Ocean Tides

TIDES AND TIDAL CURRENTS

ORIGINS OF TIDES

 Introduction
Tides are the periodic motion of the waters of the sea due to changes in the attractive forces of the Moon and Sun upon the rotating Earth. Tides can either help or hinder  mariner. A high tide may provide enough depth to clear a bar, while a low tide may prevent entering or leaving a harbor. Tidal current may help progress or hinder it, may set the ship toward dangers or away from them. By understanding tides and making intelligent use of predictions published in tide and tidal current tables and descriptions in sailing directions, the navigator can plan an expeditious and safe passage through tidal waters.

Tide and Current:

The rise and fall of tide is accompanied by horizontal movement of the water called tidal current. It is necessary to distinguish clearly between tide and tidal current, for the relation between them is complex and variable. For the sake of clarity mariners have adopted the following definitions: Tide is the vertical rise and fall of the water, and tidal current is the horizontal flow. The tide rises and falls, the tidal current floods and ebbs. The navigator is concerned with the amount and time of the tide, as it affects access to shallow ports. The navigator is concerned with the time, speed, and direction of the tidal current, as it will affect his
ship’s position, speed, and course. Tides are superimposed on nontidal rising and falling water levels, caused by weather, seismic events, or other natural forces. Similarly, tidal currents are superimposed upon non-tidal currents such as normal river flows, floods, and freshets.

Causes of Tides:

The principal tidal forces are generated by the Moon and Sun. The Moon is the main tide-generating body. Due to its greater distance, the Sun’s effect is only 46 percent of the Moon’s. Observed tides will differ considerably from the tides predicted by equilibrium theory since size, depth, and configuration of the basin or waterway, friction, land masses, inertia of water masses, Coriolis acceleration, and other factors are neglected in this theory. Nevertheless, equilibrium theory is sufficient to describe the magnitude and distribution of the main tide-generating forces across the surface of the Earth. Newton’s universal law of gravitation governs both the orbits of celestial bodies and the tide-generating forces which occur on them. The force of gravitational attraction between any two masses, m1 and m2, is given by: where d is the distance between the two masses, and G is a constant which depends upon the units employed. This law assumes that m1 and m2 are point masses. Newton was able to show that homogeneous spheres could be treated as point masses when determining their orbits.

 

Types Of Tides:

Spring Tides: 

A tide in which the difference between high and low tide is the greatest. Spring tides occur when the Moon is either new or full, and the Sun, the Moon, and the Earth are aligned. When this is the case, their collective gravitational pull on the Earth's water is strengthened is called spring Tides. Spring tides occur when the Sun and Moon are in line with the Earth and their gravitational pulls reinforce each other.

spring tides are caused when the moon and sun cause a pull on the earths waters and sence the moon is closer it creats a stronger pull. thus creating higher high tides and lower low tides.

Neap Tides:

The interactions of the sun and the moon affect all tides. The highest tides occur during the New moon or when the sun and moon are in alignment. Neap tides are the smaller tides that occur at the intermediate moon phases.A neap tide is the lowest high tide. Neap tide occurs twice per month, during the first and third quarters of the moon. The opposite of a neap tide is called a spring tide.