NavList:

I think you’re correct. I leaped before thinking it all the way through.
This - adding the right side of the vernier (0-10) up to the half way past a degree mark on the arc and then, after the halfway point on the arc, adding the left side of the vernier (10-20) until you get to the next degree mark on the arc is nicer. No subtracting.

So, why?

In spite of the eBay seller’s unproven claim of “the 1700’s”I’ll consider that the maker of this octant knew about the normal vernier scale and was trying to accomplish something else.
I wondered if he, or she, was trying to cover 90 degrees without having to accommodate the vernier to the left of zero. But- he, or she, has added 5 degrees to both ends of the arc so no net gain of space.
I like David’s idea that the maker had already made a mark and didn’t want to start over. I can imagine a 1700’s “Arrrgh!” from the workbench.
Looking cool is always a good reason too.

Any opinions on “the 1700’s” claim?
Regards, Noell

Noell

Did you notice the other difference? Most Verniers fit n divisions on the Vernier bit into n-1 divisions on the main scale.  This one does the opposite.  It fits n+1 divisions on the main scale into n divisions on the Vernier bit.  I.e. the vernier divisions are a touch wider than the main scale divisions.  This aspect is, I believe, called a ‘retrograde Vernier’.  I can claim no credit for this amazing discovery.  You can read about the ‘retrograde’ bit in Wikipedia.  DaveP