|MadSci Network: Earth Sciences|
There are a few different definitions for the word "level". When we say that "the floor is level" or "the bubble in the builder's level is correct", what we usually mean is that the surface is flat and objects set down on it will not roll around. In this sense, we determine if something is level by referencing the earth's gravitational pull. When a spirit level (or builder's level) is tilted relative to the gravitational pull, the air bubble is free to float upwards. When the level is exactly perpendicular to the gravitational pull, the air bubble doesn't move. This marks a level, or flat, surface.
Things are slightly more complicated by the fact that we live on a round planet. Using the above definition of what it means to be level (where objects don't roll around), level surfaces aren't really perfectly flat. Instead, they curve with the earth's surface as the direction of the gravitational pull changes. Luckily for builders, since the earth is so large the amount of curvature over any practical building length is negligible. For example, let's assume the earth is a perfect sphere with a radius of about 6400km. For every meter we go out perpendicular to the surface, we need to curve about 0.00007mm to stay "level". Negligible!
I hope this has helped.
James R Holliday
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