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From: George Huxtable (no email)
Date: Sun Dec 11 2005 - 16:31:21 EST
Frank Reed expressed surprise that in 1615 nobody had proposed to compare
Moon observations, made at an explorer's home base, with corresponding
observations that he had made at about the same moment, in the field. That
would avoid the need to rely on any predictions of the Moon's position, in
determining longitude.
I wrote-
> "Frank mentions "precision naked-eye observatories". But that in itself
> is
> somewhat contradictory."
Frank replied-
> Tycho's observations were accurate to around 1 or 2 minutes of arc.
> That's
> what I mean by a "precision naked-eye observatory". Tycho was a singular
> exception in Europe at this time. Why?
===========================
Tycho's observations were remarkably good for his time. He did indeed claim,
for some of these observations, that they were to within an arc-minute or
two. But that related to the precision with which his scales could be read,
and the resolution that the unaided humnan eye could achieve, viewing with
slits and peeps. The overall division of the scales of his instruments
didn't achieve a corresponding precision, however, though for Tycho's era,
it was by far the best there had ever been..
However, to go with that precision a certain degree of understanding was
required, that was achieved only later. For example, it required a much
better knowledge of how to correct for refraction, a study to which Tycho
contributed, but was vastly improved-on a century later. Tycho's Sun
observations were corrected on the erroneous belief that Sun parallax was 4
arc-minutes (based on ancient Greek notions of the distance of the Sun) and
"corrected" on that basis. We now know it is only about 9 arc-seconds. So
recent studies of Tycho's work have had to back-correct his recorded
observations on that basis.
I don't wish to denigrate or belittle Tycho's work. For his time, his
achievements were amazing. But the notion that his positions were "good to 1
or 2 minutes of arc" should be dispelled.
========================
> Clearly it wasn't a technological issue. It
> was a conceptual problem. The idea of simultaneous observation simply
> didn't
> occur to Baffin or any of his contemporaries. Or if it did, they couldn't
> convince anyone else to fund the project.
==========================
Well, that notion had occurred to the Greeks, using Moon eclipses, but their
determination of the length, in longitude, of the Mediterreanean was many
hundreds of miles in error. That notion also occured to an English
astronomer, Gellibrand, a few years after Baffin, who proposed that an
explorer, James, should view a 1631 lunar eclipse, and compare it with
Gellibrand's observations in London, where it would also be visible (if the
weather was favourable). Fortune smiled, and a reasonably good longitude was
found for Charlton Island, in James Bay, at the South end of Hudson Bay.
See Miller Christy, "Voyages of Captain Luke Foxe of Hull...", Hakluyt
Society, 1894.
=========================
> And:
> "Another difficulty that affected observatories was the lack of proper
> clocks, until Huyghens invented pendulum clocks."
>
> Both local sidereal time and local apparent time are observable
> parameters.
==========================
Not always. It depends on the latitudes of the two stations, and their
longitude difference, and the time of Moon meridian transit. Take as an
example Baffin's first "culmination lunar", in 1612. If a check had been
attempted on that, at London, to determine the time of Moon culmination
there by Sun altitude, the Sun would have been only 1 degree above the
horizon. Probably not visible above the local horizon, and certainly not
amenable to precise time determination.
==========================
Frank continued-
> And you wrote:
> "And precise calculation was impossible until the laws controlling the
> planet's motions were understood, which had to wait for Newton."
>
> No Newtonian calculations are required here. If the goal is MAPPING, then
> you don't need the ability to predict the positions of the Moon and other
> celestial bodies. You observe them in two longitudes and compare later.
===========================
Frank is right about that. You only need a good model of the Moon's motion
to predict what it will do in the future. If all that's needed is to
interpolate between observations on consecutive days, an inaccurate model
(in terms of epicycles rather than ellipses) will probably be good enough.
But the Moon's motion against the stars is so fast and so complex, that
trying to bridge between observations by interpolation gets more inaccurate
as the gap gets longer. And that's inevitable when an imprecise (or wrong)
model is used.
=============================
Frank continued-
> And you summed up:
> "So Frank is telescoping more than a century of scientific and technical
> achievement into nothing, with that "what if"!
>
> Nope. There's no question that the tools and techniques were available in
> Baffin's time. But there was no one with the insight to apply them to the
> task.
=============================
It needed more than insight. Baffin may well have realised what was needed.
But the commitment required, of having an observatory in London, devoted to
measuring Sun altitude at every visible Moon culmination, would have been
huge, and disproportionate, just for the purpose of supporting Baffin. As a
national effort, that would have been another matter: and it was, 50 years
later, when Greenwich Observatory was started.
What's more, Baffin's observations using culmination were doomed to failure
until techniques for determining the moment of Moon culmination had been
improved, or lunar distances had become possible. So any such effort, back
in London, would have gone to waste. Inaccurate Moon predictions were only a
part of Baffin's problems.
George.
contact George Huxtable at
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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