A Place to Stand

Originally proposed in the 1970's by Princeton Professor Gerald K O'Neill as a result of a class design project for living off Earth this is a cylinder up to 6 km wide which provides artificial gravity by spinning.

O'Neill has created three reference designs:

Island One
A sphere measuring one mile in circumference (1,681 feet or 512.27 meters in diameter) which rotated, and people lived on the equatorial region. See Bernal Sphere.
A later NASA/Ames study at Stanford University developed an alternate version of Island One: the Stanford torus geometry, a toroidal shape 1,600 meters (just under a mile) in diameter.

Island Two
Also a sphere, also 1,600 meters in diameter.

Island Three
Two counter-rotating cylinders each two miles (3 km) in radius, and capable of scaling up to twenty miles (30 km) long. [1] Each cylinder has six equal-area stripes that run the length of the cylinder; three are windows, three are "land." Furthermore, an outer agriculture ring, as seen in the picture on the right, 10 miles (15 km) in radius, rotates at a different speed for farming. The manufacturing block is located at the middle (behind the satellite dish assembly) to allow for minimized gravity for some manufacturing processes.

To build this requires material mined from the Moon, both metals to construct it & rock (the slag from the mined metals) as shielding at the sunward end against solar radiation. Once we have an extensive lunar settlement it would be possible to build a mass driver which, either by catapult or linear accelerator, could throw material from the Moon's surface at orbital speed - fortunately since lunar gravity is 1/6th of ours this only requires 1/36th of the speed we need. Obviously to build all this is not possible until we have such an extensive lunar settlement & even once we have such infrastructure built the cost is going to be in the many 10s if not hundreds of billions of $s. On the other hand looking at Iraq & Wall Street that is cheap at the price.

A 3km radius, 20 km long cylinder would have a surface area of 375 km, which is pretty small by planetary size of 500,000,000 km but remember that 75% of that is sea & of the rest barely a 10th isn't to cold, too hot, too hilly, too desert, too jungle or too covered in tsetse flies to be habitable & it gets to look better. Ass the fact that the weather is always going to be what we want & that no rule suggests we bring along midges or alligators & it gets really attractive. Then add the fact that there is no reason to spin it at anything close to the one gravity we are used to (indeed you always have zero G at the centre) at it gets to look like a very nice place to retire to. At Singapore's population density of 6.489 people per km we could have 2.4 million living there which would produce expensive but not outrageous real estate.