Surfers for LIFE

adapted from The Hopewell Rocks:

What causes high tides?

Commanded by the gravitational forces of the sun and moon, high tides can rise and fall over 16 meters (50 feet) in extreme circumstances…. These fluctuations are a result of many factors.

The moon’s effect on the tides

Although, because of its size, the sun exerts a gravitational force 180 times stronger than the moon, the moon is much closer, so its force across Earth’s diameter is twice as strong as the sun’s.

As the moon exerts its force on the Earth, both the Earth and its waters respond by accelerating toward the Moon; however, the fluid waters on the side facing the Moon, being closer to the gravitational force, accelerate more and fall ahead of Earth.

Gravitational force of the moon affects the high tides.

Similarly, Earth itself accelerates more than the waters on the far side and falls ahead of those waters. As a result, two aqueous bulges are produced, a larger one on the side of Earth nearest the Moon, and a slightly smaller one on the side furthest from the Moon. These bulges are, in effect, the high tides – each occurring simultaneously on each side of the Earth.

As Earth rotates on its axis beneath these two bulges, the rise and fall of the ocean results. If Earth had no rigidity, the entire planet would flex freely causing the ocean bottoms to rise and fall exponentially with the water. Consequently there would be no tides.

The sun and moon positions affect the height of the tides

The positioning of the sun and the moon also has an effect on the height of the tidal swell. During a new or full moon phase, the sun and moon are in line, pulling on the earth’s surface. This creates a higher level of tide – called the “spring tides” from the Welsh word “springan” meaning ‘to bulge’.

During a half moon phase, when the sun and moon are at right angles to the earth, a lower high tide results. These are called “neap tides” from the Anglo-Saxon term “neafte” meaning “scanty” (taken from 1914 edition of The Admiralty Manual of Navigation).

The moon’s phase has an impact on the height of the tides

Another factor is the elliptical path the moon takes during its revolution around the earth. When it is closer to Earth, it is referred to as a perigee moon. During this perigee moon phase, there can exist a 30-48% greater gravitational pull. Tides during this period would have a greater differential between high and low.

When the moon is furthest away, it’s called an apogee moon and the result is seen in a lower high tide.

To us here on Earth, the moon in perigee appears larger and brighter than an apogee because it is much closer.

How are the Tides Measured?

Tide heights vary with each tide according to various meteorological and astronomical effects.

Most people know that land contours are measured according to mean sea level (average level between high and low tides). This would not be an effective method for calculating water levels, so tide heights are referenced according to Chart Datum. Chart Datum is the lowest mean water level measured over a period of time, for a particular area.

…. The Chart Datum reference point would be “0″. Low tide references with a negative value show a lower than average low tide; this would correspondingly indicate a higher than average high tide.

Why isn’t High Tide at the Same Time Each Day?

The Earth turns on its Axis one full revolution every 24 hours. Meanwhile, the moon, being somewhat slower, takes 28 days to complete its journey around the circumference of the Earth.

For every 24-hour rotation of the earth, the moon moves counter-clockwise only 13 degrees. Translating 13 degrees to a time factor, it becomes 50 minutes. Therefore, it takes 24:50 for the same spot on the Earth to line up once more with the moon.

Twice a day, powered by the moon’s attraction, the water swells towards the coastline, reaches its crest, then begins to recedes once more. The period between high and low water is approximately 6 hours and 13 minutes.

Therefore, if it takes 6:13 for the low water to reach high water, and 6:13 for the high to reach low again, this single cycle takes about 12:25. Two cycles a day, and the total elapsed time is 24:50 minutes.

Therefore, if the tide reached its zenith today at 12:00 p.m., we could return tomorrow and expect to see the high tide at 12:50 p.m.