The Rule of Twelfths

Tide heights in your head

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Tides rise 1, 2, 3, 3, 2, 1 twelfths of the range in successive hours.
• The rule approximates a sine curve to within a few percent.
• High water comes about 50 minutes later each day.

Steps

1. Tides are not steady
2. The third hour
3. Crossing the sandbar
4. Why it works
5. Tomorrow’s tide
6. Two hours in