The astronomical tide is caused by the gravitational attraction that the heavenly bodies, mainly the Moon and Sun, have on the body of water. It can be calculated with high precision and in advance of even many years.
In Venice, the astronomical tide describes a curve of type predominantly semidiurnal, with two maxima and two minima within 24 hours.
During the new moon and full moon the effects of the sun and moon are additive, resulting in the highest tidal fluctuations (syzygies).
In times of first and last quarter, however, the tide is less extensive and less regular (square) and there may be days with only one minimum and one maximum.
According to the method of the harmonic analysis, the astronomical tide at a given location can be calculated as a superposition of sinusoidal oscillations, each characterized by its amplitude and phase.
In Venice eight harmonics are sufficient to describe accurately the astronomical order of 1 cm.