A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bob Goethe
Date: 2015 Nov 23, 15:00 -0800
I used a whole horizon mirror for 31 years and was perfectly happy with it. All of my sights were of sun/moon/planets or Sirius (i.e. the brightest star in the sky). You can do a lot with those.
Then I got to crew on a 45 foot boat sailing from Hawaii to Victoria, Canada, and I tried my hand at a number of star sights. What I found was that the whole-horizon cut the brightness of objects viewed. This was true all along, of course. But I had never particularly noticed it when shooting the sun/moon/Venus, but with any star OTHER than Sirius I discovered that it was indeed an issue. The week after I got home, I ordered a split-horizon mirror for my Tamaya.
It was a pain in the butt to switch mirrors in the frame, working with the tiny screws and clips. My fat fingers had a real problem with them.
But having gotten the new mirror in, I really LIKE being able to use pretty much any star I want. Want to try shooting Alnilam, the middle star in Orion's belt? If you have a split-horizon mirror, then help yourself. If you have a whole-horizon, good luck.
The other thing was that when using the whole-horizon, particularly if the object was rather higher up in the sky, I spent what felt like a lot of time rocking the instrument, trying to ensure that I was holding it vertically. With the split horizon mirror, it is rather more obvious if I am not holding the sextant properly...and easier to correct if I am not.
So I agree that it is a personal preference. But I think that personal preference is going to be driven by the kinds of celestial objects you elect to shoot.