FIGURE 19.23  The gravitational attraction of the Moon produces tidal bulges at two locations on the Earth’s surface. These bulges form because the intensity of the gravitational attraction declines with increasing distance from the source. One of the bulges is centered at the surface location closest to the Moon (labeled A). At this point on the Earth’s surface, the Moon’s gravitational attraction is at its strongest. We can find the second tidal bulge on the opposite side of the Earth (labeled B). At this point on the Earth’s surface, the Moon is at its greatest distance, and gravitational attraction is at its weakest intensity. Image Copyright: Michael Pidwirny.

FIGURE 19.24  The relative position of the Moon, Sun, and Earth during a spring tide. The fact that the Moon’s and Sun’s gravitational forces align creates the highest high and lowest low tides. Image Copyright: Michael Pidwirny.

FIGURE 4  The relative position of the Moon, Sun, and Earth during a neap tide. The fact that the Moon's and Sun’s gravitational forces are at right angles to each other produces the lowest high and highest low tides. Image Copyright: Michael Pidwirny.

            A tide refers to the cyclic rise and fall ofsea levelin our oceans. A high tide refers to the time when a tide reaches its maximum water level (Figure 19.22). The minimum water level is called a low tide. The period between a high and low tide is known as an ebb tide and is characterized by a falling sea level. A flood tide is the time between a low and high tide when the sea level rises. In estuaries and bays, the changing height of the ocean because of tidal forces often produces horizontal flows of seawater known as tidal currents. In addition to moving seawater, tidal currents can also transport significant amounts of sediment. Two types of tidal currents are recognized. Falling or ebb tides produce an ebb current that generally moves seawater from the nearshore out to sea. Flood currents occur withfloodorrising tides, creating a flow of seawater that moves landward.

            Tides are caused by minor variations in the gravitational attraction between the Earth, the Moon, and the Sun, as they are in geometric relationships with places on the Earth's surface. Tides are periodic because of the cyclical influence of the Earth's rotation and the orbital movements of the Moon. The Moon is the primary factor controlling the temporal rhythm and height of tides (Figure 19.23). The Moon produces two tidal bulges on Earth through its gravitational attraction. The height of these tidal bulges is controlled by the Moon's gravity and Earth's gravity, which pull the water back toward 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. This bulge is due to the fact that at the opposite point on the Earth, the Moon's gravity is weakest. 

 

            The timing of tidal events is mainly determined by Earth's rotation and the Moon's orbit around Earth. If the Moon were stationary in space, the tidal cycle would be precisely 24 hours long or the same length as one complete rotation of the Earth on its axis. However, the Moon is also in motion, orbiting the Earth. One orbit cycle takes about 27 days and adds about 50 minutes to each tidal cycle. As a result, the tidal period lasts 24 hours and 50 minutes.

            The Sun's gravity is the second factor controlling the Earth's ocean surface tides. The height of the average solar tide is about 50% of 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 gravity (Figure 19.24). During these times, the two bodies act together to create the highest and lowest tides of the year. These spring tides occur during full and new Moons every 14-15 days.

            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 19.25). These events are called neap tides, and they occur during the first and last quarter of the Moon.

Types of Tides

            The geometric relationships between the Moon and the Sun and locations on the Earth's surface create three types of tides (Figure 19.26). In parts of the northern Gulf of Mexico, the Aleutian Islands, and Southeast Asia, tides have one high water and one low water per tidal day (Figure 19.27). These tides are called diurnal tides. Semi-diurnal tides have two high and two low waters per tidal day. They are common on the Atlantic coasts of the United States and Europe. Many parts of the world experience mixed tides where the successive high water and low water stands differ appreciably. 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 the west coast of Canada and the United States are of this type.

FIGURE 19.22  Dock located on the Bay of Fundy, New Brunswick, Canada, at high tide (left) and at low tide (right).  Image Source: Image Left and Image Right, Wikimedia Commons, photo by Samuel Wantman. This image is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

FIGURE 19.26  The repetitive tidal cycles during one tidal period associated with a diurnal tide, semi-diurnal tide, and a mixed tide. Image Copyright: Michael Pidwirny.

FIGURE 19.27  The global distribution of the three tidal types. Most of the world's coastlines have semi-diurnal tides. Image Copyright: Michael Pidwirny.