How The Tides Work

Our activities as sailors are governed by the tides to a greater or lesser extent depending on where we are sailing. So what causes tides?

The gravitational pull of the Moon as the Earth rotates is the primary cause for tides. This causes a hump in the oceans which follows the path of the Moon. The Sun provides additional influence as the planets move in and out of line. In a fortnightly cycle the Sun moves between adding to and opposing the pull of the Moon causing spring and neap tides.

Influences On Tides

The shape of the ocean floor and the coastline influence the timing and range of the tides causing huge differences between locations. Also there are a multitude of variations in the rotations and orbits of the Sun, Earth and Moon over different time scales which have to be taken into account.

Consequently there are variations and anomalies at various places. The Bay of Fundy in Canada has the largest tidal range at 16m. Southhampton in the UK has a double high tide. Some areas only have one high and one low tide each day.

Accuracy Of Tide Prediction

Early tide clocks were synchronised with a full moon and could only provide a rough indication of the state of the tide. The prediction of the tides has grown more sophisticated over the years and uses information gathered from many years of tidal observations and sophisticated measurements of the interactions of the planets. With the use of computers we now have accurate tide predictions at least for the main areas of marine activity.

Australian Tide Tables

Since 2005, The National Tidal Centre has provided the predictions for the Australian National Tide Tables (ANTT) which are very accurate. However the NTC cautions that tides may be affected by by unusually high or low barometric pressure or prolonged strong winds, so it pays to allow a safety margin when navigating in shallow water.

Free Tide Prediction Program

Here is a link to a free tide prediction program for use on PC’s. Please read the authors caution regarding accuracy.