FAQ About Tide Times

What are tide times?

A tide time shows us the specific time during the day when the sea level reaches its highest and lowest point. Caused by the interaction between the Moon, Sun and Earth this movement of water can most clearly be seen as the water comes up a beach on an incoming tide or moves down the beach on a falling tide.

In most places around the world there are two high tides and two low tides each day. The time these occur changes each day, hence the importance of tide time tables.

What causes tides?

The rise and fall of the tides are caused by the gravitational pull of the moon and the sun on the Earth's oceans. The moon's gravity pulls the water towards it, creating a bulge of water - a high tide. 

On the opposite side of the Earth, the moon’s gravitational pull is weakest allowing  to bulge out in the opposite direction. Between these high tides, the water level drops, creating low tides. 

This is why we see the level of the sea rise and fall in a regular pattern every day.

Does the Sun or the moon have greater influence on the tide?

The moon has by far the larger effect on the tides and this is due to its proximity to the Earth. It is gravity which generates the tidal force and this is proportional to the product of the masses of the two bodies but, more importantly, inversely proportional to the distance between them. Therefore, although its mass is much smaller than that of the Sun the moon is far closer to the Earth. 

The tide generating force of the Sun is a little less than half that of the moon - so not insignificant.

How often do tides occur each day?

Most coastal areas experience two high tides and two low tides every 24 hours and this is referred to as a semidiurnal tide cycle. However, some parts of the world may experience diurnal tides (one high and one low tide per day) as in Karumba, Australia, or mixed tides (varying high and low tides).

In some places there is a double-high water (e.g. Southampton, UK) or double-low water. This is caused by the depth of the sea and the shape of the coastline.

How are tide times predicted?

Future tide times are calculated using a combination of historical data, astronomical observations, and complex mathematical models. Data of tide times and heights measured at a location are used to generate what are known as harmonic constants. 

This actual data is used in combination with astronomical predictions for positions of the moon, sun, and the Earth’s rotation. Referred to as harmonic constituents there are nearly 40 of these which are factored into the predictions.

How far ahead can tides be predicted?

It is possible to accurately predict tide times far in advance. This is because the tides occur as a result of the movements of Earth, the moon and the Sun and their relative positions. These celestial relationships are well understood and can be forecast well into the future with a high degree of accuracy. Not just months and years, but centuries or more.

There are other factors that can affect the accuracy of tidal predictions, both near and longer term. Changes to the coastal topography and underwater landscape (bathymetry) will alter the flow of the sea in and out. As will the overall level of the sea, whether this be from global warming or storm surges caused by localized weather.

Generally speaking, tide times can be forecast for many years in advance (or in the past). However, over time the accuracy may decline as the landscape changes.

Why does high tide happen later every day?

In most places there are two high and low tides a day - 1 semi-diurnal tide cycle. You may have noticed however, that the times these tides occur changes, and that it in fact gets later every day.

The reason for this is that the moon rises 54 minutes later every day. As it is the position of the moon and its gravitational pull that creates the tide, this directly affects the time of the tides.

This additional 54 minutes means that a location will experience high tides every 12 hours 27 minutes. There is some variation between different locations as local geography has an impact on tidal dynamics.

What factors can affect tide times?

While the primary factors is the gravitational force of the moon followed by that of the sun, local factors such as wind conditions, atmospheric pressure, and the shape of the coast can also influence tide times and the size of the tide.

Do other planets have any effect on tide times?

In theory, yes. In reality the effect is negligible.

Venus is generally accepted as the closest planet to earth but it has been calculated that its gravitational pull is only 0.005% that of the moon. This translates to around a 0.1 mm effect on tidal height.

What is a spring tide?

A spring tide occurs when the sun, moon, and Earth align, leading to higher high tides and lower low tides. This is because the combined gravitational pull is increased. The alignment happens during full and new moons, so we get spring tides twice a month.

What is a neap tide?

A neap tide occurs when the sun and moon are at right angles to each other causing a smaller gravitational pull. This results in less extreme high and low tides and happens during the first and third quarters of the moon.

What time of year do the biggest tides occur?

The largest tides of the year often coincide with the spring and autumn/fall equinoxes. These extra large spring tides occur just after the full moon closest to these dates (usually around the 21st March and 23rd September respectively). This is due to the sun and moon being aligned, with respect to the earth, and exerting a stronger gravitational pull in combination.

For the biggest possible tide to occur it is necessary for the moon to be as close as possible, i.e. lunar perigee or a supermoon. If this occurs at the same time as the sun and moon are in perfect alignment we will see the largest tides.

Weather conditions can also have an impact on the tide height - either magnifying or reducing it. Low pressure and onshore winds can combine to create storm surges which can increase the high tide hugely.

Where are the highest tides in the world?

The following locations have the six biggest mean tidal ranges around the world.

• Bay of Fundy, Nova Scotia 12.9 m (38.4 ft)
• Ungava Bay, Quebec, Canada 12.5 m (38.1 ft)
• Avonmouth, United Kingdom 12.3 m (31.5 ft)
• Granville, France 11.4 m (28.2 ft)
• Rio Gallegos, Argentina 10.4 m (29 ft)
• Cook Inlet, Alaska, USA 9.2 m (26.2ft)

Are tide times the same everywhere?

No, tide times vary widely depending where in the world you are. Factors such as the shape of the coastline, ocean basin characteristics, and to some extent, local weather conditions all influence tide times.

That said locations on the same longitude will have tide times in the same ballpark as the moon orbits the earth.

Why do tide times differ between nearby locations?

Tide times can differ between nearby locations due to variations in the shape of the coast, underwater topography (bathymetry), and local tidal flow. Even slight differences in geography can result in significant changes in tide times.

Why are tide times important?

Tide times are crucial for various industries and activities, including fishing, boating, surfing, and coastal navigation. Knowing the tide time also help in planning safe and effective outings by indicating when water levels will be favorable.

Are tide charts and tide tables the same?

You will often see these two terms used interchangeably but they are actually quite different. 

Tide charts are actually maps that show the water level in a particular location at a certain time. These are quite a specialized product and mostly used for navigational purposes.

Tide tables on the other hand show predictions for the times and heights of daily low and high tides. These are what you will find on this website and are of the most use to the general public.