__What it tells you__ is that the distance run is equal to the distance from the object when it is
abeam.

__This works because...__ Bowditch says so. The mathematics involved are a little complex, but here's
a shot at it, the explanation to follow may be entirely wrong, but this sounds like it should make sense. The example and
diagrams should be perfectly correct though.

tan x = opposite / adjacent

cot x = adjacent / opposite

(see right for further explanation)