lower culmination

The time from one upper culmination to the next is approximately 24 hours, and between an upper to a lower culmination is approximately 12 hours.

A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon (upper culmination) or midnight (lower culmination).

Generally, the sun is visible at its upper culmination (at noon) and not visible at its lower culmination (at midnight).

The most common convention starts the civil day at midnight: this is near the time of the lower culmination of the Sun on the central meridian of the time zone.

To determine absolute declinations or polar distances, it was necessary to determine the observatory's colatitude, or distance of the celestial pole from the zenith, by observing the upper and lower culmination of a number of circumpolar stars.

On this latitude, e.g. the south coast of South Africa (Cape Town to Port Elizabeth) when in lower culmination it is barely visible to the naked eye as it is only 1 degree above the horizon, but still circumpolar.

During a sidereal day, an astronomical object crosses the meridian twice: once at its upper culmination, when it is (approximately) at its highest point as seen from the earth, and once at its lower culmination, its (approximately) lowest point.