The alternate rising and falling of the waters of the
   ocean, and of bays, rivers, etc., connected therewith. The tide ebbs
   and flows twice in each lunar day, or the space of a little more than
   twenty-four hours. It is occasioned by the attraction of the sun and
   moon (the influence of the latter being three times that of the
   former), acting unequally on the waters in different parts of the
   earth, thus disturbing their equilibrium. A high tide upon one side of
   the earth is accompanied by a high tide upon the opposite side. Hence,
   when the sun and moon are in conjunction or opposition, as at new moon
   and full moon, their action is such as to produce a greater than the
   usual tide, called the spring tide, as represented in the cut. When the
   moon is in the first or third quarter, the sun's attraction in part
   counteracts the effect of the moon's attraction, thus producing under
   the moon a smaller tide than usual, called the neap tide.