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: Frank Reed CT
Date: 2003 Dec 23, 17:56 EST
From: Frank Reed CT
Date: 2003 Dec 23, 17:56 EST
Trevor, you quoted:
" "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."
Yes, I would say so.
And you quoted:
"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."
That strikes me as no explanation. Why wouldn't that same logic apply to the lunar tide? There's almost no difference between them. I think we're dealing more with the basic phenomenon of phase lag in resonant systems.
"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?"
Sorry, I don't know any sources that would help. Of course today they just grid up the whole ocean and run a numerical integration. Those calculations yield excellent results, but they don't provide much insight into overall phenomenology.
You also wrote:
"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."
I speculated to someone recently by e-mail that it became popular because it was invented in an era when compasses were more common than clocks.
And you asked:
"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?"
This is one of those things where it works when it works. Suppose I check a tide table and find that for my location, it's always high tide 1 hour after lunar transit. An LHA of +1 hour is very close to an azimuth of 195 degrees over a fairly wide range of latitudes. But suppose my time offset is +3 hours. Then it's not tough to see that you could get fairly large errors even in middle latitudes. Of course in northwest Europe, where this technique seems to have been developed, you're in even higher latitudes, so it works fairly well. But it fails completely on at least some dates anywhere between 28.5 North and 28.5 South latitude. If you would like to experiment with this on your own, I would be happy to post some basic code (it's even IN Basic) that calculates tides by the usual harmonics and also calculates Moon and Sun positions.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
" "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."
Yes, I would say so.
And you quoted:
"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."
That strikes me as no explanation. Why wouldn't that same logic apply to the lunar tide? There's almost no difference between them. I think we're dealing more with the basic phenomenon of phase lag in resonant systems.
"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?"
Sorry, I don't know any sources that would help. Of course today they just grid up the whole ocean and run a numerical integration. Those calculations yield excellent results, but they don't provide much insight into overall phenomenology.
You also wrote:
"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."
I speculated to someone recently by e-mail that it became popular because it was invented in an era when compasses were more common than clocks.
And you asked:
"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?"
This is one of those things where it works when it works. Suppose I check a tide table and find that for my location, it's always high tide 1 hour after lunar transit. An LHA of +1 hour is very close to an azimuth of 195 degrees over a fairly wide range of latitudes. But suppose my time offset is +3 hours. Then it's not tough to see that you could get fairly large errors even in middle latitudes. Of course in northwest Europe, where this technique seems to have been developed, you're in even higher latitudes, so it works fairly well. But it fails completely on at least some dates anywhere between 28.5 North and 28.5 South latitude. If you would like to experiment with this on your own, I would be happy to post some basic code (it's even IN Basic) that calculates tides by the usual harmonics and also calculates Moon and Sun positions.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
