Principal Lunar Tide

The principal lunar semidiurnal tide, designated M2, is the largest tidal constituent over most of the world ocean, with a period of 12.4206 hours and a speed of 28.9842 degrees per hour, half the 24.84-hour lunar day. It represents the tide a circular lunar orbit in the equatorial plane would raise, and its Doodson number is 255.555. M2 dominates the Atlantic and sets the semidiurnal rhythm of two highs and two lows per lunar day. Its amplitude and Greenwich phase are the first harmonic constants measured at any station, and the form number F compares it with S2, K1, and O1 to classify the local tidal regime.