Understanding Tides: The Complete Guide to How Tides Work

By TideTimes Global·March 5, 2025·Updated March 5, 2025

⚡ Quick Answer

Tides are caused by the gravitational pull of the Moon and, to a lesser extent, the Sun acting on Earth's oceans. As the Moon orbits Earth, its gravity creates two bulges of water on opposite sides of the planet — one facing the Moon, one on the opposite side. As Earth rotates through these bulges, coastal locations experience two high tides and two low tides roughly every 24 hours and 50 minutes.

What Causes Tides?

Tides are the result of gravitational forces acting on Earth's oceans. The Moon is the primary driver: its gravity pulls ocean water toward it, creating a bulge of elevated water on the side of Earth facing the Moon. Simultaneously, the centrifugal effect of Earth-Moon orbital motion creates a second, smaller bulge on the opposite side of the planet.

As Earth rotates, coastal locations pass through these two bulges — experiencing two high tides — and through the low points between them — experiencing two low tides — over the course of roughly 24 hours and 50 minutes.

The Sun also exerts a gravitational pull on Earth's oceans, about 46% as strong as the Moon's. When the Sun and Moon are aligned (new moon and full moon), their forces combine to create spring tides with greater tidal range. When they are at right angles (quarter moons), they partially cancel each other, producing neap tides with smaller range.

Spring Tides vs Neap Tides

The monthly lunar cycle produces a regular alternation between spring tides and neap tides.

Spring tides occur at new moon and full moon, when the Sun, Moon, and Earth are aligned. The combined gravitational pull creates the highest high tides and lowest low tides of the month — the greatest tidal range. Despite the name, spring tides have nothing to do with the season; they occur every two weeks year-round.

Neap tides occur at first and third quarter moon, when the Sun and Moon pull at right angles. Their forces partially cancel, producing tides with a smaller range — higher low tides and lower high tides. Currents are weaker, and the difference between high and low water is at its monthly minimum.

For practical coastal activities: spring tides offer the most dramatic conditions, the strongest currents, and the lowest low tides that expose the most beach. Neap tides offer more moderate, predictable conditions.

Why Do Tide Times Vary by Location?

While the gravitational forces driving tides are global, the actual tides experienced at any given coastal location are shaped by local geography — the shape of the ocean basin, the depth of water, the orientation of the coastline, and the presence of bays, estuaries, and continental shelves.

These factors mean that tide predictions must be calculated individually for each location. Two ports 50 kilometres apart can have tide times that differ by 2–3 hours. A sheltered bay may experience half the tidal range of the open coast nearby.

This is why TideTimes Global maintains individual predictions for over 1,000 stations worldwide — rather than estimating from regional data, each station's tide times are calculated from that location's specific tidal harmonics.

Tidal Range: From Centimetres to Metres

Tidal range — the difference between high and low water — varies enormously around the world. The Mediterranean Sea has an almost imperceptible tidal range of less than 30 centimetres in most locations. The Bay of Fundy in Canada regularly exceeds 16 metres — the greatest tidal range on Earth.

What creates such variation? The key factor is resonance. Ocean basins have natural oscillation periods, and when the tidal forcing period (roughly 12 hours) matches the basin's natural period, tidal energy is amplified dramatically. The Bay of Fundy's funnel shape creates a resonance that perfectly amplifies Atlantic tidal energy.

For practical purposes, higher tidal range means stronger currents, more exposed beach at low tide, and greater variation in navigable depth.

Diurnal vs Semidiurnal Tides

Most of the world experiences semidiurnal tides: two roughly equal high tides and two roughly equal low tides every 24 hours and 50 minutes. The UK, Australia, and most of the Atlantic coast follow this pattern.

Some regions experience diurnal tides: one high tide and one low tide per day. Parts of the Gulf of Mexico, Southeast Asia, and the western Pacific follow this pattern due to the geometry of their ocean basins.

A third category — mixed semidiurnal tides — features two high and two low tides per day, but with significant inequality between them. The US West Coast, much of the Pacific, and parts of East Asia experience this pattern, where one of the two daily high tides is noticeably higher than the other.

💬 People Also Ask

Why does the Moon cause tides but not other gravitational effects we can feel?

The Moon does exert a gravitational effect on everything on Earth, but the effect is only perceptible in fluids — specifically the ocean — because water can flow freely in response to the force. Solid land also experiences a tidal deformation of a few centimetres, but rock doesn't flow. The oceans, covering 71% of Earth's surface, respond visibly to forces that are imperceptible on solid ground.

Why are there two high tides a day if the Moon only passes once?

The Moon creates two tidal bulges simultaneously: one on the side of Earth facing the Moon (direct gravitational attraction) and one on the opposite side (due to the centrifugal effect of the Earth-Moon system's orbital motion). As Earth rotates, a coastal location passes through both bulges in roughly 24 hours, experiencing two high tides. The timing is 24h 50min rather than exactly 24h because the Moon moves in its orbit while Earth rotates, so Earth must rotate slightly further each day to catch up.

What is the highest tidal range ever recorded?

The Bay of Fundy in Nova Scotia, Canada holds the world record for tidal range, with differences between high and low tide regularly exceeding 16 metres (52 feet). The bay's funnel shape and length create a near-perfect resonance with the North Atlantic tidal cycle, amplifying the tidal range to extraordinary levels.

Can tides be predicted years in advance?

Yes. Because tides are driven by the predictable orbital mechanics of the Moon and Sun, they can be calculated years — even decades — in advance with high accuracy. Tidal prediction involves decomposing the complex tidal pattern at each station into a set of harmonic constituents (sine waves of specific periods), then summing them forward in time. Modern predictions are typically accurate to within a few minutes and centimetres under normal weather conditions.

🧭 Expert Tips

The word 'spring' in spring tides comes from the Old English 'springan' — to leap or surge — not from the season. Spring tides occur every two weeks throughout the year.
If you're ever caught out by an incoming tide, move perpendicular to the water's edge rather than running parallel to it. Headlands and points fill first; low-lying beach sections fill last.
The rule of twelfths is a useful mental model for how fast tides rise and fall: the tide moves 1/12 of its range in the 1st hour, 2/12 in the 2nd, 3/12 in the 3rd, 3/12 in the 4th, 2/12 in the 5th, and 1/12 in the 6th. The middle two hours are always the fastest.
Tide predictions assume average atmospheric pressure. A deep low-pressure system (storm) can raise actual water levels significantly above prediction — an effect called storm surge. In severe weather, always add a safety margin to predicted tide heights.