The coastline is the boundary between a body of water, such as an ocean or a sea, and the land to which it is adjacent. Coastlines are not constant features but are continuously changing as the result of geological, weather, and other forces. Perhaps the most common factor affecting a coastline is tides. Tides are periodic rises and falls of sea level caused by the gravitational attraction of the Moon and Sun. As Earth rotates on its axis and travels around the Sun, it takes different positions with respect to these two bodies. Sometimes the three bodies are lined up with Earth between the Moon and the Sun:
At other times, the Moon and Sun are on the same side of Earth:
And on still other occasions, Earth, Moon, and Sun are at right angles to each other:
Both Moon and Sun exert gravitational attraction on Earth’s surface. In some cases, those two forces act in the same direction (S—M—E), in other cases, in opposite directions (M—E—S), and in still other cases, at right angle to each other:
Those forces of attraction are not strong enough to produce noticeable effects on Earth’s lithosphere (its solid component). But they are strong enough to produce noticeable movements of ocean water. For example, when Earth lies between the Moon and the Sun, those two bodies exert an attraction in opposite directions on seawater; when Moon and Sun are both on the same side of Earth, they exert attraction in the same direction. The latter case results in a greater pull on seawater, and therefore a higher tide, than in the former case. As a consequence of these factors, coastlines constantly change throughout the day and night and from season to season.
Tides are classified according to the forces by which they are produced and the astronomical conditions causing those forces and the frequency and heights of tides that occur on a daily basis. In the first of these instances, tides that occur just after the Sun, Moon, and Earth are aligned at right angles to each other (just after the first and quarter moons) have the least difference between high and low tides; they are called neap tides. When the three bodies are lined up in a straight line relative to each other (at new and full moon), they display the greatest difference between high and low tides; they are called spring tides.
Topographic conditions may determine how often tides occur in a specific location. Tides that occur only once a day are called diurnal tides, while those that occur twice a day are called semidiurnal tides. In many locations, tides may be both diurnal and semidiurnal, a system referred to as a mixed semidiurnal tide. (For a map showing where each type of tide occurs worldwide, see Berg 2019; for a more complete discussion of tidal actions, see “Ocean Tides” n.d.)