The moon, being much nearer to the earth than the sun, is the principal cause of tides. Because the force of gravity decreases with distance, the moon exerts a stronger gravitational pull on the side of the earth that is closer to it and a weaker pull on the side farther from it. The earth does not respond to this variation in strength because the planet is rigidâ€”instead, it moves in response to the average of the moonâ€™s gravitational attraction. The worldâ€™s oceans, however, are liquid and can flow in response to the variation in the moonâ€™s pull. On the side of the earth facing the moon, the moonâ€™s stronger pull makes water flow toward it, causing a dome of water to rise on the earthâ€™s surface directly below the moon. On the side of the earth facing away from the moon, the moonâ€™s pull on the oceans is weakest. The waterâ€™s inertia (its tendency to keep traveling in the same direction) makes it want to fly off the earth instead of rotate with the planet. The moonâ€™s weaker pull doesnâ€™t compensate as much for the waterâ€™s inertia on the far side, so another dome of water rises on this side of the earth. The dome of water directly beneath the moon is called direct tide, and the dome of water on the opposite side of the earth is called opposite tide.
As the earth rotates throughout the day, the domes of water remain aligned with the moon and travel around the globe. When a dome of water passes a place on the earth, that place will experience a rise in the level of the ocean water, known as high tide or high water. Between successive high tides, the water level drops. The lowest water level reached between successive high tides is known as low tide or low water. Low and high tides alternate in a continuous cycle. The variations that naturally occur in the level between successive high tide and low tide are referred to as the range of tide. At most shores throughout the world, two high tides and two low tides occur every lunar day, the average length of a lunar day being 24 hours, 50 minutes, and 28 seconds. One of these high tides is caused by the direct-tide dome and the other by the opposite-tide dome. Two successive high tides or low tides are generally of about the same height. At various places outside the Atlantic Ocean, however, these heights vary considerably; this phenomenon, which is known as diurnal inequality, is not completely understood at the present time.
The sun likewise gives rise to two oppositely situated domes, but because the sun is far from the earth, its tide-raising force is only about 46 percent that of the moon. The sum of the forces exerted by the moon and sun result in two sets of domes, the positions of which depend on the relative positions of the sun and moon at the time. During the periods of new and full moon, when the sun, moon, and earth are directly in line, the solar and lunar domes coincide. This results in the condition known as spring tides, in which the high tide is higher and the low tide is lower than usual. When the moon is in first or third quarter, however, it is at right angles to the sun relative to the earth, and the height of the tides is subject to the opposing forces of the sun and moon. This condition produces neap tides, in which the high tide is lower, and the low tide is higher, than normal. Spring and neap tides occur about 60 hours after the corresponding phases of the moon, the intervening period of time being known as the “age of the tide” or “age of the phase” inequality. The interval of time between the crossing of a meridian by the moon at one point and the next high tide at that point is called the lunitidal interval, or the high-water interval for that point. The low-water interval is the period between the time the moon crosses the meridian and the next low tide. Average values for the high-water lunitidal intervals during periods of new and full moon are known as the “establishment of the port.” Values for the intervals during other periods of the month are often referred to as the “corrected establishment”.
Tidal Currents and Waves
Accompanying the vertical rise and fall of water are various horizontal or lateral movements commonly known as tidal currents or tidal streams, which are very different from the common ocean currents. In confined areas, a tidal current flows for about 6 hours, 12 minutes in an upstream or shoreward direction, corresponding to high tide; it then reverses and flows for approximately the same time in the opposite direction, corresponding to low tide. During the period of reversal, the water is characterized by a state of rest or calm, known as slack water or slack tide. A current flowing toward the shore or upstream is called flood current; that flowing in a direction away from land or downstream is known as ebb current.