NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Tides by bearing of the moon
From: Frank Reed
Date: 2009 Apr 7, 21:30 -0700
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 --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---