Abstract Sea level relates to the vertical change in height of the sea surface, which occurs over all time and space scales. The observed sea level (excluding wind waves) can be regarding as being the combination of tides, nontidal residuals, and mean sea level. Tides are the most predictable and dominant component of sea‐level fluctuations in many parts of the world. To fully understand, predict, and categorize them, one must understand both the astronomical forcing that gives rise to them and also the hydrodynamic response of the oceans, to this forcing. The nontidal residual is the part of the sea level that remains once the tidal component has been removed and primarily contains the meteorological contribution to sea level. The most dramatic meteorological sea‐level changes occur during storms, when low atmospheric pressure allows sea level to rise and strong winds force water toward the coastline. Mean sea level is the average height of the sea over longer periods of time with the short‐term variations associated with tide and storm surges averaged out. The main climate‐related factors that are currently causing mean sea levels to rise are: the melting of land‐based sources of ice; thermal expansion of seawater; and changes in terrestrial hydrological regimes.

Tide‐prediction tables require an understanding of the relation of tide to moon and sun, as expressed by correspondence of tide heights to phases of the moon and by the lag of high tide after local meridional transit of the moon. Such tables were compiled earlier in China than elsewhere‐more to satisfy the interests of sightseers of spectacular river bores than for convenience of shipping. We used the earliest extant Chinese tables of a . d . 1056 to compare ancient predictions with those from modern tide tables for the Qiantang river bore near Hangzhou. The fit of hour and height is excellent for the 10 d or so of highest (perigean) tides, but the ancient tables do not select the higher of the two daily apogean high tides during much of the rest of a month.

Tides are the periodic rise and fall of sea level resulting from the differential gravitational forces exerted primarily by the Moon and the Sun, combined Tides are the periodic rise and fall of sea level resulting from the differential gravitational forces exerted primarily by the Moon and the Sun, combined with inertial effects associated with the Earth–Moon system’s orbital motion and the Earth's rotation. While these astronomical forcings generate the fundamental tidal potential, actual observed tides are strongly modified by terrestrial factors Tides are the periodic rise and fall of sea level resulting from the differential gravitational forces exerted primarily by the Moon and the Sun, combined with inertial effects associated with the Earth–Moon system’s orbital motion and the Earth's rotation. While these astronomical forcings generate the fundamental tidal potential, actual observed tides are strongly modified by terrestrial factors, including the geometry of ocean basins, continental boundaries, bathymetry, the coriolis effect, frictional dissipation within shallow seas and the tidal resonance of coastlines. Tides vary on timescales ranging from hours to years due to a number of factors, which determine the lunitidal interval. To make accurate records, tide gauges at fixed stations measure water level over time. Gauges ignore variations caused by waves with periods shorter than minutes. These data are compared to the reference… The shape of the shoreline and the ocean floor changes the way that tides propagate, so there is no simple, general rule that predicts the time of high water from the Moon's position in the sky. Coastal characteristics such as underwater bathymetry and coastline shape mean that individual location characteristics affect tide forecasting; actual high water time and height may differ from model predictions due to the coastal morphology's effects on tidal flow. However, for a given location the relationship between lunar altitude and the time of high or low tide (the lunitidal interval) is relatively constant and predictable, as is the time of high or low tide relative to other points on the same coast. For example, the high tide at Norfolk, Virginia, U.S., predictably occurs approximately two and a half hours before the Moon passes directly overhead. Land masses and ocean basins act as barriers against water moving freely around the globe, and their varied shapes and sizes affect the size of tidal frequencies. As a result, tidal patterns vary. For example, in the U.S., the East coast has predominantly semi-diurnal tides, as do Europe's Atlantic coasts, while the West coast predominantly has mixed tides. Human changes to the landscape can also significantly alter local tides.

This is why, in the real world, some places have very small tides while in other places huge tides become tourist attractions. If you have been in such places, you may know that “tide tables” need to be computed and published for each location; one set of tide predictions doesn’t work for the whole planet. In this introductory chapter, we won’t delve further into these complexities. The rubbing of water over the face of Earth involves an enormous amount of energy. Over long periods of time, the friction of the tides is slowing down the rotation of Earth. Our day gets longer by about 0.002 second each century. That seems very small, but such tiny changes can add up over millions and billions of years. Although Earth’s spin is slowing down, the angular momentum (see Orbits and Gravity) in a system such as the Earth-Moon system cannot change. Thus, some other spin motion must speed up to take the extra angular momentum. The details of what happens were worked out over a century ago by George Darwin, the son of naturalist Charles Darwin. George Darwin (see Figure 4.20) had a strong interest in science but studied law for six years and was admitted to the bar.