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Of moon and tides
On 05/02/2018 11:52, Andy Walker wrote:
On 31/01/18 14:16, N_Cook wrote:
A quirk of celestial mechanics.
The high tides [...].
Nothing directly to do with this [interesting] discussion,
but the BBC's programme on the supermoon was trying to explain what
was meant by full/new/quarter Moon, why some were "super", etc.,
the usual stuff. In the middle of which they told us that when the
Moon was new, its pull reinforced that of the Sun, and we had higher
tides than usual. Nothing said directly, but any normal listener
would have inferred that when it was full, and its pull was opposed
to that of the Sun, tides would be lower. I've heard physicists,
who really should know better, say exactly that on TV.
In trying to explain this to people, they can usually accept
that we get "spring" tides when the Moon-tide and the Sun-tide are
reinforcing each other, and "neap" tides when they oppose. The hard
part is explaining why the Moon-tide bulges both towards and away
from the Moon. You can explain till you're blue in the face that the
Moon's gravity pull is stronger on the side of Earth facing the Moon
and weaker on the side facing away, so the water piles up [a little!]
on both sides, but somehow that gets confused with ellipses with the
Earth at one focus, and/or with the phase of the Moon.
I had one former colleague, a highly intelligent and competent
pure mathematician, who came to me regularly to explain this. "We
did this last year!" "Yes, but I've forgotten, and the children have
asked again, and anyway [famous name] was on TV and his explanation
was different. Surely we get lower high tides at full Moon?" "No,
because [blah]." "No, you've lost me. Are you saying that [name]
was wrong?" "Yes. Let's try again ...."
Brian Cox did an excellent visual-aided correct explanation of why
springs occur at new and full moons, and tidal "bulge" on opposite sides
of the Earth at any one time. A few months back on BBC something,
perhaps on Utube if not replayer.
Something to do with momentum/centrepetal forces I seem to remember
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