M2

M2 is the principal lunar semidiurnal tidal constituent, period 12.42 hours, set by the dominant tide-raising effect of the Moon. It is the largest constituent over most of the world ocean, so the basic twice-daily rhythm and the spring-neap beat (M2 interfering with the solar S2) are governed by it. Amphidromic points, where M2 amplitude is zero and phase rotates around, structure the tidal pattern of each basin.