Thread: Spring Tides
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Old November 29th 05, 12:53 PM posted to uk.rec.sailing,uk.sci.astronomy,alt.astronomy
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Default Spring Tides

Mike Williams wrote:

Wasn't it Ronald Raygun who wrote:

The shape of the moon's orbit has an enormous influence because
it means there is a significant variation in the Earth/Moon
distance. The tidal force in inversely proportional to the
cube of the distance, and the contribution due to eccentricity
is nearly half as strong as the contribution from the sun.


So I'd therefore expect the strength of the spring tide force to be
seasonal. The strongest force occurring at spring tides which occur near
lunar perigee. That would be at new moon in February and full moon in
August.


Indeed (actually January and July are closer than February and August,
according to http://www.fourmilab.ch/earthview/pacalc.html).

At February new moon, the Earth is closer to perihelion, so the
solar tide would be slightly enhanced, making that the biggest tidal
force of the year.


Indeed, with the emphasis on "slightly": The eccentricity of Earth's
orbit is much less than that of the Moon's (about a quarter), and so
the perihelion's effect is much less noticeable than that of perigee
(about a sixtieth).

Also, the biggest tidal forces don't directly translate to the
biggest tidal ranges, because ranges are influenced by other
factors too, notably the fact that just because the lunar and
solar forces are in phase at the exact moments of new and full
moon, they may no longer be in phase by the time the oscillations
they fuel have propagated to any particular site of observation,
since they travel at different speeds due to their different
frequencies. There are other harmonics, and their phases, to
consider too.