Explanation of the Tides LUNAR TIDE:The gravitational force, which tends to draw the Earth and the Moon towards one another, is balanced by an equal and opposite centrifugal force caused by their monthly rotation about a common centre of gravity. According to Newton's law the gravitational pull exerted by the Moon on that part of the Earth's surface nearest to the Moon must be greater (whilst that exerted on the diametrically opposite part must be less) than the average exerted on the whole Earth. The centrifugal force, however, is the same on all particles of the Earth. It follows, therefore, that the difference in attraction is directed towards the Moon at that point on the Earth's surface nearest to the Moon and away at the diametrically opposite point. These differences in attraction are termed lunar tide-generating forces. Only the water on the Earth's surface is free to move and consequently tends to be drawn away from the vicinity of those meridians under and 180* from the Moon. The magnitude and time log of the response of the water due to tide generating forces varies according to terrestrial conditions, such as depth, shape and size of the sea in which the port is located so that at no two places are the responses exactly the same. THE SOLAR TIDE:Tide-generating forces arise from the differences in attraction of the sun on parts of the Earth's surface, tending to cause low water on the meridians where Sunrise and Sunsets occur and high water on the meridians directly under and 180* from the Sun. Owing to the greatest difference between the Earth and the Sun, solar tide-generating forces are less than half of those caused by the Moon. SPRING TIDES:When the Sun and Moon are in conjunction (New Moon) or in opposition (Full Moon) and two tide-generating forces are acting on the same meridians, so the height and range of the tide will be greater than at other periods. These tides are called 'Springs'. NEAP TIDES:When the Sun and Moon are in quadrature, the two tide-generating forces are acting at right angles to each other, thus producing a tide which has a higher low water than average and a range which is smaller than at other periods. These small tides are known as 'Neaps'. EFFECT OF LAG ON SPRINGS AND NEAPS:Because a time interval is required to overcome the lag in response to the tide generating forces, Springs do not occur until about 2 days after the New and Full Moon, and for the same reason Neaps occur about 2 days after the Moon's Quarters. PERIGEE TIDES:The Moon's orbit is elliptical and so its distance from the Earth varies throughout the month. When it is nearest to the Earth the Moon is said to be in 'Perigee', and when furthest away to be in 'Apogee'. The greatest attraction between Earth and Moon occurs at Perigee and, when this coincides with the time of New or Full Moon, the resulting Spring tides are greater than average springs. As one complete orbit takes 27 1/3 days the Moon will be nearly in Apogee when the springs occur (about 15 days after the Perigee tides). In consequence these tides will be less than average springs. EQUINOCTIAL SPRINGS: The lunar and solar tide-generating forces are greater than average when the Moon and Sun have low declination and vice versa. This will occur when the Moon ( new and full) has a low declination near the Equinoxes (21st March and 23rd September) when the Sun's declination is zero. Thus, at about these times of the year, greater than average spring tides can be expected. THE GREATEST SPRING TIDES WILL OCCUR AFTER A NEW OR FULL MOON WHICH IS IN PERIGEE NEAR THE EQUINOX.