In the sky, a meridian is an imaginary great circle on the celestial sphere. It passes through the north point on the horizon, through the celestial pole, up to the zenith, through the south point on the horizon, and through the nadir, and is perpendicular to the local horizon.
Because it is fixed to the local horizon, stars will appear to drift past the local meridian as the earth spins. You can use an object's right ascension and the local sidereal time to determine when it will cross your local meridian, or culminate.
The upper meridian is the half above the horizon, the lower meridian the half below it.
ALTITUDE, also referred to as elevation, refers to the vertical angle measured from the geometric horizon (0°) towards the zenith (+90°). It can also take negative values for objects below the horizon, down to the nadir (-90°). Although some will use the term height instead of altitude, this is not recommended as height is usually understood to be a linear distance unit, to be expressed in meters (or any other length unit), and not an angular distance.
The term zenith distance is more often used in astronomy and is the complement of the altitude. That is: 0° in the zenith, 90° on the horizon, up to 180° at the nadir.
