In
geometry,
physics,
astronomy,
geography, and related sciences, a
plane is said to be
horizontal at a given point if it is perpendicular to the
gradient of the
gravity field at that point— in other words, if apparent gravity makes a plumb bob hang
perpendicular to the plane at that point. Alternatively, a
spirit level, which exploits the buoyancy of a bubble, can be used to determine if the plane is horizontal.