NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: New Moon, Perigee, and Solstice
From: Trevor Kenchington
Date: 2003 Dec 22, 21:02 +0000
From: Trevor Kenchington
Date: 2003 Dec 22, 21:02 +0000
Frank, You wrote: > 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. Interesting. I had always supposed that the Age of the Tide (to give this lag its name) resulted from the resonant systems taking some time to respond to the extra energy being poured into them when the Sun and Moon are aligned -- a matter of friction bleeding away the energy as gravity pours it in, with some inertia in the resonating system (an explanation which will not satisfy any physicists!). However, on checking the Admiralty Manual, I see that it favours your explanation: "The "age" of the tide is due to the fact that on average over the earth the solar tide lags behind the solar forces by a greater amount than the lag of the lunar tide behind the lunar forces." Which I think is the same as your explanation albeit in different words. However, the Manual continues: "It is possible that this phenomenon [meaning the greater lag of the solar tides, not the Age of the Tide directly] is due to dissipation of energy in the coastal fringes." Which approaches my supposed explanation, without being quite the same. However, the Manual is over 60 years old now and there must have been a lot of relevant research in that time. Can you suggest somewhere I could go for a fuller account of the cause(s) of the Age of the Tide? > 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). I guess some things take a long time to die. That method of tidal prediction was current in Elizabethan times and likely back into the Middle Ages if not before. But how inaccurate is it when used in temperate latitudes? Or, more usefully, how precise does your estimate of the Moon's azimuth need to be before constancy of estimated azimuth is an insufficient approximation to constancy of LHA? Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus