Pituuspiiri

Pituuspiiri is the Finnish term for a meridian or line of longitude, an imaginary north–south line on the Earth's surface that connects the North and South Poles. Each pituuspiiri is part of the geographic coordinate system used to specify a location on Earth together with latitude (leveysaste). Longitude is measured in angular degrees east or west of a chosen prime meridian.

The internationally accepted prime meridian passes through Greenwich, London, and is defined as 0° longitude. Longitudes extend 180° eastward and 180° westward from this line; the 180th meridian roughly corresponds to the International Date Line. Values of longitude are commonly given in degrees, minutes and seconds or in decimal degrees.

Pituuspiirit are not parallel circles like latitude lines, but great or nearly great circles that converge at the poles; the distance between meridians decreases toward the poles. They are essential in navigation, mapping and global positioning systems (GPS), enabling precise east–west positioning. Historically, determining longitude was a central problem in navigation until accurate marine chronometers and later radio and satellite methods provided reliable solutions.

In Finnish usage, pituuspiiri is used in technical, educational and everyday contexts to refer to longitude, often contrasted with leveyspiiri (latitude). The concept underlies time zone definitions, cartography and geodetic reference systems.