know to check the tide tables before going fishing, sailing,
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
a California-based publication featuring daily tidal
information and astronomical data.
For more information, visit
"Second Day of Creation"
--- M. C. Escher
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
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
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.
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.)
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
and we'll wave when we see you at the Beach!
Walter & Nancy Spille
weather information, go to the website
*Tidelog is a registered trademark,
graphics and original compilation
2004 Mark Alan Born.
Pacific Publishers, Box 480, Bolinas CA 94924