Introduction

An ocean tide refers to the cyclic rise and fall of seawater. Tides are caused by slight variations in gravitational attraction between the Earth and the moon and the Sun in geometric relationship with locations on the Earth's surface. Tides are periodic primarily because of the cyclical influence of the Earth's rotation.

The moon is the primary factor controlling the temporal rhythm and height of tides ( Figure 8r-1 ). The moon produces two tidal bulges somewhere on the Earth through the effects of gravitational attraction. The height of these tidal bulges is controlled by the moon's gravitational force and the Earth's gravity pulling the water back toward the Earth. At the location on the Earth closest to the moon, seawater is drawn toward the moon because of the greater strength of gravitational attraction. On the opposite side of the Earth, another tidal bulge is produced away from the moon. However, this bulge is due to the fact that at this point on the Earth the force of the moon's gravity is at its weakest. Considering this information, any given point on the Earth's surface should experience two tidal crests and two tidal troughs during each tidal period.

Figure 8r-1 : The moon's gravitational pull is the primary force responsible for the tides on the Earth. Photo taken by the Galileo spacecraft from a distance of about 6.2 million kilometers from Earth, on December 16, 1992. (Source: NASA).

The timing of tidal events is related to the Earth's rotation and the revolution of the moon around the Earth. If the moon was stationary in space, the tidal cycle would be 24 hours long. However, the moon is in motion revolving around the Earth. One revolution takes about 27 days and adds about 50 minutes to the tidal cycle. As a result, the tidal period is 24 hours and 50 minutes in length.

The second factor controlling tides on the Earth's surface is the Sun's gravity. The height of the average solar tide is about 50% the average lunar tide. At certain times during the moon's revolution around the Earth, the direction of its gravitational attraction is aligned with the Sun's (Figure 8r-2). During these times the two tide producing bodies act together to create the highest and lowest tides of the year. These spring tides occur every 14-15 days during full and new moons.

Figure 8r-2 : Forces involved in the formation of a spring tide.

When the gravitational pull of the moon and Sun are at right angles to each other, the daily tidal variations on the Earth are at their least (Figure 8r-3). These events are called neap tides and they occur during the first and last quarter of the moon.

Figure 8r-3 : Forces involved in the formation of a neap tide.

Types of Tides

The geometric relationship of moon and Sun to locations on the Earth's surface results in creation of three different types of tides. In parts of the northern Gulf of Mexico and Southeast Asia, tides have one high and one low water per tidal day (Figure 8r-4). These tides are called diurnal tides.

Figure 8r-4 : Cyclical tidal cycles associated with a diurnal tide.

Semi-diurnal tides have two high and two low waters per tidal day (Figure 8r-5). They are common on the Atlantic coasts of the United States and Europe.

Figure 8r-5 : Cyclical tidal cycles associated with a semi-diurnal tide.

Many parts of the world experience mixed tides where successive high-water and low-water stands differ appreciably (Figure 8r-6). In these tides, we have a higher high water and lower high water as well as higher low water and lower low water. The tides around west coast of Canada and the United States are of this type.

Figure 8r-6 : Cyclical tidal cycles associated with a mixed tide.

The map in Figure 8r-7 shows the geographic distribution of these three tide types on the Earth.