Expert: Tom Whiting - 5/31/2006

Question
could you give me an explanation on why the tides change. thank you


Answer
Hi Brooke,
I think most people know that the moon and its gravity
causes the ocean tides.... but they are only partially correct....
so therefore, wrong....it's actually the "differential gravity"
that produces tidal effects.  (If it were *just* gravity, we'd only have one high tide and one low tide per 24 hour period)...but as you probably know, we have 2 highs and 2 low tides per 24 hour period.

Differential gravity is the pull by the moon on the close side of the Earth, compared to the pull on the distant side of
the Earth from the moon.  This then causes a bulging of the
ocean water on the side of the Earth right underneath the moon, and another bulge on the far side of the Earth.....and as a consequence, two low tide regions at 90 degrees to these
bulges.   How so, the far side?  Picture a funnel hole straight
through the Earth to the other side of the planet from the moon....the water gathers "as if"  there is a huge hole in the floor of the ocean, on the far side of the Earth from the moon, and thus the water rushes there to run down an imaginary hole through the middle of the Earth, producing another high tide bulge on the side opposite the location of the moon.  Thus we have 2 high tides per 24 hour period, and 2 low tides.

Most of the time this extra tidal height ranges from 3-5 feet, but don't forget the Earth is rotating underneath these tidal effects, otherwise the tidal effect would be much higher,
so the water doesn't get much time to congregate,  and the tidal points are traveling at 700 mph westward, the speed of the rotation of the Earth *eastward* at around 40 degrees N. Latitude, relative to the center of the Earth.

The sun also causes some differential gravity, but not as
much as the moon  (8000/93,000,000 miles is a much smaller
ratio then 8000/238,000 miles)  8000 miles being the approximate diameter of the Earth, and the other two values are the average distance to the sun and moon, respectively, which gives us the relative strength of the tidal forces involved.
 But twice a month, when the moon is roughly aligned with the sun (full moon and new moon),  the tidal effects are additive so the Earth experiences higher than normal tides, and when the moon is at right angles to the sun, at first and last quarter phases, the smaller tidal effects of the sun are subtractive, so we have lower than normal tides at the quarter moons.

{Note:  this explanation uses the old, standard Newtonian
definition of gravity as an attractive force, a pulling, (because it's more easier to understand and explain) rather than the more modern, more correct,  explanation of gravity by Albert Einstein with his General Theory of Relativity, whereby gravity is actually a curvature of the space-time continuum produced by the simple presence of a mass. }

I used Newtonian Gravity to make the explanations more
easily understandable.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA