found: NASA.
found: Random House.
found: McGraw-Hill dict. sci. tech.(UF--Apparent horizon)
found: Van Nostrand sci. encyc.(UF--Sensible horizon; real horizon)
found: Encyc. astron.(UF--Celestial horizon)
found: McGraw-Hill encyc. astron.(Visible horizon is the apparent boundary line between sky and earth or sea. Astronomical horizon is the great circle of the celestial sphere 90⁰ from the zenith and the nadir)
found: Facts file dict. astron.
found: Dict. astron., space atmos. phen.
notfound: INIS:Thes.;INSPEC
