The lunitidal interval measures the time lag from lunar
culmination to the next high
tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another.
Tide tables ...
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 table
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 (between high and low water) can be approxi ...
s.
Tides are known to be mainly caused by the
Moon's gravity
image:MoonLP150Q grav 150.jpg, 300px, Radial gravity anomaly at the surface of the Moon in Gal (acceleration), mGal
The gravitational acceleration, acceleration due to gravity on the surface of the Moon is approximately 1.625 m/s2, about 16. ...
. Theoretically, peak tidal forces at a given location would concur when the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
reaches the
meridian, but a delay usually precedes high tide, depending largely on the shape of the coastline and the
sea floor. Therefore, the lunitidal interval varies from place to place – from 3 hours over deep oceans to eight hours at
New York Harbor. The lunitidal interval further varies within about +/-30 minutes according to the lunar phase. (This is caused by the time interval associated with the solar tides.)
Hundreds of factors are involved in the lunitidal interval, especially near the shoreline. However, for those far away enough from the coast, the dominating consideration is the speed of
gravity waves, which increases with the water's depth. (It is proportional to the square root of the depth, for the extremely long gravity waves that transport the water that is following the Moon around the Earth. The oceans are about 4 km deep and would have to be at least 22 km deep for these waves to keep up with the Moon. As mentioned above, a similar time lag accompanies the solar tides, a complicating factor that varies with the
lunar phases.) By observing the age of leap tides, it becomes clear that the delay can actually exceed 24 hours in some locations.
The approximate lunitidal interval can be calculated if the moonrise, moonset, and high tide times are known for a location. In the
Northern Hemisphere, the Moon reaches its
highest point when it is southernmost in the sky. Lunar data are available from printed or online
tables
Table may refer to:
* Table (furniture), a piece of furniture with a flat surface and one or more legs
* Table (landform), a flat area of land
* Table (information), a data arrangement with rows and columns
* Table (database), how the table d ...
. Tide tables forecast the time of the next high water.
NOAA Tides & Currents
/ref> The difference between these two times is the lunitidal interval. This value can be used to calibrate certain clocks and wristwatches to allow for simple but crude tidal predictions.
See also
* Phase (waves)
References
{{DEFAULTSORT:Lunitidal Interval
Physical oceanography
Tides