Douglas Denny ended his last posting with a farewell, writing-
"I object for instance to being dismissed out of hand with an offahnd and
glib: "clear indication I have not tried it" - when I have. So I think it
time to leave."
Well, it's a pity to lose a new member who is clearly interested in the
topic, although reluctant to consider views other than his own. I sympathise
with his objection to the phrasing of the words he complains about.
But just in case he hasn't quite closed his eyes just
yet, and to put the
record straight, I will have a go at addressing some aspects of that
departing posting that seem clearly wrong to me.
It's helpful that he has identified his Hughes sextant as a micrometer
instrument, in which case the "platinum divided scale", indicating just
whole degrees, serves not much more than cosmetic purposes. The provision of
a calibration certificate becomes useful in correcting for any scale errors,
and provided those corrections are applied and not ignored (and they vary
smoothly over the arc) imply that any such scale errors do not degrade the
overall accuracy of the instrument. The extent of the maximum correction, of
1' 30", does not of itself prevent the instrument being read and used to
10".
Although my old Vernier sextant has a Kew certificate of zero measurable
error (which I think implies no more than 10",) at all calibration points, I
would be
just as happy to use Douglas's Hughes sextant, after making the
appropriate corrections. A corrected error isn't an error at all.
We were informed that the attempted tests, which failed, were tried using a
sextant that was clamped in place, not hand-held, and I suggest that was one
reason for the lack of success. It prevents the sextant being "swung" about
, to achieve proper coincidence. Indeed, it's hard to see how a user would
achieve that alignment in the first place, before tightening a clamp. And
than, that plane will swing around the Pole, at 15º per hour. No the sextant
should have been used held in the hand, just as it's designed for. Mariners
measuring lunars at sea, for whom such clamping was impossible, had to put
up with the bodies in view "dancing around" far more than did Douglas on
land, who complains "Some form of rest at the very least is needed, or a
clamp with some degree of
freedom. The stars dance about too much for
accuracy otherwise." That's exactly the motion that a sextant is designed to
cope with. I have tried to make my view clear, that measuring star-star
lunars is tricky, calling for skill and practice, and a clamped instrument
provides no easy answer.
Douglas is clearly wrong in suggesting that - " The resolving power of the
telscope is not the issue here, but the ability to superimpose star
images - accurately. In other words: eye acuity is the issue.".
And elsewhere, he goes into different aspects of human-eye acuity in some
detail. But all that is relevant only if he is using a peep-tube rather than
a telescope. That's the purpose of using the telescope, to enhance the
acuity of observation, by a factor of approximatly the magnification of the
telescope. So those acuity values we were given should have been improved by
the factor of magnification,
3 or 5, or whatever. I am surprised that an
optics professional has failed to take such matters into account.
==================
Let's turn to Douglas' views on lunars, about which he seems less than
expert. He wrote
"Lunar distances were very poor indeed." (whatever that means) and quotes
Dyson, in 1922 stating that mariners using lunars couldn't do better than
about 20 miles. Which is in my view, is a fair assessment, but of course to
measure a lunar to 20 miles or 20' at the equator requires measuring an
angle to within 40 seconds, under at-sea conditions. Which already
undermines Douglas' claim that one can't do better than 1', even from on
land.
He continues- "And this means with use of the reflecting circle too - more
accurate than a sextant - eliminating some of the inherent errors of the
sextant.". Nonsense! Dyson was writing about sextants. The circle was a
proposed solution
hardly ever used by navigators at sea, though
occasionally, until the early 19th century, by surveyors and hydrographers.
It was doomed when machine division of sextants arrived around 1780.
He adds "It took at least an hour for the astronomers on board ship when the
method was tested to 'clear the distance' to find longitud", Also nonsense!
The clearing process wasn't what took the hour's calculation. That was the
time it took to calculate the position of the Moon in its orbit, and the
need for doing that disappeared when precalculated lunars were published in
the first Nautical Almanac of 1767.c
George.
ontact George Huxtable, at
george@hux.me.ukor at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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