A telescope mounted to point vertically in order to make positional measurements on stars passing through and near the zenith.

The hypothetical rate at which meteors of 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. The lower the altitude of the radiant, the lower the observed rate. To a first approximation the ratio of the observed rate to the zenithal rate is the sine of the altitude angle of the radiant.

For a population of stars, such as a cluster, formed at the same time, the main sequence on the Hertzsprung-Russell diagram as it would have been at the start of their hydrogen-burning phase.

See also: stellar evolution.

Abbreviation for zenithal hourly rate.

A band around the celestial sphere that is imaginary, which lies approximately 9° to each side of the elliptic the perceived annual path of the Sun, in which the Moon and all the planets apart from Pluto sometimes are found. The imaginary band is used in astrology where 12 different names are given that correspond to 12 constellations found in the band.

zodiacal light.

The popular name for a 1940 catalogue of 3,539 of the brighter stars within 8° of the ecliptic. It was compiled by James Robertson and published as Volume X, Part II, of Astronomical Papers prepared for the use of the American Ephemeris and Nautical Almanac. It includes all stars brighter than magnitude 7.0 and 313 fainter than magnitude 8.5. The catalogue is used particularly for star positions in the prediction and analysis of occultations.

A tenuous flat cloud of small silicate dust particles in the inner solar system. It is believed to be derived from comets and from collisions in the asteroid belt.

A faint cone of light extending along the ecliptic, visible in the sky on clear moonless nights in the west following sunset, and in the east just before sunrise. It is caused by sunlight scattered from micrometre-sized dust particles in the zodiacal dust cloud in the plane of the solar system. The zodiacal light is dimly present all round the ecliptic, a phenomenon sometimes called the zodiacal band, and an enhancement also occurs at the position directly opposite the Sun. This is known as the gegenschein, or counterglow.

A thirteenth century, Spanish compilation of Jewish mystic writings. The Zohar anticipates Copernicus’ heliocentric view of the Solar System, which says that the Earth revolves around the Sun. The Zohar states, the whole Earth spins in a circle like a ball; the one part is up when the other part is down; the one part is light when the other is dark; it is day in the one part and night in the other.

One of a series of Soviet space probes launched between 1963 and 1970. Zonds 1 and 2 flew by Venus and Mars, respectively, but returned no data. Zond 3 photographed the farside of the Moon in 1965. Zond 4 was a failed mission. Zonds 5, 6, 7 and 8 made flights around the Moon and were recovered on Earth.

A region of sky near the plane of the Milky Way where absorption by interstellar dust is so great that no galaxies can be seen.

Zwicky is the first person to coin the word supernova, the Swiss astrophysicist used the new term in 1934 to describe a class of stellar that were different in their size and make-up from the novae that we known of at the time. He made it apparent that there was a connection between supernovae and neutron stars, over 30 years before the first neutron stars were discovered. Zwicky was studying the Coma cluster of galaxies in 1933 when he realized that the individual galaxies were travelling at such great speeds that they should have escaped a long time ago; the fact that they had not brought him to the conclusion that there must be more matter in the universe than we can see, this unknown matter was sometimes called the ‘missing mass – it is now know as dark matter.

A pulsating, variable white dwarf star. The amplitude of variation of ZZ Ceti stars is between 0.05 and 0.3 magnitude, and their periods range between 30 seconds and half an hour.