October 5, 2017
The following tidal glossary shows the various tidal ranges and describes the qualities of each.
- Apogean Tide: A monthly tide of decreased range that occurs when the Moon is farthest from Earth (at apogee).
- Diurnal: Applies to a location that normally experiences one high water and one low water during a tidal day of approximately 24 hours.
- Mean Lower Low Water: The arithmetic mean of the lesser of a daily pair of low waters, observed over a specific 19-year cycle called the National Tidal Datum Epoch.
- Neap Tide: A tide of decreased range occurring twice a month, when the Moon is in quadrature (during the first and last quarter Moons, when the Sun and the Moon are at right angles to each other relative to Earth).
- Perigean Tide: A monthly tide of increased range that occurs when the Moon is closest to Earth (at perigee).
- Semidiurnal: Having a period of half a tidal day. East Coast tides, for example, are semidiurnal, with two highs and two lows in approximately 24 hours.
- Spring Tide: Named not for the season of spring, but from the German springen (to leap up). This tide of increased range occurs at times of syzygy (q.v.) each month. A spring tide also brings a lower low water.
- Syzygy: Occurs twice a month, when the Sun and the Moon are in conjunction (lined up on the same side of Earth at the new Moon) and when they are in opposition (on opposite sides of Earth at the full Moon, though usually not so directly in line as to produce an eclipse). In either case, the gravitational effects of the Sun and the Moon reinforce each other, and tidal range is increased.
- Vanishing Tide: A mixed tide of considerable inequality in the two highs or two lows, so that the “high low” may become indistinguishable from the “low high.” The result is a vanishing tide, where no significant difference is apparent.
The tide never goes out so far but it always comes in again