Zenith 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 head" or "path above the head", by Medieval Latin scribes in the Middle Ages (during the 14th century), possibly through Old Spanish. It was reduced to 'samt'
Nadir also used figuratively to mean the lowest point of a person's spirits, or the quality of an activity or profession. In medicine, the term nadir is used to represent the lowest level of something being measured; for example, of a blood cell count while a patient is undergoing chemotherapy, or of a blood glucose level after a baby is born. A diagnosis of neutropenic nadir after chemotherapy typically lasts 7–10 days, and a newborn's blood sugar normally reaches its nadir around 2 hours after birth. The word also refers to the downward-facing viewing geometry of an orbiting satellite, such as