z The symbol normally used for ➤ redshift.
zenith The point directly overhead.
zenithal hourly rate (ZHR) The hypothetical rate at which meteors belonging to a particular ➤ meteor shower would be observed by an experienced observer, watching a clear sky with limiting magnitude 6.5, if the radiant were located in the zenith. In practice, observed rates are always lower, because fewer meteors are detected when the radiant is lower and skies are rarely so ideally clear.
zodiac A belt of 12 constellations through which the Sun's path in the sky – the ➤ ecliptic – passes. They are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces. Formerly, the ecliptic went through only these 12 constellations, but the effects of ➤ precession and the precise definitions of constellation boundaries mean that it now also goes through a thirteenth, Ophiuchus. Since the orbits of the planets lie very nearly in a plane, their apparent paths remain in or close to the zodiacal constellations.
In traditional astrology, the zodiac is divided into 12 equal 30° portions, each of which is allocated to a “sign,” but these do not correspond exactly to the astronomical constellations, which are of varying sizes. The effect of precession has also contributed to increasing disparity between the astrological signs and the astronomical constellations.
zodiacal light A faint cone of light in the sky extending along the ecliptic. It is visible on clear moonless nights in the west following sunset, and in the east just before sunrise.