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Earth Science for Kids

Ocean Tides

Tides are the rise and fall of the levels of the ocean. They are caused by the gravitational pull of the Sun and Moon as well as the rotation of the Earth.

Cycles of a Tide

Tides cycle as the Moon rotates around the Earth and as the position of the Sun changes. Throughout the day the sea level is constantly rising or falling.

1. The sea level rises
2. High tide is reached
3. The sea level falls
4. Low tide is reached
5. Back to number 1

This cycle can happen once or twice a day depending on the location of the area to the Moon. Tides that happen once a day are called diurnal. Tides that happen twice a day are called semidiurnal. Because the Earth rotates in the same direction as the Moon, the cycle is actually slightly longer than a day at 24 hours and 50 minutes.

Tides and the Moon

While the Sun and the rotation of the Earth both have some tidal impact, the location of the Moon has the biggest affect on the tide. The gravity of the Moon causes a high tide both on the side of the Earth directly below the Moon (sublunar tide) and the opposite side of the Earth (antipodal). Low tides are on the sides of the Earth 90 degrees away from the Moon. See the picture below.

Tidal Currents

When the sea level is rising or falling, water is flowing to or from the ocean. This flow causes currents called tidal currents.

Flood current - A flood current occurs as the sea level is rising towards high tide. Water is flowing towards the shore and away from the ocean.
Ebb current - An ebb current occurs as the sea level is dropping towards low tide. Water is flowing away from the shore and towards the ocean.
Slack water - At the exact time of high tide or low tide there is no current. This time is called slack water.

Tidal Range

The tidal range is the difference in sea level between low tide and high tide. The tidal range will vary in different locations depending on the location of the Sun and the Moon as well as the topography of the shore line.

In the open ocean the tidal range is typically around 2 feet. However, tidal ranges can be much larger near the shore. The largest tidal range is on the coast of the Bay of Fundy in Canada where the tides can change by as much as 40 feet from high to low tide.

Types of Tides

High - High tide is the point in the tidal cycle where the sea level is at its highest.
Low - Low tide is the point in the tidal cycle where the sea level is at its lowest.
Spring - Spring tide occurs when the Sun and the Moon are aligned to combine for the largest tidal range of the highest high tide and the lowest low tide.
Neap - A neap tide is when the tidal range is at its smallest. This occurs during the first and third quarters of the Moon.
Semidiurnal - A semidiurnal tidal cycle is one where there are two high and two low tides each day.
Diurnal - A diurnal tidal cycle is one where there is only one high and one low tide during a day.

Interesting Facts about Tides

The same tidal forces that cause tides in the oceans affect the solid Earth causing it to change shape by a few inches.
There are typically two spring tides and two neap tides each month.
In a semidiurnal cycle the high and low tides occur around 6 hours and 12.5 minutes apart.
Local factors such as weather can also affect the tides.
The energy from tidal forces can be harnessed for electricity using tidal turbines, fences, or barrages.

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