Soderqvist1: This may be truth for all practical means, but it is imperfect when it comes to the absolute!I for one am a wood worker and a carpenter. Believe me in my trades there are straight lines, and BTW, flat surfaces. Measured by thousandths of inches.

Soderqvist1: this is also something approximate as your terms “practical sense” “practical terms” indicate!Peter, I for one do understand that a straight line is only as straight so long as it begins to take on the curvature of the earth! But for practical sense, it will take miles before the straight line will begin to curve along the curvature of the earth... so this is way out there in practical terms.

So your straight line doesn’t go on forever, since it bends after some miles. Is it possible that the curvature is always there, but builds up incrementally, and imperceptible until you can see it after some miles or so? That can account for the phenomenon if we are sitting in the same boat on a calm sea and the sea’s surface seems flat, but in the horizon a ship appear, but we can only see the top of it for the moment, but more and more of the ship will be visible as it approaches us. This was one of the evidence in pre-scientific time that the earth is spherical, not flat. The evidence that the earth surface is nowhere flat comes from non-Euclidean Geometry and note the word approximation in the quote, as I have repeatedly used.

Wikipedia Non-Euclidean geometry

Euclidean geometry is modeled by our notion of a "flat plane.

On a sphere, the sum of the angles of a triangle is not equal to 180°. The surface of a sphere is not a Euclidean space, but locally the laws of the Euclidean geometry are good approximations. In a small triangle on the face of the earth, the sum of the angles is very nearly 180°.

http://en.wikipedia.org/wiki/Non-Euclidean_geometry