Many people know to check the tide tables before going fishing, sailing, surfing, beachcombing
or other water-based activities. But how many people know what makes the tides rise and fall?

The following information is reprinted from 2004 TIDELOG (Northern California),
a California-based publication featuring daily tidal information and astronomical data.
For more information, visit www.tidelog.com.

"Second Day of Creation"
--- M. C. Escher
woodcut

   Earth's tides are produced by the gravitational attractions of the moon and sun.  Although the moon is millions of times smaller than the sun, its proximity to the earth gives it a tidal influence more than twice the sun's.  Opposing the direct gravitational forces are centrifugal forces which produce a tidal bulge on the side of the earth away from the moon, resulting in two high tides per day for most locations.

   Twice a month, when the moon is full and when it is new, the moon, sun, and earth are aligned.  The resulting "spring tides" have a greater range, with higher highs and lower lows than ordinarily.  Conversely, when the moon is in quarter phase, the moon and sun tend to counteract, rather than reinforce, each other.  The resulting "neap tides" are much weaker.

   Since the moon's orbit around the earth is elliptical, its distance carries by about 11% during a month.  At perigee, the moon is closest and its tidal influence is increased.  The opposite is true at apogee, the farthest point of its monthly orbit.  Likewise, on an annual rather than monthly cycle, the sun's influence is increased at perihelion and decreased at aphelion.  When a new or full moon coincides with perigee (augmented in winter by perihelion), the result is dramatically increased tidal ranges, called perigean spring tides.

   The moon's declination also affects the tides.  Since its orbit is inclined relative to the earth's equator, the moon appears to cross over the equator twice a month, reaching maximum North declination and maximum South declination about two weeks apart.  Either point a maximum declination tends to encourage inequality between a day's two high (or low) tides.  (This effect is more pronounced on the Pacific coast than on the Atlantic.)

   Planetary tidal influence is negligible.  Since a planet's tidal attraction varies directly with mass and inversely with distance cubed, the planet having the greatest influence on Earth's tides (Venus, the closest) exerts a force only a few thousandths of one percent of that of the moon.
Some geologists theorize that the same forces which cause the highest tides may also trigger earthquakes, with most major quakes occurring near times when a new or full moon closely coincides with perigee.  Here in earthquake country, we've seen this theory hit, and we've seen it miss.  Those who like to see for themselves will be able to identify several such periods in 2004 and form their own conclusions.
 

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Enjoy, and we'll wave when we see you at the Beach!
Walter & Nancy Spille
Celebrations
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For weather information, go to the website www.srh.noaa.gov.