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    Re: Tides by bearing of the moon
    From: Frank Reed
    Date: 2009 Apr 7, 21:30 -0700

    Hi Dave.
    
    Yep, that's an old standard! Here's what I wrote on the topic during my first 
    month as a NavList member in December 2003:
    "Speaking of tides, has anyone encountered boaters who use an azimuth rule for 
    tide prediction? There was an article last summer in "Sail" magazine that 
    suggested using an azimuth trick as a rule-of-thumb for predicting the tides. 
    The author claimed that you could check a tide table for the time of low or 
    high tide and then just record the Moon's compass bearing at the time of the 
    given tide phase, and the Moon's altitude would not matter. He claimed that 
    whenever the Moon returned to that compass bearing, the tide phase would be 
    the same (e.g. Moon at azimuth 240... it must be low tide). As many people on 
    this list know, that doesn't work in general. The quantity that matters is 
    the Moon's hour angle. Here in southeastern Connecticut, it's low tide about 
    3.3 hours after lunar transit [very convenient for digging clams]. So when 
    the Moon's local hour angle is 50 degrees, it's low tide. 
    
    Of course there are times/places when compass bearing is roughly the same as 
    hour angle. When you're in high latitudes, or when the LHA is not large, the 
    difference between azimuth and LHA is also not large. For example, at either 
    pole, a difference in azimuth is exactly equal to a difference in hour angle. 
    But of course as you reach the tropics and especially at the equator, no rule 
    based on compass bearing will work. It's just impossible. But if you can 
    picture a line of constant LHA in the sky (and I think most people can be 
    taught to do that using the North Star as a starting point), you can predict 
    the times of the tides to tolerable accuracy just by looking for the Moon in 
    the sky. The time offset is different for every location, and tricks like 
    this do NOT work in regions with "mixed tides" (like most of the Pacific 
    Coast of North America), but they do work very well in any part of the world 
    that has stable semi-diurnal or diurnal tides."
    
    By the way, the problem with this rule is essentially the same as the problem 
    with using the hour hand of a watch as a compass. Both tricks equate azimuth 
    with local hour angle. When the body involved (Sun or Moon) is relatively low 
    in the sky, it's a fair approximation. But otherwise it can be a poor 
    approximation and frequently quite useless in the tropics.
    
    -FER
    
    
    
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