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
www.shc.melb.catholic.edu.au/home/birding/index.html
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