A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: New Moon, Perigee, and Solstice
From: Dennis Koehler
Date: 2003 Dec 22, 19:58 -0500
From: Dennis Koehler
Date: 2003 Dec 22, 19:58 -0500
Hi Frank; please remove me from you e-mail list. Thanks...Dennis
----- Original Message -----From: Frank ReedSent: Monday, December 22, 2003 3:27 PMSubject: Re: New Moon, Perigee, and SolsticeGeorge H wrote:
"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?"
Every tidal component in the driving force produces a response in the ocean basins based on their particular resonances, which are quite complicated. In most parts of the world, the solar tides and the lunar tides have responses to the driving force that are very similar. That is, given that the solar driving force is about 40% of the lunar force on average and only slightly different in frequency, we can usually count on similar responses --similar driven amplitude, similar phase difference. So in most parts of the world, the effect of the Sun's tide will follow the Sun's transit by almost exactly the same number of hours that the effect of the Moon's tide follows the Moon's transit. And because of that, you usually get a maximum combined tide (Spring Tide) when the Sun and Moon are aligned in the sky at New Moon (or 180 degrees apart at Full Moon). But suppose the solar component has a very different resonant response. In extreme cases, this can lead to complicated tides that follow no simple rule. In the case of northwest Europe, the result is that the solar tides lag the lunar tides and Spring Tides occur about three days after New Moon and Full Moon.
" 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."
They just might, but my point was that this is not a general rule. It works in the North Sea and in neighboring parts of northwest Europe. There are other places in the world where it will also work for no other reason than chance. But it doesn't work in New England (where Bowditch lived and sailed), and it doesn't apply generally in other parts of the world. One can safely say that Spring Tides occur close to the time of New Moon/Full Moon with an offset of a day or even more that depends on the local tidal conditions. Bowditch wasn't aware of that fact in early editions and gave a rule derived in Europe, but it's also true that there was very little science available at that time. Maybe it was inevitable that he would defer to European sources on the tides. Still, it's hard to imagine that locals in the Boston area, after nearly two centuries of European habitation, hadn't figured out that the local rules from Europe didn't match the local rules in New England.
Norie has a chapter on tides, doesn't he? What does he say about the timing of the Spring Tides? And is there a point during the 19th century when the rules are modified?
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.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois