A tide is the periodic rise and fall of sea level, driven primarily by the gravitational pull of the Moon on Earth’s oceans, with the Sun contributing a smaller additional effect.
Why Tides Happen
The Moon’s gravity pulls ocean water toward it, creating a bulge of higher water on the side of Earth facing the Moon — and, due to inertial effects, a corresponding bulge on the opposite side of Earth as well. As Earth rotates beneath these bulges, most coastal locations experience two high tides and two low tides roughly every 24 hours and 50 minutes (a full tidal day, slightly longer than a solar day because the Moon itself is also orbiting Earth).
Types of Tidal Patterns
- Semi-diurnal — two high and two low tides of roughly similar size each day, the most common pattern globally
- Diurnal — only one high and one low tide per day, seen in some specific regions
- Mixed semi-diurnal — two highs and two lows, but of noticeably unequal size
Spring and Neap Tides
Tidal range (the difference between high and low water) itself varies over roughly a two-week cycle, driven by the Moon’s phase — larger at spring tides (new/full moon) and smaller at neap tides (quarter moons).
Why Tides Matter Practically
- Available depth in port approaches, channels, and alongside berths varies with the tide, directly affecting under-keel clearance
- Tidal streams (horizontal water movement from the tide) affect a vessel’s speed and course over ground
- Port and canal access — some ports and shallow-draft routes are only accessible around high tide
Tide Tables
Predicted tide times and heights for specific ports are published in official tide tables, essential reference material for passage planning in any tidally-affected water.
Exam Relevance
Tidal formation, patterns, and their practical application in passage planning are core Meteorology/Tides topics in deck officer competency exams.