Lunitidal Interval

Even Pirates have Birthdays; fortunately Mrs Hook has taken very seriously my woes about crossing the start-line last everytime. I now have a sailing watch to rectify this! [thank you Mrs Hook]

Now it is not only the countdown alarm which is going to be useful for telling when to cross the startline, but it has the very handy facility that shows you what the tide is where you sail. From my earlier blogs you'll know it is very important to get this right. All I had to do was input

1. GMT Differential +0.0


2. Longitude 1'0333'' E
   
3. Lunitidal Interval ??
   

If you are like me you'll have no idea what the Lunitidal Interval is and why should you because I don't think anyone does unless they buy a watch that displays what level the tide is at.

Still guessing? [so am I]

The Lunitidal Interval is the average time difference between when the
moon passes over a time meridian and the following high or low tide. If the time meridian referenced is the local time meridian, then the lunitidal intervals are called the "Local Intervals." If the time meridian referenced is the Greenwich meridian, the lunitidal intervals are called the "Greenwich Intervals."

Generally, the manufacturers of such items use the Local Intervals for their calculations. Our office does not track or maintain the
Local Intervals; we calculate and track the Greenwich Intervals for various locations. This information can be found in the Datums
section of our website.

The Greenwich Intervals we provide can be converted to the Local Interval using the following formula: Local Interval = GI - (0.069 * L)
Where L is the longitude for the location in degrees. West longitudes are positive; East longitudes are negative.Example: 122 degrees 23.7 minutes West would be
122.395 degrees. (23.7 / 60 = 0.395)
Longitude for a station is available from the Accepted Datums page by using the link provided for further station information. Where GI is the Greenwich Interval. To calculate the High Tide Local Interval, use the Greenwich Mean High Water Interval in Hours from the Accepted Datums page. To calculate the Low Tide Local Interval, use the
Greenwich Mean Low Water Interval in Hours.

Some products will require the Local Interval to be a positive number. You can convert a negative interval to a positive by adding 12.42 to the result.For example, if your interval calculated to be -4.11; you could also use 8.31 (-4.11 + 12.42 = 8.31)

The Highest Tide normally refers to the highest tide for a particular year. For practical purposes, there is no single "highest tide" for a year. Depending on the location, there will be between 2 and 8 days during the year when the high tide is at it highest predicted height for that year (with a precision of 0.1 foot). We do not attempt to track which day(s) this will occur on as the date and time may be different for each location. You may determine the date of the "highest tide" for your uses from the High/Low Tide Predictions section of our wesbite. After selecting the station you are interested in, you will be provied the times and heights of the high and low tides for that location through the entire year. In general, the highest high tides and lowest low tides occur around the dates of the New Moon and Full Moon; these are known
as the "spring tides". Dates of the New Moon and Full Moon can be found in the Astronomical Data section of our website.