Why does the sea rise and fall?

Two time scales explain different features of tides. Earth’s rotation and the Moon’s movement produce a repeating cycle at a fixed shore. An ideal lunar day is about 24 hours 50 minutes, but actual coastal tides may be twice-daily, once-daily or unequal. Over the lunar month, changing Sun–Moon geometry alters tidal range, the high-to-low height difference. Alignment near new and full Moon reinforces the range, called spring tide; quarter-phase geometry reduces it, called neap tide, without eliminating tides. Follow the observer and compare the water-level display’s vertical span separately from the number of peaks. These are idealised relative levels, not measured water heights or harbour predictions.