How to Calculate Lunitidal Interval


A delay often occurs between the time an astronomical event happens and when that event causes something else to occur. When the moon passes over a time meridian, a tide occurs at that location. However, those tides may not happen until a short time after the moon passes over. Scientists call this time delay the lunitidal interval, and you can compute it if you know when the moon rises at a location and when the corresponding high tide occurs.

Determine Moonrise Time

  • Visit the U.S. Naval Observatory’s Sun or Moon Rise/Set Table for One Year Web page.

  • Find form A on that page and enter the location of interest’s state and city in the appropriate text boxes. Even though form B contains locations outside the United States, these instructions are applicable to calculating lunitidal intervals in the United States only.

  • Type the current year in the “year” text box and select “moonrise/moonset” in the “type of table” drop-down menu. The "year" text box allows you to type any year, but you should ensure that you only enter the current year. These instructions can only be used beginning in the current month and going forward 1 year.

  • Click “compute table” to view a table that shows the moonrise times for the current year. Find the month and day for which you want to know the lunitidal interval and write down its moonrise time.

Determine High-Tide Time and Compute Lunitidal Interval

  • Visit the Saltwatertides website and click the state for which you’d like to compute the lunitidal interval. A new page opens and displays locations within the state. Click your location of interest.

  • Click the desired month in the “pick the month for the tides” box.

  • Click the desired date in the “first day for tides” box. Click a value in the “how many days of tides” box. For instance, if you only want to see tides for 1 day, click “1” in that box.

  • Click “get tides” to view a page that shows when high tides occur at the location you selected. Write down or remember the first high-tide time that occurs after moonrise. The difference between that high-tide time and the moonrise time at your location is the lunitidal interval.

Tips & Warnings

  • You can sometimes find a location’s high-tide and moonrise times from a local newspaper in the area.
  • When the moon orbits Earth, the moon’s gravity causes water 'below' it on Earth to move upward to create a high tide. A high tide also occurs on the opposite side of Earth as the moon’s gravity tugs the planet slightly in the moon’s direction.


  • Photo Credit Anson_iStock/iStock/Getty Images
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