Zodiac

a zone within the CELESTIAL SPHERE which contains the paths of the planet, the Moon and the Sun. It is divided into the signs of the zodiac, named after the constellations.

Taken from Dictionary of Science

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In astronomy, the zodiac is the ring of constellations that lines the ecliptic, which is the apparent path of the Sun across the sky over the course of the year. The Moon and planets also lie within the ecliptic, and so are also within the constellations of the zodiac. In astrology, the zodiac denotes those signs that divide the ecliptic into twelve equal zones of celestial longitude. As such, the zodiac is a celestial coordinate system, to be more precise, an ecliptic coordinate system, taking the ecliptic as the origin of latitude, and the position of the sun at vernal equinox as the origin of longitude.

It is known to have been in use by the Roman era, based on concepts inherited by Hellenistic astronomy from Babylonian astronomy of the Chaldean period (mid-1st millennium BC), which, in turn, derived from an earlier system of lists of stars along the ecliptic. The construction of the zodiac is described in Ptolemy's Almagest (2nd century AD). It was originally described in Rigveda. The term zodiac may also refer to the region of the celestial sphere encompassing the paths of the Moon and the planets corresponding to the band of about eight arc degrees above and below the ecliptic. The zodiac of a given planet is the band that contains the path of that particular body; e.g., the "zodiac of the Moon" is the band of five degrees above and below the ecliptic. By extension, the "zodiac of the comets" may refer to the band encompassing most short-period comets.

The term zodiac derives from Latin zōdiacus, in turn from the Greek ζωδιακός κύκλος (zōdiakos kuklos), meaning circle of animals, derived from ζώδιον (zōdion), the diminutive of ζῶον (zōon) animal. The name is motivated by the fact that half of the signs of the classical Greek zodiac are represented as animals as well as two mythological hybrids.

Although the zodiac remains the basis of the ecliptic coordinate system in use in astronomy besides the equatorial one, the term and the names of the twelve signs are today mostly associated with horoscopic astrology.

Taken from Wikipedia

Nadir

the lowest point -- the pole which is vertically below the observer in the CELESTIAL SPHERE. It is opposite to the ZENITH.

Taken from Dictionary of Science

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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 word is also used figuratively to mean the lowest point of a person's spirits.

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