S2

S2 is the principal solar semidiurnal tidal constituent, period exactly 12.00 hours, the Sun’s main contribution to the twice-daily tide. It is roughly half the size of the lunar M2; the slow drift between their two periods produces the fortnightly spring-neap cycle as the two move in and out of phase. The M2-to-S2 amplitude ratio is part of what sets a port’s tidal character.