At any one place the tide rises and falls twice a day. Seen globally, it does something else: it rotates. This is M2, the lunar semidiurnal constituent, the largest contributor to the tide almost everywhere. One cycle takes 12 h 25 min, which is why high tide arrives about fifty minutes later each day. On a Spilhaus projection, where the ocean appears as the single connected body it is, the structure becomes readable.

⚪ The circles are amphidromic points. At each one the M2 range is essentially zero, and the wave turns around them like water around a drain. The entire global tide is organised around them.

🌀 Why they exist. The textbook picture of two bulges following the Moon describes an ocean with no continents on a planet that doesn't spin. The real ocean is partitioned into basins, and the Earth's rotation deflects every moving parcel of water. The response organises into rotating waves, with a node at the centre of each.

🔴🔵 Red is water piled up, blue drawn down. Watch one basin for a full cycle: the lobes swing around the nodes rather than march across the ocean.

📏 The colour scale saturates at one metre, but open ocean M2 amplitude is typically two or three decimetres. The huge ranges of the Bay of Fundy or the English Channel are made locally, where a narrowing shelf piles the wave up.

⬇️ M2 is one constituent of many. The real tide is their sum, and it is the beat between M2 and the solar S2 that gives spring and neap tides over a fortnight. A different animation, perhaps.

Data: FES2022 (AVISO/CNES/LEGOS)