3. TIDES

Tides are mainly caused by the gravitational pull of the moon. This attraction-force causes the waters to swell out the moon in a high tide. At the same time, on the far side of the earth, the moon’s attraction is at its weakest and the waters on that side will swell away from the moon in an equal and opposite high tide. Within the 24-hour-period two high tides arise, with two low tides in between them. The sun has a gravitational effect on the earth, too. The effect of the sun’s gravity on the tides is approximately half of that of the moon because the sun is at a far greater distance from the earth. Every other week, at new and full moon, when the sun and the moon are both in line with the earth, the two gravitational forces are added together, and exceptionally strong tides, called Spring-tides, are produced.

In the weeks that lie in between, when the sun and the moon are at 90 degrees to each other, at the first and third quarters of the moon, the pull of the sun will cancel out half of the moon’s gravitational effect, and weak tides, called Neap tides, will occur. This theoretical pattern does not occur in every part of the world. The shape of a continent may prevent water from flowing uninterruptedly across the surface of the planet, and this has complex effects on the regularity of tides. In some places there are four high tides within 24 hours, while only one high tide and one low tide a day in other areas. There are places on the ocean’s surface where all the tidal forces cancel each other out. In restricted areas the tides may be almost imperceptible, while in other areas enormous ranges – differences between high and low tides – may occur.

Tides are measured from chart datum, and levels are always given at average ( mean ) heights. The Lowest Astronomical Tide (LAT) is the lowest tide that has ever been recorded in that area. Highest high tides and lowest low tides are called Spring tides. Highest low tides and lowest high tides are called Neap tides. When the tide is turning we speak of Slack tide. So, at Low Slack the tide is about to rise; at High Slack the tide is about to fall. This latter tide is especially important to the mariner, since it is the best tide to leave and proceed to open sea.