In measuring positions of stars with reference to the horizon astronomers use the following circles: Any great circle of the celestial sphere whose plane passes through the zenith and nadir is called a _vertical circle_. The verticle circle SPNZ', passing through the poles and meeting the horizon in the north and south points, N and S, is called the _meridian circle_, because the sun is on this circle at true mid-day. The _meridian_ is the plane in which this circle lies. The vertical circle, EZ'WZ, whose plane is at right angles to the meridian, is called the _prime vertical_ and it intersects the horizon at the east and west points, E and W. These circles, and the measurements of positions of heavenly bodies which involve their use, were all employed in Chaucer's time and are referred to in his writings.[195] The distance of a star from the horizon, measured on a vertical circle, toward the zenith is called the star's _altitude_. A star reaches its greatest altitude when on the part of the meridional circle between the south point of the horizon, S, and the north pole, P. A star seen between the north pole and the north point on the horizon, that is, on the arc PN, must obviously be a _circum-polar star_ and would have its highest altitude when between the pole and the zenith, or on the arc PZ. When a star reaches the meridian in its course across the celestial sphere it is said to _culminate_ or reach its _culmination_. The highest altitude of any star would therefore be represented by the arc of the meridional circle between the star and the south point of the horizon. This is called the star's _meridian altitude_. The _azimuth_ of a star is its angular distance from the south point, measured westward on the horizon, to a vertical circle passing through the star. The _amplitude_ of a star is its distance from the prime vertical, measured on the horizon, north or south.