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From: bob@islenet.UUCP (Robert P. Cunningham)
Newsgroups: net.astro
Subject: Re: StarDate: April 5 Spring Tides
Message-ID: <1054@islenet.UUCP>
Date: Wed, 10-Apr-85 16:15:19 EST
Article-I.D.: islenet.1054
Posted: Wed Apr 10 16:15:19 1985
Date-Received: Sun, 14-Apr-85 02:09:15 EST
References: <43@utastro.UUCP> <24204@lanl.ARPA>
Organization: Hawaii Institute of Geophysics
Lines: 38

> Am I missing something?  If, as described above, the earth, moon and
> sun are in a line with earth in the middle, won't the gravitational
> fields of moon and sun subtract rather than add?

Tidal forces are the resultant difference between gravitational
attraction and centrifugal forces.

At the surface of the Earth, around the point directly underneath the
Moon, the gravitational attraction is stronger.  The net effect is to
create a bulge pointing towards the Moon.

At the antipode, on the other side from the Moon, centrifugal forces are
stronger than the Moon's gravitational pull.  The net effect is to
create another bulge, pointing away from the Moon.  [yes, this seems
counter-intuitive]

Thus, you get a bulges (high tides) point both towards and away from the
Moon.  This is the major tide cycle, with a period of 1/2 a lunar day
(12 hours, 25.235 minutes).

The tidal potential due to the Sun creates two similar bulges and a
tidal cycle of 1/2 the solar day.  When these are in line (both Sun &
Moon on the same side of the Earth, or on opposite sides), that's when
the highest tides occur.

Important secondary effects influencing the tidal forces involve the
relative declinations of the Sun & Moon, and the actual current distance
of the Moon from Earth.

The actual resultant rise (or fall) in sea level depends greatly on the
local configuration of the ocean basin and shoreline configuration.

And, of course, there are tidal effects in the Earth's mantle and
atmosphere, not just the oceans.  In fact, there are measurable tidal
effects even in various lakes.
-- 
Bob Cunningham   ..{dual,ihnp4,vortex}!islenet!bob
Honolulu, Hawaii