Z

zenith: (astronomical) the point on the celestial sphere vertically above the observer. In everyday parlance,
~ usually implies the highest point. This gives rise to confusion as the mid-day Sun is often described as
its ~, irrespective of the latitude.

zenith distance

[z] the complement of the altitude i.e. (90º
– a)

zodiac

An imaginary band, centred on the ecliptic, across the celestial sphere and about 16º wide, in which the Sun, Moon and the planets Mercury, Venus, Mars, Jupiter and Saturn are always located. The band is divided in 12 intervals of 30º
, each named (the Signs of the Zodiac) after the constellation of stars which it contains. The sun’s ecliptic longitude may be measured against this scale. The names (and/or signs) of the constellations are given in Appendix I and are often used in sundials, instead of the date, to specify declination lines etc. Because of the effects of precession over the period of 2,300 years since the constellations were first named, the signs of the zodiac have slipped by a whole sign, i.e. at the vernal equinox (defined as the first point of Aries), the Sun is actually in the constellation of Pisces.

Zonwvlak

A major suite of computer programs for calculating sundial lines. Written by Fer de Vries
(Netherlands) it is available at www/iaehv.nl/users/ferdv. The name is short for zonnewijzer (sundial) vlak (plane).