Lunitidal interval

The lunitidal interval,^[1] measures the time lag from the moon passing overhead, to the next high or low tide. It is also called the high water interval (HWI).^[2][3] Sometimes a term is not used for the time lag, but instead the terms age of the tide or the establishment of the tide are used for the entry that is in the tidal tables.^[4]

Tides are known to be mainly caused by the moon's gravity. Theoretically, peak tidal forces at a given location occur when the moon is at the meridian, but there is usually a delay before high tide that depends largely on the shape of the coastline, and the sea floor, therefore, the lunitidal interval varies from place to place. It varies from 3% over deep oceans to 8 hours at New York harbour.^[5] 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 shorelines. However, if you are far enough away from the coast, the dominating consideration is the speed of gravity waves, which increases with the depth of the ocean. (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.^[6] As mentioned above, a similar time lag accompanies the solar tides, a complicating factor that varies with the phases of the moon.) 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 moon-rise, moon-set and high tide times are known for a location. In the northern hemisphere, the moon is at its highest point when it is southernmost in the sky. Lunar data are available from printed tables and online. Tide tables tell the time of the next high water.^[7][8] 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)
• Tide clock

References

1. Australian Hydrographic Service definition
2. NOAA HWI definition
3. Proudman Oceanographic laboratory definition
4. 'Beyond the Moon: A Conversational, Common Sense Guide to Understanding the Tides, p. 89, by James Greig Mccully, World Scientific Publishing Company, Jan 13, 2006
5. Beyond the Moon: A Conversational, Common Sense Guide to Understanding the Tides, p. 89, by James Greig Mccully, World Scientific Publishing Company, Jan 13, 2006
6. M.Grant Gross, Oceanography, second edition, Charles E. Merrill Publishing Co., p.114, 1971, Columbus, Ohio.
7. UK Tidal Predictions
8. NOAA Tides & Currents