Greenwich Avenue is a thoroughfare in the northern city of Coriopolis running the length of the habitat along zero degrees longitude. It is the habitat’s prime meridian, and the datum from which all circumferential position in Coriopolis is measured.

Function as a datum#

A cylindrical habitat has a natural origin for axial position — the midpoint between the end-caps — but none at all for circumferential position, since every line around the cylinder is equivalent. A datum must therefore be chosen by convention, and in Coriopolis it was fixed to the avenue laid out along the meridian during the city’s original survey.

Circumferential position in Coriopolis is stated in degrees of longitude east or west of Greenwich Avenue, running from 0° at the avenue to 180° at the antimeridian on the opposite side of the cylinder. At the habitat’s ten-kilometer radius one degree of longitude is about 175 meters at the floor, and proportionally less on the sub-levels.

The naming is a deliberate borrowing from the terrestrial prime meridian, and carries the same implication: the datum is arbitrary, it is fixed by agreement rather than by nature, and once fixed it is not moved.

Relation to metric statements of position#

Circumferential distances are also quoted in kilometers for engineering and transit purposes, measured along the floor in the same sense as the longitude. Because a kilometer of arc corresponds to a fixed angle only at a fixed radius, metric circumferential positions are ambiguous between levels in a way that degrees are not, and the degree form is preferred wherever a position must be unambiguous. See coordinate system of Coriopolis.