NavList:

Frank,

Excellent questions, and welcome to NavList!

Those are two perfect examples for learning how to read a vernier. In the first example, the reading is actually 14°2.0' - not 13°3.0'. Your logic is sound, it's just that it doesn't work that way. Unlike the other four short lines, the fifth one is not added to the minutes indicated by the long line. You can think of the fifth short line as a sort of "double check" that the long line is exactly on the whole minute. Or, you can think of it as 0.0' instead of 1.0'.

In the second example, the reading is actually 17°0.6'. Even though it looks like the index line is to the right of the 17° mark from this viewpoint, we know the reading should be 17° because the minutes are just above 0. This is the same reason we know that the first example is 14°, not 13°. The long line in the second example is roughly halfway between 0 and 1. The short line which aligns best with the minutes scale is the third one, so we do add 0.6'.

It is very easy to mis-read the vernier and even the main scale sometimes. Great care must be taken when reading and recording a sextant measurement. The good thing is: once you take your hand off the micrometer, the reading shouldn't change. So, while you do need to record the time of the shot very quickly, you can record the sextant reading at your leisure. So take your time and double chack that it all makes sense.

It's also a good idea to make sure the worm screw has completely engaged the rack, otherwise you'll get an incorrect measurement. If you've ever turned the micrometer and felt or heard the index arm jump a little, that's the worm screw settling into the rack after not engaging correctly when the quick adjust lever was released. It has been my experience that this happens slightly more often with my Davis than with my Astra or Tamaya. I think that's because of the way the rack is made on the Davis.

Lastly, you probably already know this but I just want to make sure you know that you cannot use a nearby object, such as a roof, as a horizon for actual altitude measurements. You need to use either a large body of water or an artificial horizon of some sort.

Anyway, I hope that helps!

Cheers,

Sean C.