Item: They’re renovating the house across the street from us; a
big job with the basement enlarged and the whole structure raised a few
feet. The trouble is, from our front porch it looks like it’s
ever-so-slightly tilted. Item: I was doing some photo-editing and
having a little trouble getting one shot satisfactorily leveled. Those who’ve
worked with Photoshop or equivalent know that a tilt of much less than 1º is
obvious to an attentive eye.
[The comments on this piece are remarkable; you might a perusal rewarding.]
I wonder what evolutionary pressure drove the human mind’s exquisite sensitivity to the condition where a line deviates slightly from the “horizontal” or “vertical”, that is to say whose angle is almost but not quite 0 or π/2 radians from the vector representing the local gravitational field, best thought of as an arrow pointing at the centre of the earth.
This raises another question that’s long troubled me (unlike the first one, I suppose it has an easy answer): Where do straight lines come from?
More concretely: How do they make rulers? And every other artifact such as those that fill my living room whose edges are straight enough to satisfy the demanding human eye? I know how to stretch a string, but asked to construct, starting with raw materials, any object whose edge needed to be straight, I wouldn’t know where to start.