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Joel you wrote: Using an artificial horizon, my sights at angles of 20 to 15 deg. (40 to 30 deg. sextant) have been significantly in error (40 min. to a full degree). Readings at greater angles have been relatively accurate.

Hello Joel

I’m guessing that you’re attempting to observe the Sun from temperate latitudes in the Northern winter.  It’s not easy is it?  All the answers so far have made good suggestions, and probably all are affecting your observations to a greater or lesser extent.  However, they don’t answer your question about why your errors are greater the lower the altitude of the Sun.

I don’t know about you, but I find that the lower the altitude, the harder it is to frame the image, because the light is arriving at such a glancing angle.  It’s a bit like tossing a ball.  If you’re throwing about 60 degrees downward it’s much easier to hit a mark than if you’re throwing at a much shallower angle.  That means that you’ll be happier to accept an image of the sun from anywhere on the mirror, and the bigger the mirror the better. 

That means that if the suggestions about mirror bend (for want of a better description) or the image being well away from the spirit level are correct, your observations are more likely to be incorrect for lower altitudes.  Brad’s correct about needing a solid support, unlike in my photo below.  If you look at the measuring devices of the ancients such as Tycho Brahe, some are as much civil engineering projects as they’re scientific instruments.

Does it matter?  If you’re just building up experience using your sextants, and practicing procedures it probably doesn’t although it would be less frustrating if the error was all too high or too low rather than random.  A liquid isn’t much better.  At low altitudes the edge of the container gets in the way; water gets ripples, oil gets dust, and mercury, if you can get it, is a bit of a liability around the home. My mirror’s silver backed; is about 6mm thick; and was rescued from inside an airman’s wardrobe (they don’t make them like that anymore).  It sits on ¾” birch multiply.  For a level, I use a simple circular level placed on the mirror.  Using this system with my Hughes Mates Three Circle sextant I don’t remember ever getting worse than around 6 minutes of arc.  The good thing is, now practised, when you finally get to a south facing coast and install yourself inside one of those windbreak shelters that people sit in to look out to sea, you should start getting one minute of arc observations from the natural horizon almost immediately.  DaveP

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