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In the 17th century, English physicist and mathematician Isaac Newton described the tidal effects that the moon and the sun have on the Earth. Newton explained that the gravitational attraction of the moon, and to a much lesser extent, the sun, causes water swelling, or tides on the opposing surfaces of the earth. The opposing tides are different because the distance between their location and the moon are different.

Newton’s law of universal gravitation states that the force of the gravitational attraction between two celestial bodies is proportional to their masses and inversely proportional to the square of the distance between them. This is why the sun has a lesser effect on Earth’s tides than the moon; it is 149,785,000 km (93,072,084 miles) away as compared to the moon, which is only 384,835 km (239,125 miles) away. Based on the sun and moon’s masses and distances from earth, we find that the sun has approximately 46% of the tide-generating force that the moon does.

The sun and moon exert their effects on the entire composition of the Earth: the water, air, ground , trees, rocks, etc. The effects on everything but the Earth’s large bodies of water are so small that they are only discernible using sensitive instruments.

In the oceans, seas, and large lakes and rivers the force of the gravitational attraction appears as tides. Every 24 hours and 50 minutes, any exact, given point on Earth rotates under the moon. The inertia caused by the rotation of the Earth also has an effect on tides. In fact, it is this inertia that causes the tidal bulge on the side of the Earth that is opposite of the moon, also called the far side. The gravitational force of the moon is less on the far side, and the inertia of the Earth’s rotation causes the water to continue moving in a straight line, away from the earth, thus, causing the tide on the far side.

This means that every 12 hours and 25 minutes that location will experience a tidal bulge. This is also why coastal areas experience two high tides every day. Depending on the location on Earth, the angle between it and the moon changes, which directly affects tidal strength at that point.

Six hours and 12.5 minutes after high tide, the moon has moved to a point where it has the least amount of impact on the given location, which is referred to low tide. The sun and the moon are aligned with each other when a new moon or full moon are in line with a given location on Earth. When this happens, the sun and moon’s forces of attraction are combined, producing the maximum tidal effect at that location. This is referred to as spring tide.

The variance in the height of the water changes due to tides at any given location are caused by multiple contributing factors including the depth of the water during slack time, the physical features at, and near the location, and the place itself. For example, in the Mediterranean Sea, the tides generally have a 50 cm / 20 in variation, while in the Adriatic Sea; the variation can be up to twice that. There are places on Earth where the tides change significantly. For instance, in the Bay of Fundy, Canada and at the mouth of the Rio Gallegos in Argentina, the maximum change can be as much as 18-20 meters / 60-65 feet.

When high tide is at its maximum, and low tide is at its minimum, there is approximately a two-hour period in which there is little to no tidal movement. This is referred to as slack tide. Slack tide is the ideal time to perform a dive in regard to movement and visibility.

Based on all of the variables we have mentioned, as well as many we have not, you can imagine that guessing what is going to happen with the tides at any given location is very difficult. However, because all of the variables are well defined, we are able to calculate the time of high and low tide, and therefore slack tide at any given location. This information is available from tidal tables that are offered at local dive shops and online. This information is very valuable when planning your dive. When planning, make sure you reference the tidal tables for the location in which you will be diving.

When water masses move towards shore due to tidal currents, the tide is referred to as flowing. As the tide changes from high to low, the water moves away from the shore. This is referred to as ebbing. The currents created by flowing and ebbing tides can be extremely powerful in narrow passes. The Great Barrier Reef has many examples of these passes. The effect can be compared to a full, open water bottle that is forcefully squeezed. The massive body of water is forced through a small opening, which causes very fast, turbulent water motion on the other side of the opening. The Strait of Messina in Italy, which is formed by the proximity of the coasts of Sicily and Calabria is also a good example of the type of bottleneck that can cause powerful ebb and flow.


Advanced Open Water Diver

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