This circle divides the sky into an eastern and a western half. When an object is rising, it is to the east of the meridian, and when it is setting, it is to the west of the meridian. The meridian above the horizon is called the upper meridian and that below the horizon, the lower meridian . When an object crosses the upper meridian and has its greatest altitude, it is said to be at upper transit . At lower transit an object has its smallest altitude and usually lies below the horizon on the lower meridian.
Objects are at upper and lower transit once each day.
A local apparent day is the time interval between two lower transits of the real Sun. Likewise, a local mean day is the time interval between two lower transits of the Mean Sun. Both types of days begin when the respective suns are below the horizon at lower transit. On the other hand, the local sidereal day is the time between two upper transits of the vernal equinox, which means a sidereal day starts when the vernal equinox is above the horizon. At any instant, local time depends on the chosen object's position relative to the observer's astronomical meridian.