![]() ![]() Increased gravitational influences and tide-raising forces are produced when the Moon is at position of perigee, its closest approach to the Earth (once each month) or the Earth is at perihelion, its closest approach to the Sun (once each year). The Lunar Parallax and Solar Parallax Inequalitiesīoth the Moon and the Earth revolve in elliptical orbits and the distances from their centers of attraction vary. Similarly, in the earth-sun system, when the earth is closest to the sun (perihelion), about January 2 of each year, the tidal ranges will be enhanced, and when the earth is farthest from the sun (aphelion), around July 2, the tidal ranges will be reduced. Approximately two weeks later, when the moon (at apogee) is farthest from the earth, the lunar tide-raising force will be smaller, and the tidal ranges will be less than average. Once each month, when the moon is closest to the earth (perigee), the tide-generating forces will be higher than usual, thus producing above-average ranges in the tides. The moon's tide-producing force acting on the earth's waters will change in inverse proportion to the third power of the distance between the earth and moon, in accordance with the previously mentioned variation of Newton's Law of Gravitation. 4), the distance between the earth and moon will vary throughout the month by about 31,000 miles. Since the moon follows an elliptical path (Fig. Such tides of diminished range are called neap tides, from a Greek word meaning "scanty".Ģ. High tides are lower and low tides are higher than average. In the tidal force envelope representing these combined forces, both maximum and minimum forces are reduced. Each force tends in part to counteract the other. Such greater-than-average tides resulting at the syzygy positions of the moon are know as spring tides - a term which merely implies a "welling up" of the water and bears no relationship to the season of the year.Īt first- and third-quarter phases (quadrature) of the moon, the gravitational attractions of the moon and sun upon the waters of the earth are exerted at right angles to each other. This means that the tidal range is greater at all locations which display a consecutive high and low water. Since the resultant or combined tidal force is also increased, the observed high tides are higher and low tides are lower than average. When the moon is at new phase and full phase (both positions being called syzygy) the gravitational attractions of the moon and sun act to reinforce each other. 3) during the monthly cycle of phases (29.53 days) the gravitational attraction of moon and sun may variously act along a common line or at changing angles relative to each other. It is obvious that, because of the moon's changing position with respect to the earth and sun (Fig. It has been noted above that the gravitational forces of both the moon and sun act upon the waters of the earth. Lunar Phase Effect: Spring and Neap Tides. Those influences of astronomical origin will first be described.ġ. The range of the tides at any location is subject to many variable factors. 6, the difference in the height, in feet, between consecutive high and low tides occurring at a given place is known as the range. Chapter 4 - Variations in the Range of the Tides: Tidal InequalitiesĪs will be shown in Fig. ![]()
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