🚀 go-pugleaf

mayor@ee.umanitoba.ca (Cam "You didn't see me!" Mayor) writes:

[ Reformatted to < 80 columns; Tsk! ]

> During the new moon, the moon is on the same side of the earth as the
> sun.  During full moon, the moon is on the OPPOSITE side of the
> earth as the sun.  So shouldn 't the gravitational tugs oppose each
> other during full moon and reinforce each other during the new moon
> only?
>
> I think so.  Maybe I'm wrong.  Think about it.

My understanding is that tidal forces are a second order effect.
Ignore the Sun for the moment and consider the Earth as a sphere and
the moon as a point mass located (of course) at its centre of
gravity.

Although the Earth will indeed experience the pull of the moon, the
earth/moon are in free fall about each other and, like astronauts in
orbit, don't actually "feel" any force.

However, since gravity weakens as you get further away, the force at
the side of the earth farthest away from the moon will be slightly
less than the force on the side close to the moon and this will be
felt as a force acting as if to "stretch" the earth along the
earth-moon axis.

Meanwhile, on the sides perpendicular to the direction of the moon,
there will be a slight inward force since the moon is not at infinity
and so the direction of the force is not quite parallel to the line
joining the *centre* of the earth to the moon (it's a point,
remember?) This force can be resolved into a large component parallel
to the aforementioned axis and a small inward component.

So the presence of the moon causes the earth to be both stretched at
the ends and squashed in the middle.

Needless to say, these forces are negligable on the scale of a human
being but under extreme conditions can be quite powerful. If you
jumped into a black hole you would be torn to shreds by the tidal
forces, in most cases before you got anywhere near the event horison.

--
Tony Lezard IS tony@mantis.co.uk OR tony%mantis.co.uk@uknet.ac.uk OR things
like tony%uk.co.mantis@uk.ac.nsfnet-relay OR (last resort) arl10@phx.cam.ac.uk
      "The two most common things on Earth are hydrogen and stupidity."
              -- Geoff Miller (geoffm@purplehaze.Corp.Sun.COM)