How Do Tides Work? The Moon, Sun, and Moving Oceans

The Moon pulls gravitationally on every part of Earth, but the pull is slightly stronger on the side closest to the Moon and slightly weaker on the far side. Those differences create tidal forces. Ocean water responds by forming broad tidal bulges relative to Earth's centre. As Earth rotates through this pattern, many coastlines pass through regions of higher and lower water level.

This is more accurate than saying the Moon simply pulls one pile of water toward itself. The entire Earth-Moon system is involved, and real oceans are broken up by continents and shaped by seafloor depth. Even so, the difference in gravitational pull is the key to how tides work at a basic level.

The Sun is much more massive than the Moon, but it is also much farther away. Its tidal effect is therefore smaller than the Moon's, yet still strong enough to change the tidal range. When the Sun, Earth, and Moon are roughly aligned around new moon and full moon, their tidal effects reinforce each other. These are spring tides, which usually have a larger difference between high and low water.

When the Sun and Moon are roughly at right angles as seen from Earth, their tidal effects partly oppose each other. These are neap tides, which usually have a smaller tidal range. Understanding spring and neap tides adds an important layer to how tides work because the height difference changes through the lunar month rather than staying constant.

The timing is slightly later from one day to the next because the Moon is moving around Earth while Earth rotates. A location has to rotate a little farther to line up with a similar Moon-related tidal position, so many tide cycles follow a lunar day of about 24 hours and 50 minutes rather than an ordinary 24-hour day. Local basin effects can shift that pattern further, which is another reason printed or digital tide predictions are location specific.

Local geography strongly changes the simple global pattern:

• •The Moon provides the strongest tidal influence on Earth's oceans.
• •The Sun changes tidal range as its position relative to the Moon changes.
• •Spring tides have larger ranges and occur near new and full moon.
• •Neap tides have smaller ranges and occur near first and third quarter moon.
• •Coastlines and seafloor shape change the timing and height of local tides.

A bay can funnel water and increase the tidal range, while a narrow channel can delay or speed local currents. Continents block the idealised bulges, and ocean basins have their own patterns of oscillation. That is why some coasts experience roughly two high tides each day, others have one dominant high tide, and many have mixed patterns. To use how tides work in real life, you need a local tide table because coastline shape and ocean depth matter.

Tides do not only move water; they also move energy. As Earth spins, friction between the tidal bulges and the seafloor, especially in shallow seas, acts like a gentle brake on the planet's rotation. The effect is tiny on a human timescale, adding roughly two thousandths of a second to the length of the day each century, but it builds over geological time. Fossil corals from hundreds of millions of years ago show growth bands suggesting there were around 400 days in a year then, because each day was shorter.

The same interaction is pushing the Moon away. The bulges raised on Earth are dragged slightly ahead of the Moon by Earth's faster spin, and their gravity tugs the Moon forward along its orbit, which makes it spiral slowly outward. Scientists measure this directly by bouncing laser beams off reflectors left on the lunar surface by Apollo astronauts. The Moon is receding by about 3.8 centimetres a year, about as fast as your fingernails grow. Earth's gravity long ago did something similar to the Moon, which is why it always shows us the same face.

Because Earth rotates and coastlines block the water, the tide in an ocean basin does not simply rise and fall in place. It travels around the basin as a huge, slow wave, circling a point where the tidal range is almost zero, called an amphidromic point. Lines joining places that have high tide at the same moment radiate from that point like the spokes of a wheel, and the range generally grows with distance from it. The North Sea has three such systems, with the tide sweeping anticlockwise around each, which helps explain why tide times vary so much along its coasts.

Some places have enormous tides because of resonance. Every enclosed body of water has a natural period at which it sloshes back and forth, like water in a bath. The Bay of Fundy in eastern Canada has a natural period close to the twelve hours and twenty five minutes between successive lunar high tides, so each tide pushes the water in step with its own motion, much like well-timed pushes on a swing. The result is a tidal range that can reach around 16 metres, the largest in the world, where the open ocean nearby rises and falls far less.

Tide tables are not calculated from the Moon's gravity directly. Instead, scientists record sea level at a port for a long period, ideally a year or more, and break the record into a set of regular cycles called harmonic constituents. Each comes from a particular astronomical rhythm. The largest is usually M2, the main lunar tide, repeating every 12 hours and 25 minutes. Another, S2, follows the Sun every 12 hours, and others capture effects such as the Moon's changing distance and its position north or south of the equator. Each port has its own mix of strengths and delays.

Once those constituents are known, predicting the tide means adding the cycles back together for any future date. In the 1870s Lord Kelvin built a mechanical tide-predicting machine that did this with pulleys and a single wire, each pulley turning at the rate of one constituent, so the wire traced a year of tides in a few hours. Machines descended from his design were used well into the twentieth century, including by Arthur Doodson in Liverpool to predict tides for the D-Day landings in 1944, before electronic computers took over the job.

Differences in the Moon's gravity across Earth create the main tidal forces, while the Sun strengthens or weakens the pattern depending on alignment. Earth's rotation and the shape of ocean basins turn those forces into local cycles of high and low water. Think global force first, local coastline second, and tides become much easier to understand.