The tides

Fishermen, harbormasters, and coastal dwellers around the world can't help but notice an astronomical phenomenon that affects their region, sometimes dramatically, every day: the ebb and flow of the tides. The Earth's daily rotation, along with the Moon's incremental eastward revolution, produces two high tides and two low tides during each 24-hour period. Because the Moon rises approximately 50 minutes later each day, high tides occur about 50 minutes later on successive days, at intervals of 12 hours and 25 minutes.

Tides are governed by the Moon's gravitational effects on the Earth and its oceans. The Sun, too, has an effect, as does the centrifugal force of Earth's revolution, but it is the Moon that exerts the most influence. Though the Sun is nearly 400 times larger than the Moon, it is also about 400 times further away, which is why both bodies appear to have the same diameter in the sky. Since gravitational attraction varies inversely with the square of the distance between two bodies, the Moon exerts 46 times more gravitational pull than the Sun.

Contrary to what many people think, the Moon does not 'pull' water away from the Earth. Rather, high tides result when the Moon's gravity causes the ocean to flow over the Earth's surface toward the region where the Moon appears directly overhead. However, because of the friction between the ocean and the ocean floor, the impeding effects of land masses, differences in water depth, wind, and Earth's rotation, the water 'heaps up,' not directly below the Moon, but somewhat behind the Moon's zenithal position. Taken together, these circumstances conspire to create oscillations in the ocean surfaces, so that the water over a large area rises and lowers in cycles.

Additionally, as high tide occurs on the Moon side of the Earth, a corresponding high tide takes place on the opposite side of the Earth. This apparent paradox is explained by the fact that the Earth's surface on the lunar side of our planet is much closer to the Moon than its center. Consequently, the Moon's gravity attracts the nearer surface more strongly than its center. At the same time, however, the Moon attracts the center more strongly than the ocean on Earth's far side. Thus, the Moon's gravity pulls the rigid Earth away from the less-rigid ocean on the opposite side, causing a high tide there. It's akin to what happens when you pull a plastic bowl of water toward yourself a little too quickly. The bowl itself flexes slightly from the force of the pull, but the water in the bowl, being less resilient, is more easily displaced in the opposite direction.

On a daily basis, one tidal cycle is very much like another. From its lowest point, the water level rises gradually for about six hours, then lowers again for another six hours until it reaches low tide. When the Moon is new or full, and nearest to the Earth for the month (perigee), exceptionally high tides, called spring tides, occur. They are most noticeable in funnel-shaped bays and estuaries. In places like the Bay of Fundy, between Nova Scotia and New Brunswick, Canada, the tides can rise by as much as 15 meters (50 feet).

When the Moon is at right angles to Earth and the Sun, as when the Moon is at first or last quarter, and is also at its greatest distance from Earth for the month (apogee), the attraction is not as strong and exceptionally low tides, called neap tides, occur.

Exceptionally high spring tides occur when the new or full Moon is at perigee and Earth is also nearest the Sun (perihelion). Under those circumstances, both the Moon and Sun act together to increase the tide levels. With the relevant information, you can predict you own 'super spring tide' event. Times of high and low tide usually can be found in your local newspaper in the weather section. The dates of lunar perigee and apogee for the year, as well as the dates of the phases, can be found in any current almanac. The date of perihelion falls at nearly the same time every year, usually around January 2 or 3. (Aphelion, Earth's greatest distance from the Sun, is around July 4 or 5.)

The more coincident least-lunar perigee, perihelion, and the new Moon are, the greater the height of the spring tide. For example, in the year 2005, the Moon reaches its least perigee distance on January 10, when it will also be at the new phase. Moreover, the Earth-Moon system will be nearest the Sun that year on January 2. With all these events happening within nearly the same time period, the spring tides of January 2005 should be exceptionally high.

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