Y
Four different types of year are of significance to tides.
Sidereal year – the period taken by earth complete a single orbit of the sun, 365.2564 mean solar days (msd).
Tropical year – measured in relation to the beginnings of the various seasons (successive vernal equinoxes). It is slightly shorter than the sidereal year as a consequence of precession. The axis of the earth is tilted at about 23½° degrees to the perpendicular of the orbital plane. The axis slowly precesses about the perpendicular, in the manner of a 'sleeping top'. Each precession takes 26,000 years. If a single precession was completed in just one day, then we would experience four seasons in a single day. This means that the seasons advance 1/26,000th part per sidereal year faster than they would without precession, and the tropical year is therefore only 365.2422 msd.
Anomalistic year – the period between successive perihelions. Just as the anomalistic month is slightly longer than a sidereal month, the anomalistic year, 365.2596 msd, is slightly longer than a sidereal year. Of the three types of year, the anomalistic is of greatest importance in tides. The longitude of the sun undergoes a complete cycle in one tropical year.
Julian year – defined as 365.25 days.