This section describes the celestial meridian and zenith.
The relative coordinate system on the celestial sphere is defined relatively to an observer on the Earth:
The Zenith is defined as the line running the observer straight up to the sky reaching the celestial sphere.
The Horizon is defined as the plan touching the observer and perpendicular to the Zenith line. The half celestial sphere above the
Horizon the visible sky of the observer.
The Celestial Meridian is defined as the plan that running from the North Pole to the South Pole passing through the Zenith. The
Celestial Meridian divides the visible sky into two part: east part and west part.
The following picture provides a good illustration of the coordinate system on the celestial sphere:
The word "Meridian" is an ancient word meaning midday.
When telling time, we use the letters a.m. to indicate morning for ante meridiem, which is Latin for "before midday." We use p.m. for
post meridiem, or "after midday."