definition 1: the lowest or most depressing point or stage.
example: His disastrous fall at the world championships five years ago brought him to the nadir of his career.

definition 2: the point on the celestial sphere┬╣ directly below a specified position on the earth’s surface and at the other end of the diameter drawn from the zenith.

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┬╣celestial sphere
the infinitely large, imaginary sphere around the earth in which the stars, planets, and other celestial objects seem to be located.