How Tide Predictions Work

Tides look complicated at the shoreline, but the prediction problem starts with a simple idea: gravity gives the ocean small, regular nudges. The shape of each coastline and basin determines how those nudges turn into local water levels.

Gravity gives the ocean a regular push

The Moon and Sun apply small gravitational forces to the ocean. Those forces repeat in familiar rhythms, including the roughly twice-daily pull behind many tides.

A useful way to picture it is a swing: one small push does not look like much, but a push at the right time keeps adding energy.

The Sun and Moon pulling two playground swings with Earth sitting on each swing.

Why both sides feel lighter

For someone directly beneath the Moon:

  • The Moon pulls you upward a little more than it pulls the Earth.
  • Relative to the Earth beneath your feet, you experience a tiny upward acceleration.
  • Your apparent weight decreases slightly.

For someone on the opposite side of the Earth:

  • The Moon pulls the Earth more strongly than it pulls them.
  • The Earth is effectively pulled away from them.
  • They also experience a tiny reduction in weight.

The pushes add up into rising and falling water

Over time, repeated forcing produces a repeating water-height signal. The water rises and falls at the same frequencies that are driving it.

This is why tide prediction is mostly a frequency problem: identify the repeating cycles, then project them forward.

Coastlines and basins resonate differently

The land does not just contain the water; it shapes the response. Bays, shelves, inlets and ocean basins can resonate at some tidal frequencies and dampen others.

That is why two places at the same latitude can have very different tide ranges and timing.

Measurements reveal the local recipe

By sampling water height over time, harmonic analysis estimates the amplitude and phase of each tidal frequency at that location.

Once those local constituents are known, future tides can be predicted by adding the waves back together.

Safety note: Tide predictions do not include weather, river flow, storm surge or seismic activity. They should not be relied on as the only source for navigation or other critical safety decisions.