Tide clock
Encyclopedia
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 percent) of the combined lunar and solar tide
s.
Tide clocks keep an approximation of the average time between high lunar tides: 12 hours 25 minutes per revolution. As the precise average time between high lunar tides is actually half of a lunar day
, or 12 hours 25 minutes 14 seconds, tide clocks gain about 15 minutes per month and must be reset periodically.
The bottom of the tide clock dial is marked "low tide" and the top of the tide clock dial 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 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".
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"). 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. Along shorelines where both components are important, 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. All navigators use tide table
s either in a booklet or on a computer
.
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...
that keeps track of the Moon's apparent motion around the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. Along many coastlines the Moon contributes the major part (67 percent) of the combined lunar and solar tide
Tide
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....
s.
Tide clocks keep an approximation of the average time between high lunar tides: 12 hours 25 minutes per revolution. As the precise average time between high lunar tides is actually half of a lunar day
Lunar day
In space exploration, a lunar day is the period of time it takes for the Earth's Moon to complete one full rotation on its axis with respect to the Sun. Equivalently, it is the time it takes the Moon to make one complete orbit around the Earth and come back to the same phase...
, or 12 hours 25 minutes 14 seconds, tide clocks gain about 15 minutes per month and must be reset periodically.
The bottom of the tide clock dial is marked "low tide" and the top of the tide clock dial 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 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".
Tides have an inherent lead or lag, known as the lunitidal interval
Lunitidal interval
The lunitidal interval, measures the time lag from the moon passing overhead, to the next high or low tide. It is also called the high water interval /Tides are known to be mainly caused by the moon's gravity...
, 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
Lunar month
In lunar calendars, a lunar month is the time between two identical syzygies . There are many variations. In Middle-Eastern and European traditions, the month starts when the young crescent moon becomes first visible at evening after conjunction with the Sun one or two days before that evening...
, 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
Full moon
Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...
and the new moon
New moon
In astronomical terminology, the new moon is the lunar phase that occurs when the Moon, in its monthly orbital motion around Earth, lies between Earth and the Sun, and is therefore in conjunction with the Sun as seen from Earth...
. The two tides are unsynchronized near the first and last quarter moon
Lunar phase
A lunar phase or phase of the moon is the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun...
(or "half moon"). 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. Along shorelines where both components are important, 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. All navigators use tide table
Tide table
A tide table, sometimes called a tide chart, is used for tidal prediction and shows the daily times and height of high water and low water for a particular location...
s either in a booklet or on a computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
.