Tides are produced by the gravitational and centrifugal forces operating between the earth and the moon and, to a lesser extent, between the earth and the sun. The horizontal components of the earthmoon forces tend to produce two bulges in the oceans, one on the side of the earth facing the moon and the other on the earth's opposite side. Where these bulges occur the tide is high, and at points midway between these regions the tide is low. Since the earth rotates once per day, however, the points at which the greatest horizontal forces occur will vary, and any point on the earth's surface will experience rhythmic forces which complete two cycles per day. The forces between the earth and the sun cause smaller tides in a similar way. In some places the sea responds more freely to small components of force of a daily period, which are present in the tidal generating forces, with the result that only one tide per day is experienced. The interval between two successive lunar tides is almost 12 hours 25 minutes, whereas the main component of the solar tide completes one cycle in exactly 12 hours. When these tides are superimposed, the high-water times will depend mainly on the lunar tides which do not occur at the same time every day. The tides will vary in range (the difference in height between low and high water) depending on whether the lunar and solar tides are in or out of phase. When the earth, moon, and sun are in line, both the lunar and solar high tides occur together and big tides, called “spring tides”, result. This happens near the times both of new and of full moons. At quarter moon the lunar high tide coincides with the solar low tide, and small tides, called “neap tides”, result. With a little thought it may be seen that spring tides and, similarly, neap tides, occur at approximately the same time of day at any particular place. The tidal range also varies from place to place; for example, at Wellington the maximum range is 4 ft 6 in., while at Auckland it is 12 ft.
Tide gauges, which automatically trace a graph of the sea level against time, are widely used. Such graphs are wave-shaped curves showing that the tides are long waves. These tidal waves move regularly in set patterns around the oceans, patterns dictated in part by the shape of the ocean basins, and in part by the effects of the rotation of the earth. Adjacent places on the coast will have high tides at slightly different times. Proceeding in an anti-clockwise direction around the New Zealand coast, high-tide times generally become progressively later.