By David Foulds
Updated Mar 24, 2022
Ocean tides are the result of gravitational forces from the Moon and the Sun acting on Earth’s water. Because the Moon is much closer, its pull dominates, creating two bulges in the ocean: one facing the Moon and another on the opposite side of the planet. These bulges give rise to high tides at their peaks and low tides at their troughs. When the bulges align with a shoreline, we observe the familiar rise and fall of coastal waters.
A solar day lasts 24 hours – the time for Earth to complete one rotation relative to the Sun. The Moon orbits Earth in the same direction as Earth’s rotation, so a full lunar cycle takes slightly longer: 24 hours and 50 minutes. This extra 50 minutes means that the ocean bulges shift a bit each day.
Because the bulges are driven by the Moon’s gravity, they travel around the globe every 24 hours and 50 minutes. With two bulges present, there are typically two high tides and two low tides in that period, occurring roughly every 12 hours and 25 minutes. That explains why the timing of high and low tides changes from day to day.
If Earth were a smooth, water‑covered sphere, the tidal pattern would be simple: two equal high tides and two equal low tides each lunar day. In reality, continents and ocean basins disrupt this flow, creating three main tidal regimes: diurnal, semi‑diurnal, and mixed.
Most coastal areas experience semi‑diurnal or mixed tides. Semi‑diurnal tides feature two high and two low tides of roughly equal height. Mixed tides, on the other hand, have one high tide noticeably higher than the other, and similarly for the low tides.
Diurnal tides occur when geographic features force the water to cycle only once per day, producing a single high tide and a single low tide. In the United States, diurnal tides are found in the Gulf of Mexico and along Alaska’s coast.
