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Subject: Visibility at rising and setting
From: George Huxtable (george@XXX.XXX)
Date: Tue Apr 25 2000 - 11:24:19 EDT
The May issue of the UK magazine Practical Boat Owner carries a short
article entitled "sights without a sextant" by Alastair Buchan. This
suggests that you can make worthwhile astro observations of Sun or Moon,
perhaps even stars and planets, to obtain a position line without a
sextant, by timing the moment at which they are tangent to the horizon at
rising or setting.
There are well-known weaknesses in this approach, which I propose to write
in to point out.
However there is one claim made that goes way outside my own experience,
and perhaps I can call on the collective wisdom of this mailing list for
help. The author states "I've never tried it myself, but given a dark sky
and a clear hard horizon, it should be possible to take horizon sights of
stars and planets - although deciding when they're exactly at tangency may
involve more of a guess than observation."
Clearly, there's a special problem at rising, in that the star or planet is
invisible before rising, and you have little idea beforehand just when and
whre it's going to happen, so let's confine ourselves to settings.
What I'm asking is whether any of you enjoys clear enough skies that they
can observe stars or planets, even Venus at its brightest, right down to
being able to time a sudden moment of extinction as it sets below the
horizon. As a navigator in the waters of NW Europe, with its notably hazy
horizons, I am rather at a disadvantage in this respect. I am somewhat
sceptical that it is ever feasible elsewhere, but I look forward to
receiving the comments of others. Presumably, one could be certain that the
light from star or planet really was being intercepted by the now-dark
horizon itself, if it were being occulted on and off a few times, by
distant waves or slight swell, as it set, similar to observing a distant
lighthouse.
I am setting aside, for the moment, the unpredictability of the refraction
correction which bedevils all low-altitude observations.
Yours, George Huxtable
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george@XXX.XXX
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel, or fax, to 01865 820222 or (int.) +44 1865 820222.
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