An important organizing theme in architectural theory is the notion of principal directions, which imbue space with an inherent structure. The identification of these directions is powerfully influenced by gravity, which has - till now - been taken as a universal constant in architectural design.
Six directions on three axes are innately perceptible: up-down (height), left-right (breadth), and front-back (depth). In terrestrial architecture, the up-down axis is normally tied to the force of gravity - the plumb line; the other axes are free to rotate around it. The up-down axis is called "vertical", while all possible left-right and front-back axes are called "horizontal". The anisotropic character of this space is judged by the effort required to move in any given direction: up and down are distinct irreversible poles; left, right, front, and back are interchangeable simply by turning around. Thus, gravitationally, there are three principal directions - up, down, and horizontal - and three basic architectural elements - ceiling (or roof), floor, and wall. The walls, which bound the horizontal dimensions, are not inherently distinct. North, south, east, and west walls are all "walls"; none of them is a "floor" or a "ceiling", nor are "floor" and "ceiling" interchangeable.
Distinctions between walls arise from higher-level analytical considerations that are not operative at the deepest levels of meaning. This is reflected in language: there is no simple linguistic term for the concept "east wall" or "west wall". The cardinal directions acquired their distinct characters through observations of celestial phenomena, reflected in the etymology of "east", "west", "south", and "north" - respectively: "dawn", "evening", "sun", and "lower" . Those distinctions are not inherent in architecture. A building may isolate its occupants from all celestial cues to cardinal orientation, but it cannot isolate them from gravity.
Studies indicate that familiarity with gravity is not innate, but is learned in infancy. At 4 months, infants begin to realize that a rolling ball cannot pass through an obstacle, but are not yet aware that an unsupported ball will fall. At 5 months, they discriminate between upward and downward motion. At 7 months, they show sensitivity to gravity and the "appropriate" acceleration of a ball rolling upward or downward. By adulthood, falling objects are judged to move naturally only if they accelerate downward on a parabolic path. These judgments are based not on mathematical reasoning, but on visual experience; when asked to reason abstractly about such motion, many adults are prone to error [29, 30, 31].
Misalignment with the vertical axis is psychologically disturbing in a way that horizontal misalignment is not. The perception of a picture hung awry on a wall produces an urge to straighten it. And, according to Thiis-Evensen, visitors at the Leaning Tower of Pisa "very seldom pause immediately beneath the leaning side. They feel safe only when at a certain distance and preferably on the opposite side of the tilt" . Again, this is reflected in the language: there is no horizontal equivalent of "lean" or "tilt".
These common-sense ideas, rooted in the experience of terrestrial gravity, permeate architectural theory. Thiis-Evensen builds his entire grammar around the three elements of floor, wall, and roof. Architectural design for a gravitational environment distinctly different from Earth's requires a fundamental reexamination of design principles which until now have been taken for granted.
In a normal gravity environment, with the head up and the feet down, there are four principal body orientations. In micro gravity, without a strong up-down reference, there are twenty-four - four rotations for each of six orientations of the body axis. It is no wonder that such an environment can be disorienting. Again, it is misalignment or realignment of the perceived vertical axis (as compared to the horizontal axes) that is most disorienting. Skylab astronaut Ed Gibson reported that "being upside down in the wardroom made it look like a different room than what we were used to" . The wardroom furnishings provided a reference for that volume that was lacking in the larger workshop volume. In the wardroom, as a matter of etiquette when taking one's place at the dinner table, one did not float over the table, but instead squeezed past one's crew mates in an Earth-like fashion. Nevertheless, when looking out the window, the internal reference was abandoned in favor of an Earth-down reference, even if this meant floating sideways or upside down relative to the furnishings .
As was mentioned above, the large workshop volume in Skylab did not provide any particular vertical reference. According to Oberg and Oberg, "the Skylab astronauts would sometimes become a little disoriented in it and preferred the lower [sic] decks with their smaller but more familiar spaces" . Even though micro-gravitational space may be amorphous and isotropic, the human body is not. It's hard to collaborate with your crew mate when you're looking at his ankles and his task light is shining in your eyes. Modern space station modules, such as Spacelab and Freedom, are designed with a strong standard vertical reference - arbitrary perhaps, but beneficial.
On Earth, the normal stimulus to the perception of vertical is the plumb line. But even here there are exceptions. Hesselgren describes the case of a large concert hall in Gothenburg. By design, the walls are not plumb, but lean slightly. Apparently, there is a tendency in the occupants to misread this leaning as vertical perspective. Consequently, when light fixtures were installed to hang freely, they were perceived to be leaning the other way. This disturbing illusion was overcome by arranging the fixtures to hang parallel to the walls . Once again, this shows that perception is not deducible from stimulus.