How to calculate lunitidal

Written by soren bagley
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How to calculate lunitidal
(Image courtesy U.S. National Oceanic and Atmospheric Administration/Department of Commerce)

The lunitidal is the time difference between when the moon passes over a meridian and when the corresponding tide reaches its peak. In theory the two should happen simultaneously, but due to factors such as friction there is some lag time between when the moon passes the meridian and when the tide meets its highest point. The amount of this lag is what is known as the lunitidal or the lunitidal interval. Being able to calculate the lunitidal interval will allow you to predict more accurately when high tide will occur.

Skill level:

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Things you need

  • Internet access
  • Paper
  • Pen or pencil

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    Select your desired location, then click "See moonrise/moonset."

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    The resulting table lists moonrise/moonset times for the next seven days. To see an entire month's worth of moonrise/moonset times, go to the "Modify Parameters" box at the top of the table and click "Show." You can also use this box to select a different month or year from the current one(s).

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    Once you have generated a table that lists your desired date, look at the "Time" column under "Meridian Passing" (immediately to the right of the "Moonset" column). The time listed for the date in question is when the moon will pass over the meridian on that date. Write down this time, along with the date.

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    From the sites listed, click your desired location.

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    The resulting table lists tidal information for the next two days. If you want information for a different date, scroll down to "Prediction Options" and reset the fields under "Starting time and time display options" for your desired date. Then scroll to the bottom of the page and click "Make Prediction Using Options."

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    Once you have generated a table that lists your desired date, check it to find the first high tide occurring after the 24-hour meridian time you wrote down earlier. Write down the time of the high tide, along with the date.

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    In the "First date and time" field, enter the 24-hour meridian date and time that you wrote down earlier.

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    In the "Second date and time" field, enter the high tide date and time that you wrote down earlier.

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    Click "Calculate." The resulting time will be the lunitidal interval.

Tips and warnings

  • Remember, the moon often rises on one date and sets on another. (For instance, it may rise at 5:10 PM on August 1 and set at 2:55 AM on August 2.) To minimise confusion, keep careful track of dates when checking tables and noting times.
  • Weather conditions can have a profound effect on tides and currents.

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