eagre (eager)—Same as tidal bore.

earth tide—Periodic movement of the Earth's crust caused by gravitational interactions between the Sun, Moon, and Earth.

East Africa Coast Current—Same as Somali Current.

East Australian Current—A South Pacific Ocean current setting southward along the east coast of Australia.

East Greenland Current—A North Atlantic Ocean current setting southward and then southwestward along the east coast of Greenland.

ebb axis—Average set of the current at ebb strength.

ebb current (ebb)—The movement of a tidal current  away from shore or down a tidal river or estuary. In the mixed type of reversing tidal current, the terms greater ebb and lesser ebb are applied respectively to ebb tidal currents of greater and lesser speed each day. The terms maximum ebb and minimum ebb are applied to the maximum and minimum speeds of a current running continuously ebb, the speed alternately increasing and decreasing without coming to a slack or reversing. The expression maximum ebb is also applicable to any ebb current at the time of greatest speed. See ebb strength.

ebb interval—The interval between the transit of the Moon over the meridian of a place and the time of the following ebb strength.

ebb strength (strength of ebb)—Phase of the ebb tidal current at the time of maximum speed. Also, the speed at this time. See strength of current.

eccentricity of orbit—Ratio of the distance from the center to the focus of an elliptical orbit to the length of the semimajor axis. The eccentricity of orbit = %1 - (B / A)2: where A and B are respectively the semimajor and semiminor axes of the orbit.

ecliptic—The intersection of the plane of the Earth's orbit with the celestial sphere.

eddy—A quasi-circular movement of water whose area is relatively small in comparison to the current with which it is associated.

edge waves—Waves moving between zones of high and low breakers along the shoreline. Edge waves contribute to changes in water level along the shoreface which helps to control the spacing of rip currents. See longshore current and rip current.

Ekman spiral—A logarithmic spiral (when projected on a horizontal plane) formed by the heads of current velocity vectors at increasing depths. The current vectors become progressively smaller with depth. They spiral to the right (looking in the direction of flow) in the Northern Hemisphere and to the left in the Southern with increasing depth. Theoretically, in deep water, the surface current vector sets 45° and the total mass transport sets 90° from the direction toward which the wind is blowing. Flow opposite to the surface current occurs at the so-called "depth of frictional resistance". The phenomenon occurs in wind drift currents in which only the Coriolis and frictional forces are significant. Named for Vagn Walfrid Ekman who, assuming a constant eddy viscosity, steady wind stress, and unlimited water depth and extent, derived the effect in 1905.

electric tape gauge—A gauge consisting of a graduated Monel metal tape on a metal reel (with supporting frame), voltmeter, and battery. Heights can be measured directly by unreeling the tape into its stilling well. When contact is made with the water's surface, the circuit is completed and the voltmeter needle moves. At that moment the length of tape is read against an index mark, the mark having a known elevation relative to the bench marks.

elimination—One of the final processes in the harmonic analysis of tides in which preliminary values for the harmonic constants of a number of constituents are cleared of the residual effects of each other.

equation of time— Difference between mean and apparent time. From the beginning of the year until near the middle of April, mean time is ahead of apparent time, the difference reaching a maximum of about 15 minutes near the middle of February. From the middle of April to the middle of June, mean time is behind apparent time but the difference is less than 5 minutes. From the middle of June to the first part of September, mean time is again ahead of apparent time with maximum difference less than 7 minutes. From the first of September until the later part of December, mean time is again behind apparent time, the difference reaching a maximum of nearly 17 minutes in the early part of November. The equation of time for each day in the year is given in the American Ephemeris and Nautical Almanac.

Equatorial Countercurrent—A current setting eastward between the North and South Equatorial Currents of the Atlantic, Pacific, and Indian (in northern winter) Oceans. In the Atlantic and Pacific, its axis lies about latitude 7° north and in the Indian, about 7° south.

equatorial tidal currents—Tidal currents occurring semimonthly as a result of the Moon being over the Equator. At these times the tendency of the Moon to produce a diurnal inequality in the tidal current is at a minimum.

equatorial tides—Tides occurring semimonthly as a result of the Moon being over the Equator. At these times the tendency of the Moon to produce a diurnal inequality in the tide is at a minimum.

Equatorial Undercurrent—A subsurface current setting eastward along the Equator in the Pacific, Atlantic, and Indian Oceans. In the Pacific, its core of maximum velocity lies at a depth of about 100 meters within the South Equatorial Current.

equilibrium argument—The theoretical phase of a constituent of the equilibrium tide. It is usually represented by the expression (V + u), in which V is a uniformly changing angular quantity involving multiples of the hour angle of the mean Sun, the mean longitudes of the Moon and Sun, and the mean longitude of lunar or solar perigee; and u is a slowly changing angle depending upon the longitude of the Moon's node. When pertaining to an initial instant of time, such as the beginning of a series of observations, it is expressed by (Vo+ u).

equilibrium theory—A model under which it is assumed that the waters covering the face of the Earth instantly respond to the tide-producing forces of the Moon and Sun to form a surface of equilibrium under the action of these forces. The model disregards friction, inertia, and the irregular distribution of the land masses of the Earth. The theoretical tide formed under these conditions is known as the equilibrium tide.

equilibrium tide—Hypothetical tide due to the tide producing forces under the equilibrium theory. Also known as gravitational tide. equinoctial —The celestial equator.

equinoctial tides—Tides occurring near the times of the equinoxes.

equinoxes—The two points in the celestial sphere where the celestial equator intersects the ecliptic; also, the times when the Sun crosses the equator at these points. The vernal equinox is the point where the Sun crosses the Equator from south to north and it occurs about March 21. Celestial longitude is reckoned eastward from the vernal equinox. The autumnal equinox is the point where the Sun crosses the Equator from north to south and it occurs about September 23.

equipotential surface—Same as geopotential surface.

establishment of the port—Also known as high water full and change (HWF&C). Average high water interval on days of the new and full Moon. This interval is also sometimes called the common or vulgar establishment to distinguish it from the corrected establishment, the latter being the mean of all the high water intervals. The latter is usually 10 to 15 minutes less than the common establishment.

estuary—An embayment of the coast in which fresh river water entering at its head mixes with the relatively saline ocean water. When tidal action is the dominant mixing agent it is usually termed a tidal estuary. Also, the lower reaches and mouth of a river emptying directly into the sea where tidal mixing takes place. The latter is sometimes called a river estuary.

Eulerian measurement—Observation of a current with a device fixed relative to the flow.

eustatic sea level rate—The worldwide change of sea level elevation with time. The changes are due to such causes as glacial melting or formation, thermal expansion or contraction of sea water, etc.

evection—A perturbation of the Moon depending upon the alternate increase and decrease of the eccentricity of its orbit, which is always a maximum when the Sun is passing the Moon's line of apsides and a minimum when the Sun is at right angles to it. The principal constituents in the tide resulting from the evectional inequality are <2, 82, and D1.

extreme high water—The highest elevation reached by the sea as recorded by a water level gauge during a given period. The National Ocean Service routinely documents monthly and yearly extreme high waters for its control stations.

extreme low water—The lowest elevation reached by the sea as recorded by a water level gauge during a given period. The National Ocean Service routinely documents monthly and yearly extreme low water for its control stations.