2. The complexity of tide patterns
d. Other factors
(1) The sun - The effect of the sun on the earth's tides
is much less than that of the moon. Although the sun is 500,000 times as
massive as the moon, it is 3900 times further away. The gravitational
effect of a body on another body falls off rapidly as distance increases.
The sun's effect on tides is less than 1/100 that of the moon.
The sun only has an effect by adding to or subtracting from the moon's
effect. See the drawings below. As the moon goes around the earth,
it occupies successively each four positions. In positions 1
and 3
the sun's gravity adds to the moon's effect; the high tides are higher
and the low tides are lower than average.
In positions 2 and 4, the sun's gravity opposes the effect of
the moon; the high tides are lower and the low tides are higher than
average. Every month the earth goes through all four of these
conditions. Number 1 is new moon,
number 3 is full moon. Numbers 2
and 4 are the quarter moons.
For some reason, the tides at new and full moon are called spring tides,
and those at quarter moon are called neap tides. (The origin of the word neap
is obscure, according to the Oxford English Dictionary.) Remember:
Spring tides are of greater range (higher highs, lower lows), and neap tides are
of lesser range (lower highs, higher lows).
(2) Tides in the solid earth and the atmosphere
The solid earth and the atmosphere are subject to the same tidal forces as
the liquid oceans. In the case of the earth, there are small bulges
that can be measured, but the earth's solidity limits these tides to less than 1
cm in height (compared to about 1 meter for ocean tides).
In the case of the atmosphere, the effects are small also, in this case
because (1) there is no clear line at which we can say the atmosphere ends, and
(2) a gas, being much less dense, is less responsive to tidal forces than a
liquid or solid.
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3. Where to get more information
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