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Go With the Flow: Telling the Tides Apart

As a diver you probably already know that both tides and currents influence when your dives begin and what happens once you’re underwater.

What causes the tides? How often do they change? What’s the difference between tides and currents? Here’s what to expect from each type.

What’s the difference between tides and currents?

Put very simply, tides are slow, long waves moving from the open ocean to the shore. The sun and moon’s gravitational pull on the earth and its bodies of water creates them. When the highest part of the wave reaches the shore we call it high tide. When the lowest part of the wave comes to shore we call it low tide.

Finally, when the tide moves toward (approaching high tide) or away from (approaching low tide) the shore, it also causes a horizontal flow of water. This is called a tidal current. Usually the tidal current is strongest closer to the time of the high and low tides. Currents are usually weaker when the water has reached either the high or the low tide, before it ‘turns’ to move in the opposite direction. This is called the slack tide.

 What causes them?

What causes the tides? As I mentioned earlier, the sun and moon’s gravitational pull on the water creates them. Although the sun is larger and might pull harder, the moon is closer to Earth and thus affects the tides more. The water on the side of the Earth that’s facing the moon gets pulled towards the moon, creating a bulge of water on that side. A phenomenon called inertia (the tendency for a moving object to keep moving in that direction) creates a water bulge on the opposite side of the Earth. The area with the bulge experiences the high tide, while the area with no bulge experiences low tide.

As the Earth moves around the sun, and the moon around the Earth, the position of the Earth, sun and moon in relation to each other changes constantly. These changes influence the location of the water bulges. As a result of that there is a change in the heights of the tides and consequently a change in the strength of the tidal currents. These changes occur on a daily, and even hourly basis.

Tides caused by the gravitational pull of the sun are called solar tides, while the tides caused by the moon are called lunar tides, which are about twice as big as solar tides. Generally when we speak about tides we are referring to the combination of lunar and solar tides.

Types of tides

When the sun, moon and Earth are in a straight line (during a full or new moon) the sun and moon pull together, creating particularly high tides and particularly low tides. This phenomenon is called spring tide. Because there’s a big change in the height of the tides in a small time, the water moves fast, creating stronger tidal currents.

About a week after the spring tide, the sun and moon are at a right angle to each other, which means each partially cancels out the other’s pull, leading to moderate tides. This phenomenon is called neap tide. Smaller changes in the height of the tides means less water moves during the specific time and thus there are weaker tidal currents.

Both spring tides and neap tides occur twice in each lunar month. The distance between the Earth and the moon and between the Earth and the sun influences the tides, as there is a greater pull when either the sun or moon is closer to the Earth. Once a month the moon moves close enough to Earth to create above-average changes in the tides. Two weeks later the moon is further away from Earth, leading to below-average changes in the tides.

Once a year, on January 2nd,  the Earth is closer to the sun, leading to exaggerated tides, while six months later, on July 2nd, the Earth is the furthest away from the sun, leading to smaller changes in the tides.

Frequency of tides: the lunar day

We are all familiar with the 24-hour solar day, during which the Earth makes one compete rotation. When a specific place on the Earth moves from an exact point under the moon to the same point under the moon it’s called a lunar day, which is 24 hours and 50 minutes long.

During this lunar day the Earth revolves through two tidal bulges and thus most coastal areas experience two high tides and two low tides every 24 hours and 50 minutes. Thus high tides are around 12 hours and 25 minutes apart while it takes about 6 hours and 12.5 minutes to change from high tide to low tide, and the same from low to high tide.

Other influences

A few other things, such as the shoreline, the shape of bays and inlets, and local wind and weather patterns influence the tides besides the sun and the moon.

What does all this mean for divers? If you’re looking for some swift drift diving, book your next dive trip during the full or new moon; if you want to take it easy, plan your trip for the phases in between.