Definition
Around 21 March and 23 September the Sun's declination is zero, aligning the solar tidal bulge with the equator and maximising the solar contribution at moderate latitudes; when this coincides with syzygy the resulting springs exceed those of other seasons. In practice the highest astronomical tides of the year at many European ports occur within a few weeks of the equinoxes, a factor worth allowing for when planning shallow-water passages.
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