A tide clock is a specially designed clock that keeps track of the Moon's apparent motion around the Earth. Along many coastlines, the Moon contributes the major part (67%) of the combined lunar and solar tides. The exact interval between tides is influenced by the position of the Moon and Sun relative to the Earth, as well as the specific location on Earth where the tide is being measured. Due to the Moon's orbital prograde motion, it takes a particular point on the Earth (on average) 24 hours and 50.5 minutes to rotate under the Moon, so the time between high lunar tides fluctuates between 12 and 13 hours. A tide clock is divided into two roughly 6 hour tidal periods that shows the average length of time between high and low tides in a semi-diurnal tide region, such as most areas of the Atlantic Ocean.
The bottom of the tide clock dial (6 o'clock position) is marked "low tide" and the top of the tide clock dial (12 o'clock position) is marked "high tide." The left side of the dial is marked "hours until high tide" and has a count-down of hours from 5 to 1. There is one hand on the clock face, and along the left side it points to the number of hours "until" the (lunar) high tide. The right hand side of the clock is marked "hours until low tide" and has a count-down of hours from 5 to 1. The number pointed to by the hand gives the time "until" the (lunar) low tide. Some tide clocks incorporate time (using standard quartz movement) and even humidity and temperature in the same instrument.
Some tide clocks count down the number of hours from high or low tide, as in "one hour past high or low tide". When the clock reaches the half way point ("half-tide"), it then counts the hours up to high tide or low tide, as in "one hour until high or low tide". Generally, there is an adjustment knob on the back on the instrument which may be used to set the tide using official tide tables for a specific location at either high or low tide.
Tides have an inherent lead or lag, known as the lunitidal interval, that is different at every location, so tidal clocks are set for the time when the local lunar high tide occurs. This is often complicated because the lead or lag varies during the course of the lunar month, as the lunar and solar tides fall into and out of synchronization. The lunar tide and solar tide are synchronized (ebb and flow at the same time) near the full moon and the new moon. The two tides are unsynchronized near the first and last quarter moon (or "half moon"). Also, in addition to the relative position of the moon and the elliptical pattern of the sun, the tide can be affected to some degree by wind and atmospheric pressure. All of these variables have less impact on the tide at the time of the full moon, so this is usually the best time to set a tide clock. If the tide clock is mounted on a moving boat, it will need to be reset more frequently. The best time to set the clock is at the new moon or the full moon, which is also when the clock can most reliably indicate the actual combined tide. A simple tide clock will always be least reliable near the quarter moon.
Tide range is the vertical distance between the highest high tide and lowest low tide. The size of the lunar tide compared to the solar tide (which comes once every 12 hours) is generally about 2 to 1, but the actual proportion along any particular shore depends on the location, orientation, and shape of the local bay or estuary. Along some shorelines, the solar tide is the only important tide, and ordinary 12-hour clocks suffice since the high and low tides come at nearly the same time every day. Because ordinary tidal clocks only track a part of the tidal effect, and because the relative size of the combined effects is different in different places, they are in general only partially accurate for tracking the tides. Consequently, all navigators use tide tables either in a booklet, computer or digital tide clock.
Analog tide clocks are most accurate for use on the Atlantic coasts of America and Europe. This is because along the Atlantic coastline the moon controls the tides predictably, ebbing and flowing on a regular (12- to 13-hour) schedule. However, in other parts of the world such as along the Pacific Coast, tides can be irregular.The Pacific Ocean is so vast that the moon cannot control the entire ocean at once. The result is that parts of the Pacific Coast can have 3 high tides a day. Similarly, there are areas in the world like the Gulf of Mexico or the South China Sea that have only one high tide a day. Mechanical tide clocks used on the Pacific Coast must be adjusted frequently, often as much as weekly, and are not useful in diurnal areas (those with one tide per day).
Digital tide clocks are not married to the 24 hour 50.5 minute tide cycle and thus track tides beyond the Atlantic coast. Smart digital tide clocks can work across all locations in North America without any adjustments. This is achieved by storing all the variations of tides at numerous locations. Given a particular location and date/time, a digital tide clock can display the previous tide, next tide and current absolute tide height. Thus, they are able to track semi-diurnal, diurnal and mixed diurnal tides.
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth.
A lunar node is either of the two orbital nodes of the Moon, that is, the two points at which the orbit of the Moon intersects the ecliptic. The ascending node is where the Moon moves into the northern ecliptic hemisphere, while the descending node is where the Moon enters the southern ecliptic hemisphere.
An amphidromic point, also called a tidal node, is a geographical location which has zero tidal amplitude for one harmonic constituent of the tide. The tidal range for that harmonic constituent increases with distance from this point.
Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.
The lunitidal interval measures the time lag from lunar culmination to the next high tide at a given location. It is also called the high water interval (HWI). Sometimes a term is not used for the time lag, but instead the terms age or establishment of the tide are used for the entry that is in tide tables.
An astronomical clock, horologium, or orloj is a clock with special mechanisms and dials to display astronomical information, such as the relative positions of the sun, moon, zodiacal constellations, and sometimes major planets.
Slack water is a short period in a body of tidal water when the water is completely unstressed, and there is no movement either way in the tidal stream, and which occurs before the direction of the tidal stream reverses. Slack water can be estimated using a tidal atlas or the tidal diamond information on a nautical chart. The time of slack water, particularly in constricted waters, does not occur at high and low water, and in certain areas, such as Primera Angostura, the ebb may run for up to three hours after the water level has started to rise, and the flood may run for three hours after the water has started to fall. Thornton Lecky, writing in 1884, illustrates the phenomenon with an inland basin of infinite size, connected to the sea by a narrow mouth. Since the level of the basin is always at mean sea level, the flood in the mouth starts at half tide, and its velocity is at its greatest at the time of high water, with the strongest ebb occurring conversely at low water.
Tide tables, sometimes called tide charts, are used for tidal prediction and show the daily times and levels of high and low tides, usually for a particular location. Tide heights at intermediate times can be approximated by using the rule of twelfths or more accurately calculated by using a published tidal curve for the location. Tide levels are typically given relative to a low-water vertical datum, e.g. the mean lower low water (MLLW) datum in the US.
Tidal range is the height difference between high tide and low tide. Tides are the rise and fall of sea levels caused by gravitational forces exerted by the Moon and Sun and the rotation of Earth. Tidal range is not constant but changes depending on the locations of the Moon and Sun.
The Moon orbits Earth in the prograde direction and completes one revolution relative to the stars in about 27.32 days and one revolution relative to the Sun in about 29.53 days. Earth and the Moon orbit about their barycentre, which lies about 4,600 km (2,900 mi) from Earth's center. On average, the distance to the Moon is about 385,000 km (239,000 mi) from Earth's center, which corresponds to about 60 Earth radii or 1.282 light-seconds.
Earth tide is the displacement of the solid earth's surface caused by the gravity of the Moon and Sun. Its main component has meter-level amplitude at periods of about 12 hours and longer. The largest body tide constituents are semi-diurnal, but there are also significant diurnal, semi-annual, and fortnightly contributions. Though the gravitational force causing earth tides and ocean tides is the same, the responses are quite different.
The Zimmer tower is a tower in Lier, Belgium, also known as the Cornelius tower, that was originally a keep of Lier's fourteenth century city fortifications. In 1930, astronomer and clockmaker Louis Zimmer (1888–1970) built the Jubilee Clock, which is displayed on the front of the tower, and consists of 12 clocks encircling a central one with 57 dials. These clocks showed time on all continents, phases of the moons, times of tides and many other periodic phenomena.
Atmospheric tides are global-scale periodic oscillations of the atmosphere. In many ways they are analogous to ocean tides. Atmospheric tides can be excited by:
The theory of tides is the application of continuum mechanics to interpret and predict the tidal deformations of planetary and satellite bodies and their atmospheres and oceans under the gravitational loading of another astronomical body or bodies.
A king tide is an especially high spring tide, especially the perigean spring tides which occur three or four times a year.
New Zealand has large ocean energy resources but does not yet generate any power from them. TVNZ reported in 2007 that over 20 wave and tidal power projects are currently under development. However, not a lot of public information is available about these projects. The Aotearoa Wave and Tidal Energy Association was established in 2006 to "promote the uptake of marine energy in New Zealand". According to their 10 February 2008 newsletter, they have 59 members. However, the association doesn't list its members.
A tide-predicting machine was a special-purpose mechanical analog computer of the late 19th and early 20th centuries, constructed and set up to predict the ebb and flow of sea tides and the irregular variations in their heights – which change in mixtures of rhythms, that never repeat themselves exactly. Its purpose was to shorten the laborious and error-prone computations of tide-prediction. Such machines usually provided predictions valid from hour to hour and day to day for a year or more ahead.
The Exeter Cathedral Astronomical Clock is a fifteenth-century astronomical clock in Exeter Cathedral, England. It displays the hour of the day, the day of the lunar month and the phase of the moon. The modern clock mechanism was installed in 1885 by Gillett & Bland of Croydon, and restored in 1910.
Long-period tides are gravitational tides with periods longer than one day, typically with amplitudes of a few centimeters or less. Long-period tidal constituents with relatively strong forcing include the lunar fortnightly (Mf) and lunar monthly (Ms) as well as the solar semiannual (Ssa) and solar annual (Sa) constituents.
The Zytturm is a 13th-century tower in Zug, Switzerland, which houses an astronomical clock. The tower, which is 52 metres high, is located on Kolinplatz in the old town centre.