🚀 go-pugleaf

To: alt.folklore.science
From: steve.crabtree@synapse.org

mayor@ee.umanitoba.ca writes:

M(> In article 1gigbeINNk69@gap.caltech.edu, carl@SOL1.GPS.CALTECH.EDU
M(> (Carl J Lydick) writes:

M(> >The moon is not the sole source of tides.  The tidal effects of
M(> >the sun are also significant.  During new and full moons, the
M(> >tidal effects of the sun and the moon reinforce each other.
M(> >Hence higher tides during new and full moons, ceteris paribus.

M(> During the new moon, the moon is on the same side of the earth as
M(> the sun.  During full moon, the moon is on the OPPOSITE side of
M(> earth as the sun. So shouldn't the gravitational tugs oppose each
M(> other during full moon and reinforce each other during the new
M(> moon only?

M(> I think so.  Maybe I'm wrong.  Think about it.

The reason I don't think this is so is that there are two raised
humps of water on the earth's surface, one facing the moon (offset by
effects of the earth's rotation) and one on the opposite side. This
gives the normal two high tides per day (roughly).

When I first thought about this, it gave me some trouble until I
realized that the earth-moon system does not revolve around the
earths centre, but around it's common centre of gravity.

This means that water on the far side of the centre of gravity
experiences a centrifugal pseudo-force which causes it to rise up on
the side facing away from the moon.

Similarly, the earth-sun system revolves around a common centre of
gravity which similarly creates a hump on the far side of the earth in
addition to a hump caused by the differential gravitational force on
the side facing the sun.

Clearly either the "gravitational attraction" humps or the
"centrifugal" humps can add together and therefore the sun and moon
should cause greater tides together when they line up on opposite
sides of the earth or on the same side.

One thing I've never tried to work out is whether the centrifugal hump
and the gravitational hump are the same size, and further, if they
are, whether they have to be or whether it's coincidence. I know from
personal experience (living by the banks of a tidal river) that they
are approximately equal.