For centuries, the space away from the Earth’s surface — “outer” space — has confounded attempts to make sense of it with terrestrial geometric schemes. Human occupation of and movement through space on our home planet has been dominated by the horizon and the apparent flatness of the ground plane. Meanwhile, the open sky has been conceived alternately as an unattainable place of infinite freedom or as a hard dome that limits the world, like a cake lid over a Flat Earth. For Nikolai Fyodorov and the Cosmists, in pre-revolutionary Russia, horizontality was the ultimate barrier. They believed humankind had to become vertical — had to resurrect the dead who lay flat in their graves, even — in order to escape the Earth’s surface and achieve immortality. The sky starts at your feet. Think how brave you are to walk around.

Fyodorov’s most famous follower was Konstantin Tsiolkovsky, whose astronautic theory shaped the Russian (and later the Soviet) space program. A drawing in his manuscript Free Space might be the first depiction of humans in orbital weightlessness. Four figures float in a spherical spaceship, each pointed in a different direction, disoriented. Tsiolkovsky’s ship seems better equipped than its passengers to operate in a fully three-dimensional environment. It has engines at both ends of a primary spine and gyroscopes on the other two axes, so that it can spin round and fire rocket thrust in any direction. This basic design — primary thruster, secondary retro rockets, axial gyros for orientation — has been used by all crewed Russian and American spacecraft to date, including the International Space Station.

Buckminster Fuller had an unusual way of talking about stairs. Instead of downstairs and upstairs , he encouraged people to say in stairs and out stairs . “They all laugh about it,” he wrote, “But if they try saying in and out for a few days in fun, they find themselves beginning to realize that they are indeed going inward and outward in respect to the center of Earth, which is our Spaceship Earth. And for the first time they begin to feel real reality.”  at the dawn of extra-planetary space travel, Fuller identified a break in humans’ spatial perception. Standing on Earth, we see the ground plane as flat, but we know the planet is a sphere. To describe motion and existence in a vast universe, where planetary surfaces are the exception, we would need a new language.

The International Space Station, which orbits about 250 miles up (or out), is designed to mitigate these tidal complexities. Gyroscopes continuously modify the station’s orientation, or “attitude,” to keep its mostly flat grid of modules parallel to Earth’s mostly flat surface, so that scientific instruments and observation windows look down (or in). Other gyros keep the station pointed forward. And as the station is slowed by the slight drag of the upper atmosphere, an engine periodically fires to keep it from falling in toward Earth.

Space is a place, but to be anywhere in that place is to be in motion. Astronauts on the ISS say “on orbit” instead of “in orbit,” and they use the nautical terms forward , aft , port , and starboard . Fuller’s “in” toward Earth is deck and “out” is overhead. These standard directions are labeled at every module junction, and they determine the uniform orientation of wall-mounted equipment. The ISS has ten main capsules, built in four countries, and their alignment on the same spatial axis helps avoid confusion, mitigate motion sickness, and promote community among the international crew. This architectural scheme also reaffirms a connection to Spaceship Earth. Since the bodies of the crew are in a familiar relationship with the ground plane far below, we can imagine that they are in a very tall building with all the intermediate floors removed.

In the counter-intuitive mechanics of orbital space, objects are continually falling on a trajectory that misses the spherical ground of the planetary body below. A spacecraft that accelerates forward moves to a “higher” orbit — up, or as Fuller would have it, “out.” Firing retro rockets to decelerate, it moves “in.” But these dynamics only apply to a dimensionless point. With a large, massy, complicated object like a spacecraft, we have to deal with gravity gradients and spin motion. Areas of the ship that are farthest from the planetary center are subject to less gravitational tug, and they move faster than the center of the ship’s mass, so objects there drift outward with respect to Earth. On the side of the ship closest to Earth, objects drift inward.