Next message: George Huxtable: "Re: Global oceanic tides,"
In the discussion of gravity effects on the ocean profile George Huxtable
wrote (in response to Sharon Casey):
<..snipped>
" The DYNAMIC effect of other bodies in the universe (effectively just the
" Moon and Sun combined, other planets contributing rather negligibly,
" "space" itself having no effect) is to add a local gravity-gradient which
" tries to pull the ocean surface, as described above, by an ellipsoid,
" pointing in the general direction of that combined force. That ellipsoid
" would shift the water-surface by no more than a foot or so: a very small
" effect compared with the hills and valleys we have been discussing.
" However, that ellipsoid points in a direction in space which changes only
" slowly, shifting only as the Moon and Sun move against the stars. On the
" other hand, the Earth, with its oceans, is spinning with respect to that
" ellipsoid once a day, so with respect to the Earth these tidal forces are
" trying to move an wave-ellipsoid of water, 1 foot or so high and low, right
" around the Earth each day..
<snipped..>
The magnitudes of these theoretical free ocean tides are calculated in the
Admiralty Manual of Tides (1941) as a range of 1.76ft due to the Moon and
0.80ft due to the Sun (each calculated for zero zenith distance and average
lunar parallax).
I find students who have been brought up on the notion that "the tides are
caused by the Moon" find the magnitude of the solar component surprisingly
large. However, picturing these influences coming into and out of alignment
every 14 days or so makes the understanding of spring and neap tides easy to
grasp. And also, if you picture the vectors, why the interval between high
tides alters according to the phase of the Moon.
.. only leaving the question of why there are two tides per day rather than
one to be explained!
Geoff Butt
