Re: RE: Tides: Why do seas 'bulge' on the side of earth away from the moon?

Hi Lou!



Tides are strange things, and not at all easy to explain! You must remember that 
the earth is in free fall around the sun, just like the moon or 
satellites are around the earth. We are always falling down towards the sun, 
except that we're moving so fast "sideways" (tangentially) that we go around
instead of straight down. If you think of the earth as somehow 'nailed down'
then you get the wrong idea about tides. They arise from the difference
in the strength of 
gravity at the 'front' and 'back' of the earth. Although it's only a slight 
difference because we're so far away from the sun, the liquid seas get stretched
gently into a new shape. The solid earth finds it a lot harder to change shape, 
so the effect is to make the water deeper along the line from the sun to the 
earth, and appear shallower around the middle.



Did you know that the tidal force on YOU if you went too near to a black 
hole would stretch you like spaghetti?

Tidal forces slow down the earth's spin. The solid earth gets stretched a 
little, and the position of the bulges changes as the earth spins round. This
squashing and stretching makes the rock warm up, turning the rotational 'spin'
energy of the earth into heat. It's only a small effect, though. Our day is
a few minutes longer than it was when the dinosaurs were around!

The moon also suffers from tides (not water ones, though!). The reason that
we only never see the other side of the moon is that tidal forces have been 
grinding away at it, slowing it down to a dead stop. It now just spins once
in its orbit, which means that the tidal force always acts along the same 
line and there is no more tidal grinding.



Hope this helps.


Good question!

Best Wishes,

Max Sang

(CERN, Geneva)


Added by the moderator to address the specific questions raised by Lou:



Indeed, one tide gets raised because the part of the Earth closest to the 
Sun feels a stronger gravitational attraction than the rest of 
the Earth. Sea water is gathered up and pulled towards the direction of the
Sun. On the other hand, the "backside" of the Earth, away from
the direction of the Sun feels a reduced gravity. As a result
sea water has tendency to escape the attraction of the Sun and
therefore gathers up in a tide on the backside of the Earth.



Although the use of the words "centrifugal force" tends to make physicists
uneasy, it can be a useful concept to figure out what is going on. In effect,
the Earth has a circular orbital velocity around the Sun which leads to 
a centrifugal force which exactly balances the gravitational attraction 
from the Sun at the center of the Earth. Regions of the Earth closer to the
Sun feel more gravitational attraction which is insufficiently compensated by
the "centrifugal force" from the orbit. Regions of the Earth further from the
Sun feel less gravitational attraction which is overcompensated by the orbital
"centrifugal force".



Marc Herant
Astronomy Mad Scientist Moderator