Topic 17 Tides. GEOL 2503 Introduction to Oceanography. Tides. Periodic waves caused by the gravitational pull of the moon and sun on the earth Timing set by earth's rotation Long wave lengths—up to 1/2 the circumference of the earth. Major tidal influences on Earth.
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Introduction to Oceanography
Major tidal influences on Earth
The tide-producing force (F) is proportional to the product of the masses (m1 and m2) over the CUBE of the distance (r).
C = Centrifugal Force G = Gravitational Attraction
The opposing gravitational and centrifugal forces create two tidal bulges
Earth’s rotation under the tidal bulge gives the observer two high tides and two low tides each day
Lunar Day (or Tidal Day)= Time for completion of each day’s entire tidal cycle, diurnal or semidiurnal, from high tide to the next day’s high tide. See the observer on the diagram below.
When the moon is closest to Earth (perigee), the tide-producing force is increased by 20%
When the moon is farthest from Earth (apogee), the tide-producing force is decreased by 20%
This drawing is to scale. Note the variation in the distance of the moon from the earth and the distance of the moon above and below the orbital plane.
As the Moon revolves around the Earth, the Earth-Moon system is revolving around the Sun. In a lunar month, the moon passes through a series of “phases” as seen from Earth.
Which tidal records show diurnal tides? Semidiurnal? Semidiurnal mixed?
Note the timing of spring and neap tides and lunar phases.
Courtesy of Dr. Tom Garrison
Amphidromic Point (node)—no change in water level
Cotidal lines—tide occurs at the same time along each line
Corange lines—tides of same amplitude along each circle
A tidal bore is a wall of water that surges upriver with the advancing high tide.