The transit instrument is also used in the prime vertical for the determination of latitudes.
The prime vertical is that vertical circle which passes through the east and west points.
The amplitude of a star is its distance from the prime vertical, measured on the horizon, north or south.
The amplitude of a body is its distance, on the horizon, from the prime vertical to a vertical circle passing through the body.
Take the morning sight for longitude when the sun is on or as near as possible to the prime vertical.
The imaginary circle which passes immediately over your head as you face the South is called the prime vertical.