = p.atan2(this.center.y - this.pos.y, this.center.x - this.pos.x);
                            p.rotate(angleToEarth);
                        } else if (rotationMode === 'fast') {
                            p.rotate(this.selfRotationAngle);
                        }

                        p.fill(0, 0, 50);
                        p.noStroke();
                        p.arc(0, 0, this.size, this.size, 90, 270);

                        p.fill(0, 0, 85);
                        p.arc(0, 0, this.size, this.size, -90, 90);

                        p.stroke(0, 100, 100);
                        p.strokeWeight(3);
                        p.line(0, -this.size * 0.5, 0, this.size * 0.5);

                        p.pop();
                    }
                }
            };

            new p5(sketch);
        });
    </script>
</body>
</html>