Earth facing the Moon (A), gravity is greatest, because the side of Earth facing the Moon is closer to the Moon. The Moons gravitational pull acts to pile water up on this side of Earth, producing the highest of the daily tides.

The Moons gravitational attraction is weaker on the side of Earth facing away from the Moon (B), because that side is farther away from the Moon. High tides occur on this side of Earth as well, but they are not as high as those on the side facing the Moon.

On those parts of Earth that are not in line with the Moon (C and D), the water level of tides is lower than that occurring at A and B. The reason for the low tides over these regions is related to tractive forces. Tractive forces act tangentially to Earths surface and cause the horizontal movement of water associated with tidesthe tidal currents.

The Moon revolves around Earth once every 24 hours and 50 minutes, producing a daily tidal effect. In oceanography, this time period is referred to as a tidal day. When the Moon is new or full, the Sun and the Moon are aligned with Earth (fig. 6-6-2, view A). At these times, the gravitational pull on Earth is greater because of the combined gravitational attraction of the Sun and the Moon.

When the Moon is in its new and full phases, the high tides are higher than normal and the low tides are lower than normal. These tides are referred to as SPRING TIDES.

When the Moon is in its first- and third-quarter phases, the Sun and the Moon are at right angles to each other (fig. 6-6-2, view B). This alignment produces lower-than-normal high tides and higher-than-normal low tides. In other words, the range of the rise and fall of the water level is less than normal. These tides are referred to as NEAP TIDES.

From the above information, it would appear that the rise and fall of the tides is pretty straightforward based on the position of the Moon and the Sun in relation to Earth. But this is not the case. The daily tides are not uniform.

Some places have two high and two low tides a day, while others may have one high and one low. Some places have tidal ranges well above 25 feet, while others have minimal ranges.

There are many variables in the processes that control the time and height of a tide at a particular maritime location. In addition to the relationship of the Moon, the Sun and Earth, Earths coastline configurations, ocean bottom configurations, and the interaction of water (wave mechanics) all affect tides.

Mathematical models have been developed that will allow desk top computers to forecast tides based on the motions of the Moon, the Sun, and Earth. These programs are expected to be available for your use in the near future. The Tide Tables produced by the National Ocean Service computers must be used until then.

Learning Objective: Identify the four volumes of Tide Tables and recognize how the information is used to compute tides.

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