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Source edition 1965. Please read the Introduction to find out about this dictionary and our plans for it. Caution, many entries have not been updated since the 1965 edition.
Greek symbols may not appear correctly in some browsers. For example a gamma may appear as γ.

That point of the celestial sphere vertically overhead.
The point 180 degrees from the zenith is called the nadir.
Of or pertaining to the zenith.
zenith attraction
The effect of the earth's gravity on a meteoric body, which increases the velocity and moves the radiant toward the zenith.
zenith distance
Angular distance from the zenith; the arc of a vertical circle between the zenith and a point on the celestial sphere, measured from the zenith through 90 degrees, for bodies above the horizon.
This is the same as colatitude with reference to the celestial horizon.
Referring to positions on Jupiter measured in latitude from Jupiter's equator and in longitude from a reference meridian.
zero gravity = weightlessness.
zero-g = weightlessness.
zero launch
The launch of a rocket or aircraft by a zero-length launcher.
zero-length launcher
A launcher that holds a vehicle in position and releases the rocket simultaneously at two points so that the buildup of thrust, normally rocket thrust, is sufficient to take the missile or vehicle directly into the air without need of a take-off run and without imposing a pitch rate release.
The term is not normally applied to a pad used for a vertical launch.
zero-length rocket
A rocket with sufficient thrust to launch a vehicle directly into the air.
Said especially of a rocket used to launch an aerodynamic vehicle.
zero-lift chord
A chord taken through the trailing edge of an airfoil in the direction of the relative wind when the airfoil is at a zero-lift angle of attack.
zeta machine
An experimental thermonuclear device which generates a plasma inside a torus and employs the pinch effect for heating and compression.
zip fuel
A boron-base high-energy liquid propellant.
The band of the sky extending 8 degrees either side of the ecliptic.
The Sun, Moon, and navigational planets are always within this band, with the occasional exception of Venus. The zodiac is divided into 12 equal parts, called signs, each part being named for the principal constellation originally within it.
zodiacal counterglow = gegenschein.
zodiacal light
A faint cone of light extending upward from the horizon in the direction of the ecliptic (zodiac). It is seen from tropical latitudes for a few hours after sunset or before sunrise.
The spectrum of the zodiacal light is similar to that of sunlight, so it has been suggested that it is due to the scattering of sunlight by extraterrestrial dust particles. Some scattering is also due to electrons.
zone time
See time.
z-time = Greenwich mean time.
Zurich number = relative sunspot number
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