The constituent produced by the Moon's tide-generating force acting on a rotating Earth, completing two cycles per lunar day with a period of about 12 hours 25 minutes. At most stations it has the largest amplitude, and its period explains why high water comes roughly 50 minutes later each day. Maps of M2 amplitude and phase are the standard way to show how the tide moves around an ocean basin.
M2 amplitude is 1.4 metres here and everything else is small change, so it's about as semidiurnal as it gets.
Also heard as
- principal lunar semidiurnal
Related terms
S2
The principal solar semidiurnal constituent, period exactly 12 hours, which beats with M2 to make springs and neaps.
Harmonic constituent
One of the regular cosine waves, each with a period fixed by astronomy, that together make up the tide.
Semidiurnal tide
A tide with two high waters and two low waters of similar height in each lunar day.
Amphidromic point
A point in the sea where a tidal constituent has zero range and around which its high water rotates.
Lunar day
The time for the Moon to return to the same meridian, about 24 hours 50 minutes, which sets the daily tide rhythm.