79 



(S2°— M2°)/(s2— m2) hours after the time of full or new moon. There- 

 fore : 

 Phase age (in hours) 



= (S2°-Mo°)/(30°-28°.984) =0.984(S2°-M2°) (106) 



It is easily shown that tliis expression gives also the time of neap tides 

 after the instants at which the moon is in quadrature. 



For example, at Fort Hamilton, New York Harbor, 82° = 248° and 

 M2°=221°. The phase age is therefore 0.984 (248-221) hours = 26.5 

 hours. At this station therefore spring tides occur a little more than 

 one day after the moon is at full or change (new), and neap tides at 

 the same interval after the moon is at quadrature. 



The phase age at tidal stations tlu-oughout the world ranges up 

 to 3 days. It rarely is negative. 



145. The N2 component — perigean and apogean tides. — The ampli- 

 tude of the N2 component generally is between one-sixth and one- 

 third of that of the M2 component. At stations on the Atlantic 

 coast of the United States, the N2 component usually has a larger 

 amplitude than the S2 component, but at stations on the Atlantic 

 coast of Europe, and along the British Isles, the amplitude of the 

 N2 component is materially less than that of S2. 



It is evident from the preceding paragraphs that the resultant of 

 the M2 and N2 components fluctuates between a maximum of M2 + N2 

 and a minimum of M2— N2, the period from maximum to maximum 

 being 



360°(m2-n2) = 3607(28.9841 -28.4397) = 36070.5444 = 661 hours. 



This is the length of the lunar anomalistic month (par. 62). 



The maximum amplitude of the resultant obviously is due to the 

 maximum attraction of the moon at perigee, and is called the perigean 

 tide. Its minimum amplitude results from the minimum attraction 

 of the moon at apogee, and is called the apogean tide. The average 

 perigean range of the entire tide slightly exceeds 2 (M2+N2) and the 

 average apogean range 2 (M2— N2). 



146. Parallax age. — The interval between lunar perigee and the 

 time of perigean tides is called the parallax age. Since the M2 and 

 N2 components of the equilibrium tides are in conjunction at lunar 

 perigee, the phases of these components of the actual tides then differ 

 by the difference in their epochs, M2°— N2°; and these components 

 of the actual tides come into conjunction (M2°— N2°)/(m2— n2) hours 

 later. The expression for the parallax age is then: 



Parallax age (in hours)=(M2°-N2°)/0.5444 = 1.837 (M2°-N2°) (107) 

 As is readily shown, the parallax age gives also the interval between 

 lunar apogee and apogean tides. 



