52
min, the high water of each tide in the Southern Ocean being at
12 hr. 26 min intervals. As a matter of fact, the tidal day
varies from 24 hr. 35 min at new and full moon to 25 hr. 25 min
at the quarters. Although the moon revolves around the earth in
approximately 27-1/3 days, the earth has moved 27 degrees on
its elliptical orbit around the sun, which it completes once in
365+ days, so that the period which elapses before the moon
again occupies the same relative position to the sun is 29 days
12 hr. 43 min, which is the time occupied by the moon in
completing her phases, and is known as a lunar month or a
lunation.
Considered from the point of view of a person on the earth,
this primary tidal wave constantly travels round the Southern
Ocean at a speed of 13,500 miles in 24 hr. 52 min, thus having
a velocity of 543 miles per hour, and measuring a length of
13,500/2 = 6,750 miles from crest to crest. If a map of the
world be examined it will be noticed that there are three large
oceans branching off the Southern Ocean, namely, the Atlantic,
Pacific, and Indian Oceans; and although there is the same
tendency for the formation of tides in these oceans, they are
too restricted for any very material tidal action to take
place. As the crest of the primary tidal wave in its journey
round the world passes these oceans, the surface of the water
is raised in them, which results in secondary or derivative
tidal waves being sent through each ocean to the furthermost
parts of the globe; and as the trough of the primary wave
passes the same points the surface of the water is lowered, and
a reverse action takes place, so that the derivative waves
oscillate backwards and forwards in the branch oceans, the
complete cycle occupying on the average 12 hr.
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