Zenith On the International Space Station, "zenith" and "nadir" are used instead of "up" and "down", referring to directions within and around the station, relative to the earth. Zenith The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e. the direction in which gravity pulls, is toward the nadir. The zenith is the "highest" point on the celestial sphere. The word "zenith" derives from an inaccurate reading of the Arabic expression (), meaning "direction of the
Overhead line are constant, fixed termination (FT) equipment may be used, with the wires terminated directly on structures at each end of the overhead line. The tension is generally about . This type of equipment sags on hot days and is taut on cold days. With AT the continuous length of overhead line is limited due to the change in the position of the weights with temperature as the overhead line expands and contracts. This movement is proportional to the tension length, the distance between anchors. Tension length has a maximum. For most 25 kV OHL equipment in the UK, the maximum