g'Day Irene and others.
Tidal movement in some areas are not quite as straight forward as the
theoretical model suggests. Now some might argue this
subject is off topic. But if one relates tidal movement to wader watching
along the Kimberley coast and then considers what
might happen to the observer if a 8 m. tide comes in when you expect it to ebb,
it may still be worth reading about tides, that
is unless one is a strong swimmer or prefers to hunt crocodiles rather than
birds.
At Cape Londonderry, the most northern point of mainland Western Australia,
when the tide is near high on the eastern side of the
Cape, only 25 km to the west at Cape Talbot, it is near low tide. The actual
difference is about 4 hours and this tidal
variation in such close proximity generates notoriously strong currents and
rough seas around this Cape, especially when wind is
blowing against the tide. (compare tide tables for Leseuer Island with Geranium
Harbour)
When flying over Cape Londonderry at spring tide peaks, this phenomenon is
easily observed. Large areas of exposed sand flats
are visible around the Drysdale River estuary near low tide when, in the same
field of vision, water on the west side of the Cape
can be seen lapping the high water line on the sandstone rocks
Another example of opposite tides occurring simultaneously within the same
region is found either side of the Joseph Bonaparte
Gulf; comparing Darwin @ S 12.28', E 130.51' (Time Zone -09.30 within Timor sea
influence) with Cape Voltaire (Krait Bay) @ S
14.15', E 125.36' (Time Zone -08.00 within Indian Ocean influence).
Why do we observe two opposite tides in the same region, when the model
suggests that gravitational and centrifugal influences on
tidal movement in any region should be uniform and result in mainly two bulges
of water being high tides that follow the moon
across the surface of our planet.
Regards
Ian May
PO Box 666
Enfield Plaza SA 5085
Tel: (08) 8380 9553
Mob: 0409 474 575
Irene wrote:
> Hi everyone
>
> A very good book I have (The Environment, by Chris Park) states the following
> (extracts only):
>
> "Lunar Tides
> Whilst the Moon is only a quarter of the size of the Earth and has a gravity
> a sixth that of the Earth's, it still exerts
> gravitational attraction on the surface of the Earth. As a result when the
> Moon is directly over a particular point on the
> Earth's surface, it exerts a pull on the water. The pull is powerful enough
> to raise the water above its normal level,
> creating a high tide. This is called a direct tide.
>
> "Water on the opposite side of the Earth, furthest away from the Moon, is
> also subject to the gravitational pull. This
> creates a second high tide, called an opposite tide. As the water is raised
> to create these two high tides, it is lowered
> around the circumference of the Earth at right angles to the tidal axis, to
> create phases of low tide.
>
> Solar Tides
> "Like the Moon, the Sun also exerts a gravitational attraction on the Earth
> and its waters. This also gives rise to two high
> tides on opposite sides of the Earth.
>
> "The forces exerted by the Sun and the Moon sometimes complement one another,
> and sometimes they partly counteract one
> another, because the relative positions of the Sun, Moon and Earth vary
> through the year." (and the book explains about neap
> tides and spring tides)
>
> The Moon
> "each orbit (around the Earth) takes 27 days 7 hours and 43 minutes.
> A lunar day lasts 24 hours 50 minutes and 28 seconds - and this means that
> high and low tides are experienced nearly an hour
> later each day".
>
> Earth's Rotation
> "Each rotation of the Earth around its axis takes about 24 hours (23 hours 56
> minutes 4.1 seconds to be precise)."
>
> With some good diagrams in the book, it's pretty easy to understand why we
> get two high tides and 2 low tides about every 24
> hours.
>
> Cheers and Happy Times with Nature
>
> Irene Denton
> Concord West, 12 km from Sydney city, NSW Australia
> 33°50.278'S 151°05.406'E
>
> -----Original Message-----
> Andrew Taylor wrote:
>
> But can you explain how there can be two high tides a day in many places,
> including much of
> Australia's coast? Few people seem to know and incorrect explanations are
> common, even in books. [ If you want to know, do
> a web
> search for barycenter + tide ]
>
> Andrew Taylor
>
> Birding-Aus is on the Web at
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