Koen, I'm not certain, because I haven't looked into it, but it would make sense to me that the visiblity vs obscured, and full armlength -i.e. cubit, or half-cubit etc. might refer to distance .. from the horizon, or from the zenith or whatever..
A little unusual because the 'isba or finger-breadth was more usual. But I think something connected with measures might be the go.
