Jersey How the tides work
Tides are the regular rise and fall of the sea, caused mainly by the Moon’s gravity, with the Sun playing a smaller part. This guide explains why Jersey and most other coasts have two high tides a day, why spring tides are bigger than neap tides, and why tidal ranges vary so much from place to place.
In a simple model, the Moon’s gravity produces two broad areas of higher water on opposite sides of the Earth. As the Earth turns, this helps explain why Jersey and most other coasts have two high tides and two low tides each lunar day. The Sun changes the tidal range through the month, while ocean basins, coastlines and the seabed shape the timing and size of the tides at each place.
The Moon makes the tides
How the Moon creates a stretching effect, and how the simple two-bulge model helps explain two high tides a day.
The Moon’s gravity pulls on the whole Earth, but not quite equally. Its pull is strongest on the side nearest the Moon and weakest on the far side. This small difference creates a stretching effect across the Earth and its oceans. In a simple model, this produces two areas of higher water: one facing the Moon, and one on the opposite side.
Press play and watch a coast pass through both bulges and get two high tides each turn. Simplified and not to scale; real oceans do not form two neat, fixed bulges.
The near-side bulge
The Moon pulls the nearest ocean slightly more strongly than it pulls the centre of the Earth. This creates the near-side bulge in the simple model.
The far-side bulge
On the opposite side, the Moon pulls the centre of the Earth more strongly than it pulls the more distant ocean. That difference creates the far-side bulge in the simple model. It is not the Moon pushing water away.
Why there are two high tides
In the simple model, a point on the turning Earth passes through both areas of higher water. This helps explain why Jersey and most other coasts have roughly two high tides and two low tides during each lunar day of about 24 hours and 50 minutes, and why a similar tide is often around 50 minutes later the following day, although the actual difference varies.
Real oceans are more complicated. Continents divide them into separate basins, and tides travel through those basins as extremely long waves. Coastlines, water depth and the shape of the seabed affect the exact time and height of the tide, so some places have two unequal high tides and a few have only one a day.
Why the tidal range changes
How the changing positions of the Sun and Moon affect the tidal range through the month.
The Sun makes tides too, though its effect is only about half as strong as the Moon’s. Around new and full Moon, the Sun, Earth and Moon line up, and their effects work together to make a larger difference between high and low water. These are spring tides, with higher high waters and lower low waters. The name has nothing to do with the season.
Around the first and last quarter Moon the Sun and Moon sit at roughly right angles as seen from Earth, and their effects partly offset each other, making a smaller difference between high and low water. These are neap tides. The two alternate roughly every seven days, with about two weeks from one spring period to the next.
Move the Moon around the Earth in the diagram below to see how its position and phase change the tidal range.
The biggest and smallest local ranges often arrive a little after the exact Moon phase, because the oceans take time to respond.
Why some coasts have much bigger tides
The Moon and Sun start the tide, but local geography decides how large it becomes.
Tidal range is the difference in height between high water and low water. The shape of the coastline, the depth of the water and the shape of the seabed can all change it. Narrow or shallow bays and estuaries can make the rise and fall larger. Where a bay or sea basin has a natural rhythm close to the rhythm of the incoming tide, each movement reinforces the next and the range grows larger. This is called resonance.
Local waters shape the size of the tide
As the ocean tide travels into shallower coastal seas, the shape and depth of the sea basin, together with resonance, can amplify the difference between high and low water. Around Jersey this produces a huge tidal range, up to ~12 m on the biggest spring tides.
- Bay of Fundyup to about 16 m
- Severn Estuaryup to about 14 m
- Jerseyup to about 12 m
Even nearby places can have high and low water at different times and heights. Tide predictions are produced for a named port or location, so always check which place they apply to. You can see the figures on the tide table, and there is more on staying safe around big tides in our sea safety guide.
Estimating the rise and fall
A rough way to picture how the water level changes between low and high tide.
The sea level does not rise or fall at a steady rate. The rule of twelfths is a traditional way to make a rough estimate: divide the time between low and high water into six equal parts, then estimate the change during each part as a fraction of the total tidal range.
When the rise takes close to six hours, each part is roughly one hour. In this simplified pattern, the water level changes least near high and low water and most quickly around the middle of the rise or fall. That faster middle period matters particularly on a rising tide: beaches, rocks and causeways can be covered much faster than the slow rise just after low water might suggest.
Suppose low water is 1.5 m and high water is 9.5 m, giving a tidal range of 8 m. During the third part of the rise, the water level increases by an estimated 3/12 of that range, or about 2 m. If the full rise takes close to six hours, that part will be roughly one hour long.
The rule is only a rough guide. Many local tides do not follow it closely, and it tells you nothing about currents, waves, wind or weather. Do not use it to decide when a beach, reef or causeway is safe to cross. Use the published tide times and local guidance for that spot.
Common questions
Why are there two high tides a day?
The Moon’s gravity creates a stretching effect across the Earth and its oceans. In a simple model, this produces two areas of higher water on opposite sides of the Earth. In that model, a point on the turning Earth moves through both areas, which is why many coasts have roughly two high tides and two low tides during each lunar day. A lunar day lasts about 24 hours and 50 minutes, which is why a similar tide is often around 50 minutes later the following day, though the exact difference varies. Real tides travel through ocean basins as waves and are shaped by coastlines and the seabed, so some places have two unequal high tides, while a few have only one high and one low tide during a lunar day.
What is the difference between spring and neap tides?
Spring and neap tides describe changes in the tidal range, which is the difference in height between high and low water. Spring tides have a larger range, with higher high waters and lower low waters; they happen around new Moon and full Moon, when the effects of the Sun and Moon work together. Neap tides have a smaller range and happen around the first and last quarter Moon, when the effects of the Sun and Moon partly offset each other. Spring and neap periods alternate roughly every seven days, with about two weeks from one spring period to the next.
Why are tides bigger in some places than others?
The size of the tide depends on the shape of the coastline, the depth of the water and the shape of the seabed. Narrow or shallow bays and estuaries can make the tidal range larger. Resonance can increase it further where the natural rhythm of a bay or sea basin is close to the rhythm of the incoming tide. The range can reach about 16 metres in parts of the Bay of Fundy and around 14 metres in parts of the Severn Estuary. Jersey’s range can approach 12 metres during the largest spring tides.
What is the rule of twelfths?
The rule of twelfths is a rough way to estimate how the water level changes between low and high water. Divide the time between them into six equal parts. The level is estimated to change by 1/12 of the range in the first part, followed by 2/12, 3/12, 3/12, 2/12 and 1/12. In the simplified pattern used by the rule, the water level changes most quickly during the middle of the rise or fall. It is only a rough guide and does not work reliably everywhere. It also tells you nothing about tidal currents, waves, wind or weather. Do not use it to decide when a beach, reef or causeway will be safe to cross. Use the tide table and guidance for that particular location.
This page is a simple introduction to tides. Real tides are more complicated and vary from place to place. Tide predictions on this site use ADMIRALTY® Tidal Data for St Helier. For predicted times and heights see the tide table; for guidance around the water see our sea safety guidance.