It has been recognised for over a thousand years that there are relationships between the moon, its phases and times of transit over the local meridian, and the occurrence of the tides. One such relationship is the lunitidal interval.
There is some variability in the meaning of the term lunitidal interval. Accordingly it is necessary to be very careful when using the lunitidal interval. The various meanings are as follows:
1. Lunitidal interval (see also establishment)
- The average time interval between the transit of the moon over the local or Greenwich meridian and the next local high water or low water. It can be described as local or Greenwich, depending on the transit reference meridian; assumed to be local unless otherwise specified.
- The average of all high water intervals is called mean high water lunitidal interval, high water interval, or corrected establishment. Hence, high water (lunitidal) interval (HWI).
- The average of all low water intervals is called mean low water lunitidal interval, or low water interval. Hence, low water (lunitidal) interval (LWI).
- The expressions higher high, lower high, higher low and lower low water intervals can be used when there is considerable diurnal inequality.
2. Establishment (also known as high water lunitidal interval)
- An old term for the interval of time between the transit of the moon and the next high water at a place.
- The average establishment at the time of full or new moon is called vulgar or common establishment or high water full and change (HWF&C).
By virtue of its definition, lunitidal interval can only be used to estimate the tides at places where the semidiurnal component of the tide is dominant.
The nature of the tide in Queensland waters varies from semidiurnal through to diurnal. The semidiurnal tides are closely related to the moon while the diurnal tides are closely related to the sun. Accordingly as the nature of the tide becomes more diurnal, the lunitidal interval becomes less reliable as an estimator of tidal times.
In south east Queensland the tides are semidiurnal. The tide becomes increasingly diurnal toward the north. It is considered that it is not prudent to use the lunitidal interval north from Port Douglas, in the Torres Strait, or in the Gulf of Carpentaria.
Local lunitidal intervals
|Place||Local lunitidal interval||Latitude||Longitude|
|Gold Coast Seaway
|Waddy Point Fraser Island
|Bundaberg (Burnett Heads)
|Mackay Outer Harbour
The times given above are the local lunitidal intervals, that is, they are referred to the local standard time of the moon's transit over the meridian of the place. The time of tide calculated using the local standard time of moon’s transit and the given lunitidal interval is relative to the Australian Eastern Standard Time (150 degrees or 10 hours east from Greenwich).
The lunitidal interval changes through the lunar phase cycle. The variation about the mean is approximately +/- 30 minutes.
Lunitidal interval constants should not be used to estimate the tides for any critical purpose. They are obsolete and do not give tidal predictions which are as accurate as modern tide tables.
There are standard port tidal predictions available all of the places listed above. Accordingly it is no longer necessary to calculate the tidal times from the lunitidal interval constants.
Tidal predictions for Queensland waters are published in the Queensland Tide Tables.
For instructions on how to calculate a rough estimate of the Lunitidal interval refer to http://www.lunitidal-interval.com*.
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