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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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