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From: Peter Fogg (no email)
Date: Fri Mar 10 2006 - 18:18:45 EST
Greg wrote:
> > Can anyone tell me why my sextant has two sunshades?
> > I have a MAC sextant with four shades:
> > 1 'redish' , 1 'greenish' , and 2 that are so dark as to only
> > be for the sun - why two? , also what is the reason for the red
> > & green ones?
and Bill responded helpfully, pointing out how different colours in varying
combinations can reduce light transmission by differing amounts.
To that can be added that all the possible combinations of celestial objects
and horizons that may need brightness adjustment could well be almost
infinite, and the tools to hand are the shades - just as a trumpet has few
keys to press but can emit an almost infinite variety of tunes.
As a practical example, measuring the altitude of the moon above a sea
horizon can be difficult. The moon, while much less bright than the sun, is
still potentially too bright to view in combination with the horizon, which
can be a line separating a dark sea from a sky almost as dark or even
possibly darker. When this horizon line is some way beyond the shimmering
reflections of the moon the task is even more difficult (why this happens
has been discussed in the past here).
So in this case some experimentation with the shades is called for to find
the best combination for the job. And this, I think, is the bottom line -
experience with your sextant and celestial objects will in time make you an
expert on what works best with your outfit.
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