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The Moon's Effect on Ocean Tides

The gravitational pull of the Moon and the Sun makes the water in the oceans bulge, causing a continuous change between high and low tide.

Illustration showing how the gravitational force of the Sun and Moon act together and create spring tides at New Moon and Full Moon.

The oceans bulge. (Not to scale.)

At New Moon and Full Moon the gravitational force of the Sun and the Moon act together and create spring tides. (Not to scale.)

While both the Moon and the Sun influence the ocean tides, the Moon plays the biggest role because it is so much closer to our planet than the Sun.

In fact, the tidal effect of the Moon on Earth is more than twice as strong as that of the Sun, even though the Sun's gravitational pull on Earth is around 178 times stronger than that of the Moon.

Oceans Are Pulled Up

The gravitational force of the Moon and the Sun pulls the water in the oceans upwards making the oceans bulge, which creates high tide in the areas of Earth facing the Moon and on the opposite side. (See illustration.)

At the same time, in other parts of the planet, the ocean water drains away to fill these bulges, creating low tides. However, the oceans' water is also constrained by the continents and varying ocean depths. As a result, the tides behave more like water sloshing around in an oddly shaped bathtub than in a smooth and even basin.

High and Low Nearly Twice a Day

Tides are one of the most reliable phenomena in the world, and we know that they move in and out around twice a day, but not exactly. So, why is that?

A day on Earth is the time it takes our planet to spin once around its own axis and return to the same point under the Sun. This is known as a solar day, and it lasts around 24 hours.

However, the time it takes Earth to reach the same position in relation to the Moon, takes, on average, 24 hours and 50 minutes, known as a lunar day. The reason the lunar day is longer than a solar day is that the Moon revolves around Earth in the same direction as Earth rotates around its axis, so it takes Earth, on average, an additional 50 minutes to “catch up” to the Moon.

Because the tidal force of the Moon is more than twice as strong as the Sun's, the tides follow the lunar day, not the solar day. It takes half a lunar day, on average 12 hours and 25 minutes, from one high tide to the next, so we have high and low tides nearly twice a day.

According to the National Ocean Service, there are some exceptions to the main rule of 2 tides every lunar day: there are a few places, for instance along the coastline of the Gulf of Mexico, where there is only 1 tide per day. This is caused by the local shoreline topography, among other things. This tidal cycle is called a diurnal cycle, as opposed to the normal semidiurnal cycle, where diurnal means daily and semi means half.

Gradual Ebb and Flow

The change from low to high tide is known as flood tide, while the change from high to low tide is called ebb tide. The technical term for the difference in water level between high tide and low tide is tidal range.

The flow and ebb are gradual, so it is not accurate to say that a high or low tide lasts around 6 hours and 12 minutes, i.e. a quarter of a lunar day. The speed of the water flow varies during this period, and it also varies from place to place.

The Rule of 12ths

People who have to take the tides into account in their daily life, like sailors, fishermen, and surfers, often use what is called the rule of 12ths to calculate the expected water level.

Illustration image
The water level changes gradually.
You can calculate the expected water level according to the rule of 12ths.

This rule states that in the 1st hour after low tide the water level will rise by 1/12 of the predicted tidal range in any given area. In the 2nd hour, it will rise 2/12, and in the 3rd hour, it will rise 3/12. In the 4th hour, it will also rise 3/12, in the 5th, it will rise 2/12, and in the 6th hour, it will rise 1/12.

The sequence to remember is 1-2-3-3-2-1.

So, let’s say the predicted tidal range is 12 feet. In the 1st hour, the tide would rise 1 foot. In the 2nd hour, it would rise 2 feet. In the 3rd and 4th hours, it would rise 3 feet. In the 5th hour, the tide would rise 2 feet, and in the 6th hour, 1 foot.

Storm Tides and Surge

The astronomical forces which drive the tides can be predicted very accurately, and these predictions are published in local tidal tables. However, different weather conditions also affect the sea level and may cause both lower and higher tides than expected. If there is a storm, the seawater level often increases. This is called a storm tide and is caused by a combination of storm surge and normal tidal movement.

Strong offshore winds can move water away from coastlines, exaggerating the low tide, while onshore winds may cause the water to pile up onto the shoreline, making the low tide higher than normal.

High pressure weather systems can lead to days with exceptionally low tides, while low pressure systems may contribute to causing much higher tides than predicted.

Geography and Topography Important

The depth and shape of the ocean and the distance between continents are also important in determining the water level along the shores. In the northern parts of North America, Europe, and Asia, the continents are close together, which creates a bigger difference between high and low tides than in areas farther south, where the continents are farther apart.

Average and Extreme Tides

The average tidal range in mid-ocean is around 1 meter or 3 feet. However, in some coastal areas, the tidal range can be more than 10 times higher in the most extreme areas. To give an average for tidal range along the world's coastlines doesn't make much sense, as they vary so much from place to place.

Illustration image
Boats stranded at low tide in Devon, UK.
In Brixham, Devon in the UK, boats can not leave the port at low tide.
©iStockphoto.com/Gary Perkin

The world's highest tide is in the Bay of Fundy in Canada, where the difference between low and high tide can be up to 16.3 meters (53.5 feet). The highest tides in the US can reach 12.2 meters (40 feet) near Anchorage, Alaska. Along the coast of the UK, the tidal range varies from as little as 0.5 meters (1.6 feet) to a maximum of 15 meters (50 feet).

Spring Tides

The Moon’s phase also plays a part in the tidal range. The greatest difference between high and low tide is around New Moon and Full Moon. During these Moon phases, the solar tide coincides with the lunar tide because the Sun and the Moon are aligned with Earth, and their gravitational forces combine to pull the ocean’s water in the same direction. These tides are known as spring tides or king tides. The name has nothing to do with the season spring, but rather it is a synonym for jump or leap.

If a spring tide coincides with either the March equinox or the September equinox, it is called an equinoctial spring tide. At these times we can expect the largest tidal range of the year because, at the equinoxes, the Moon and Sun are aligned with the equator.

Perigean and Apogean Spring Tides

Several times a year, the Full Moon or New Moon happens as the Moon is around its closest point to Earth, called perigee. This is popularly known as a Supermoon and leads to even larger variation between high and low tides, known as perigean spring tides. However, the difference from a normal spring tide is only around 5 cm or 2 inches.

The opposite happens when the Full or New Moon is around its farthest from Earth, apogee, also known as Micromoon. The apogean spring tides are around 5 cm (2 inches) smaller than regular spring tides.

Illustration image
Neap tides at Quarter Moon. (Not to scale.)
Neap tides occur at the 2 Quarter Moons, when the tidal range is smallest. (Not to scale.)

Neap Tides

The tidal range is smallest around the 2 Quarter Moons (Half Moons) because the gravitational force from the Moon and the Sun counteract each other at these 2 points of the lunar month. These tides are called neap tides or neaps, from Anglo-Saxon, meaning without the power. Neaps always occur about 7 days after spring tides.

Oceans and Some Rivers

There is a difference between having noticeable tides and having true tides. For tides to be noticeable, the body of water has to be huge, like an ocean. Even though true tides also occur in smaller water basins, like big lakes, the tidal variations here are too small to notice.

For example, in the Great Lakes in the US, the largest tidal range is less than 5 cm or just under 2 inches. Different weather conditions, such as wind and barometric pressure, creates bigger differences in the water level than tides on these lakes. This is also the case in the Baltic Sea, the Black Sea, the Caspian Sea, and even the Mediterranean.

Many rivers connecting to the ocean do have high and low tides. In some of these tidal rivers, the water drains away almost entirely at low tide, making it possible to walk across the bottom of the river. If a part of a larger river is affected by the tides, the section affected is known as tidal reach.

Illustration image
Low tide in the river Malta, India.
At low tide people walk across the riverbed of the river Malta in Canning Town, India.

In a few areas, where the tide comes into a narrow bay or river, tidal bores can form. Created by the incoming tide, tidal bores are waves which travel against the direction of the water current.

Tides in the Human Body?

Many people believe that the Moon’s gravitational force also affects humans, as our bodies are made up of approximately 70% fluid. However, there is no scientific evidence supporting this belief. The amount of liquid in a human body is far from big enough to experience tides.

Topics: Moon, Earth, Sun, Astronomy

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