How to Explain the Tides

Tides are some of the most obvious movements of the ocean to the human observer.

While often subtle to an aloof observer, tides actually represent some of the most obvious movements of the ocean. They also affect other parts of the planet, like inland lakes, to a lesser extent. Owing their existence to the gravitational influence of the moon and sun, they power ocean currents and shift the appearance of coastlines as water rises and falls with regular patterns. Explaining the tides doesn't only highlight aspects of oceanography and physics it also helps flesh out an understanding of the relationship between the Earth and other planetary objects.

Instructions

1. 

   The moon's ultimate gravitational effect on the Earth is greater than that of the sun, located much farther away.

   Discuss the gravitational relationship of the Earth and the moon, the "pull" of which is the main trigger of the tidal cycles the planet experiences on a daily basis. Basically, two high tides occur on opposite sides of the Earth: one on the side closest to the moon because its gravitational tug pulls ocean water toward it and one on the other side, caused by centrifugal force directed away from the moon and gravitational tug on the planet itself. Low tides occur halfway between these two high tides.

2. Explain the "spring" and "neap" tides, which deal with the interactions of solar and lunar gravitational force upon the Earth. During a spring tide, the sun and the moon are aligned such that their gravitational forces tug on the same plane; this results in the greatest tidal ranges -- the highest high tides and the lowest low tides. A neap tide refers to the phenomenon of sun and moon acting at right angles to one another; then the tidal range is least.

3. Touch on the complexity of tidal behavior by defining a few of the different types of tides. All sorts of factors, not least the contour and extent of a given ocean basin, affect tidal action. A particular coast may experience diurnal tides, with one high and one low tide per tidal day (some 50 minutes more than a 24-hour day); semidiurnal tides, with two each per tidal day; and mixed tides, with the two high and low tides respectively differing from one another in measure.

4. 

   Tides manifest themselves as flood and ebb currents.

   Discuss the actual mechanisms of tides as they appear to a coastal observer. For this you will deal with the "flood current" or flood tide of a rising tide, with waters approaching shore and heading upriver; the "ebb current" or ebb tide of retreating waters; and the "slack water" in between. The high-tide mark comes after the flood current, and the low-tide mark comes after the ebb current.

5. Demonstrate tidal action. You can do this with a diagram -- one showing the Earth with its tidal bulges and lows in relation to the moon's position. You could also show a video of dramatic tides -- like the massive shifts in eastern Canada's Bay of Fundy, which experiences the biggest tidal differences anywhere. Head down to a coastline or estuary if you have that access to show the difference between high and low tides.

References

• "Oceanography"; M. Grant Goss; 1995
• "Physical Geography: A Landscape Apprecation"; Tom L. McKnight; 1999
• NOAA: Our Restless Tides - Detailed Explanation of the Differential Tide Producing Forces

Resources

• "Oceanography: Tides"; J. Floor Anthoni; 2000