is opposite to the ZENITH.The nadir (from Arabic نظير nathir, "opposite") is the direction pointing directly below a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there. Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous terms. Specifically, in astronomy, geophysics and related sciences (e.g., meteorology), the nadir at a given point is the local vertical direction pointing in the direction of the force of gravity at that location. The direction opposite of the nadir is the zenith.
Nadir also refers to the downward-facing viewing geometry of an orbiting satellite, such as is employed during remote sensing of the atmosphere, as well as when an astronaut faces the Earth while performing an EVA.
The term nadir can also be used to represent the lowest point reached by a celestial body during its apparent orbit around a given point of observation. This can be used to describe the location of the Sun, but it is only technically accurate for one latitude at a time and only possible at the low latitudes. The sun is said to be at the nadir at a location when it is at the zenith at the location's antipode.
Taken from Wikipedia
the centre of the horizon circle. Zenith is opposite to the NADIR in the CELESTIAL SPHERE.In general terms, the zenith is the direction pointing directly "above" a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there. The concept of "above" is more specifically defined in astronomy, geophysics and related sciences (e.g., meteorology) as the vertical direction opposite to the net gravitational force at a given location. The opposite direction, i.e. the direction of the gravitational force is called the nadir. The term zenith also refers to the highest point reached by a celestial body during its apparent orbit around a given point of observation. This sense of the word is often used to describe the location of the Sun, but it is only technically accurate for one latitude at a time and only possible at the low latitudes.
Strictly speaking, the zenith is only approximately contained in the local meridian plane because the latter is defined in terms of the rotational characteristics of the celestial body, not in terms of its gravitational field. The two coincide only for a perfectly rotationally symmetric body. On Earth, the axis of rotation is not fixed with respect to the planet (for example due to constant displacements of its fluid components) so that the local vertical direction, as defined by the gravity field, is itself changing direction in time (for instance due to lunar and solar tides).
