NavList:

   

Reply

Gary LaPook [6732] and Bill Morris [6734] have both added insights about the 
upper limit of angle that can be measured by sextants (or quintants).

Some of what follows has been said before, on this list or its predecessor, 
but it may bear repeating.

When the horizon view-line of a sextant is, as usual, set to horizontal, 
objects close to the direction of the line joining the two mirrors would be 
nearly edge-on to the index mirror, and the view in that mirror would have 
shrunk to a letter-box slot of almost nothing. So the slope of that line is 
the limiting factor, depending on how narrow a slot the observer is willing 
to tolerate. There's no point in extending the calibrated arc further than 
that practical limit, though for many sextants, that's (pointlessly) the 
case. That slope is governed by the design of the frame; the nearer the line 
between the mirrors is to the horizontal, the greater the range of angles 
that can be observed. The tilt of the horizon mirror must, of course, be 
preset to correspond.

To reduce the slope of the line between the mirrors requires, first, 
minimising the vertical offset between the telescope centre-line and the 
pivot-axis of the index arm. There's a practical lower limit, depending on 
the aperture of the telescope, which still allows an unobstructed light path 
with room for the index shades. Having done that, the next step is to shift 
the horizon mirror further away from the observer. And that's the feature 
that usually distinguishes a quintant, for which, to earn its name, it 
should be practical to measure angles to at least 144�. The horizon mirror 
is often mounted on a small outrigger, so that it's offset; still centred in 
the telescope view-line, of course, but now further away along that line.

You can often identify a proper quintant, at a glance, just by the 
positioning of the horizon mirror. But don't be misled into judging it by 
the number of degrees divided on the arc. Find the angle setting, near 
maximum, at which the mirror-view, though shrunk, is still usable. That's 
what matters. Of course, the divisions must extend at least that far.

============================

However, it seems to me at least plausible for a 2-mirror instrument (which 
couldn't, any longer, be called a sextant) to be designed, which could 
measure angles from zero to 180�, though nobody ever seems to have done so. 
That's material for another posting.

George.

contact George Huxtable, now at george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. 


--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply