longitude

The imaginary (but very important) lines of longitude run from the North Pole to the South Pole. Each is identified by the number of degrees it lies east or west of the so-called prime meridian in Greenwich, England (part of London). A circle is divided into 360°; so, for example, the longitude of the Egyptian city of Cairo is about 31°E—that is, about 31° east of London. The "long" sense of the root may be easier to see in some uses of the adjective longitudinal: A longitudinal study is a research study that follows its subjects over many long years, and a longitudinal engine is one that drives a crankshaft that runs lengthwise under a vehicle (as in rear-wheel-drive cars) rather than crosswise.