Hi Phil

Keep up the good work! But there is an error on your pages where you
describe how the moon forms two tides. You cannot explain this correctly
without noting that the earth-moon pair rotate about a common point. It
is this rotation that causes the two tides. If you like, you can explain
it as a centrifugal force acting to throw water out while gravity pulls
the water to water the moon. Since the gravity term is weaker on the
outside of the earth the centrifugal term dominates and the result is
two tides.

Many regards

Mark Cannell
Professor,
University of Auckland.