A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: New Moon, Perigee, and Solstice
Date: 2003 Dec 22, 08:06 -0500
Date: 2003 Dec 22, 08:06 -0500
Here are tide tables for the Bay of Fundy. Living in downeast Maine, I occasionally reference them, but will allow someone else to calculate any lag. Sharon ----- Original Message ----- From: "George Huxtable"
To: Sent: December 22, 2003 6:16 AM Subject: Re: New Moon, Perigee, and Solstice Frank Reed made the interesting comment- >These tides bring to mind something I noticed in old editions of Bowditch. If >I am remembering correctly, Bowditch from c.1800 says that the Spring Tides >(tides with maximum range) occur about three days after New Moon and three days >after Full Moon. That's not accurate in New England where the difference is >less than a day, but it is fairly accurate in northwest Europe which has unique >resonances. Bowditch, of course, copied earlier works and carried over lore >regarding European tides. So when did they fix it? Is there a 19th century >edition of Bowditch that has better basic descriptions of tidal phenomena? Response from George- I agree with Frank that coastal resonances must play a significant part in this tidal lag, where in many parts of Europe the highest tides follow a couple of days after the time of maximum tide-generating force. But are those in Europe "unique resonances"? I doubt it. What about that resonance toward the head of the Bay of Fundy, which produces the biggest tide-range in the World? I would guess that the spring tides, at the head of the Bay of Fundy, might well lag behind new-moon and full-moon just as much as they do in parts of Europe. But I don't have tide-tables for that part of the world. Does anyone have data to confirm (or disprove) that guess? Many think about electricity as an analogy to hydraulic flow, in terms of currents and pressures. On the other hand, as I have been involved in electrical measurements for most of my life, I draw my analogies the other way round, and think of such tidal phenomena in terms of electrical tuned circuits, such as you get in a radio. A tuned circuit has a factor, always known as its "Q", which describes its sharpness of tuning, and also its ability to enhance a driving voltage that's right at the frequency it's tuned to. It also shows a time-lag in its response to changes in such a driving waveform, which as I recall is just Q radians (which would correspond to about Q/12 days for a normal semidiurnal tide). In more complex situations, when one tuned circuit (or tidal basin) transfers energy to another, their lags can add. This information may be particularly useless for many Nav-l listmembers, but may ring a bell with others familiar with electrical matters. Someone may be able to tell me if my memory of these things is still about right, after many years of retirement. George ================================================================ contact George Huxtable by email at george---.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================