Expert: Courtney Seligman - 3/25/2011

Question
Mr. Seligman:
I was hoping you could speak to me about the relationship between the moon and tides.
I am a reporter from The Palm Beach Post writing a dispute over a damaged boat. The boat owner sought the services of a tow company to move his boat. He says that because of a "super moon" the tide was higher than usual and the tow company damaged his boat by carrying it under a bridge it ordinarily would have fit under EXCEPT for this unusual moon.

Of course, I would not expect you to comment on damage to the boat, only about the moon and water level.

I am on a deadline today, Friday March 25.

Thank you.
Laura Green  

Answer
The super full moon (or for that matter, any full moon which takes place near perigee) is only about 6% closer to us than normal, and only causes lunar tides about 18% larger than normal. The average lunar tide is only a foot (although it varies from place to place, so you might need to consult a tide table for the harbor in question), so the extra height of the tide couldn't have been more than two or three inches greater than a typical "spring" tide (which occurs at every new or full moon, regardless of the Moon's distance).

So unless the boat clipped the bridge by less than 3 inches, the super full moon wouldn't have had anything to do with it. Instead, it would be stupidity on the part of the tower, as you're supposed to do such jobs when the tide is near or below normal, not when it's at high tide.

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Added comment: Re-reading your question, if you wanted a more detailed discussion of the tides, the average tide varies by about a foot from average sea level. When it is near new or full Moon, the solar tide (which is half as large as the lunar tide) adds to the lunar tide, and the average tide is about a foot and a half (and is called a spring tide). When it is near quarter Moon, the solar tide subtracts from the lunar tide, and the average tide is only half a foot (and is called a neap tide). So as noted above, towing a boat under a bridge is safer between high tides, and a little safer when at normal or neap high tide, instead of near spring tides. But we're talking about a change in height (from high tide to low tide) of up to three feet, so the two or three extra inches from a high tide at a spring tide at perigee (the situation for the "super" full moon) is insignificant in comparison to other (normal) variations in the height of the tides.

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Finally, although it doesn't apply directly to your question, there is a discussion of the (Not So Super) Full Moon of March 19, and some illustrations of perigee and apogee moons on my web page, Pictures of the Moon, at http://cseligman.com/text/moons/earthmoonpix.htm