They are periodic variations in the level of marine waters, due to the combined gravitational effect of the Moon and the Sun, which occur twice a day.
Because the Moon is much closer to Earth than the Sun, its tidal effect is almost double that of the Sun. The tidal effect consists of two rises in the waters of the oceans, which are verified one in the part in which the Moon is located and the other in the exact opposite part.
Twice a month, at the time of the New Moon and the Full Moon, when the Sun and the Moon meet on the same line, the effect of high tide becomes stronger; on the other hand, in correspondence to the First and Last Fourth phases, when the effects of the gravitational forces of the Moon and the Sun tend to neutralize, the high tide minimum is recorded.
The atmosphere and the solid crust of the Earth also experience to some extent the effects of lunisolar attraction: this is why we also talk about atmospheric tides and terrestrial tides.
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