GULF OF MEXICO 



105 



Figure 26. — Tide curves at Coatzacoalcos, Progreso, and Habana, June 23-30, 1948. 



symbol Mj, the M indicating that it is a com- 

 ponent derived from the moon's motion and tlie 

 subscript 2 indicating that it is a semidiurnal 

 component, that is, its period is half a day. The 

 principal solar component has a period of exactly 

 12 hours and therefore an angular speed of 30.00° 

 per hour. It is symbolized by S2. 



The actual moon does not move in a circular 

 orbit but in an elliptical one. This means that 

 its distance from the earth is not constant, and 

 hence its tide-producing force varies, being less 

 than average at apogee and greater at perigee, 

 the period from one perigee to another being on 

 the average, 27^2 days. Analysis shows that for 

 this reason it is necessary to introduce a simple 

 component with a period of 12.66 hours or a 



speed of 28.44° per hour. It is designated as 

 N2 and is known as the larger lunar elliptic semi- 

 diurnal component. 



Still another prominent feature of the relative 

 movements of sun and moon with respect to the 

 earth must be taken into consideration, namely, 

 the fact that the planes of their orbits are inclined 

 to the plane of the equator. This means that the 

 declinations of sun and moon are constantly 

 changing. It is found that two components must 

 be introduced, symbolized by Ki and Oi, the 

 former arising from both the sun's and moon's 

 tide-producing forces and called the luni-solar 

 diurnal component and the latter arising from the 

 moon's tide-producing force and called the lunar 

 diurnal component. 



