Museo Galileo
Virtual Museum
Meridians / parallels
Meridians / parallels

Imaginary circles drawn on the celestial sphere and on the terrestrial globe to form an ideal orthogonal reference grid for determining the latitude and longitude coordinates that give the position of celestial bodies and localities on Earth. The meridians are great circles intersecting both poles; they serve to measure longitude relative to a zero meridian—at present, the meridian of Greenwich Observatory, England. Parallels are circles running parallel to the equator and measuring latitude. Meridians and parallels are coordinated in a gnomonic projection.