O

Oberon - moon of Uranus
object - source of diverging light energy
objective - the main (primary) light gathering mirror or lens of a telescope
occultation - when a planet moves in front of one of its moons, or a star, as seen from Earth
ohm - unit of electrical resistance
Olbers' paradox - why does the sky not look bright all over at night due to all the stars
Olympus Mons - huge, extinct volcano on Mars
omega minus - an elementary particle (W^-)
Oort cloud - a cloud containing millions of comets with a diameter of about 1 light year
op amp - operational amplifier – amplifier in microchip form
opaque - material that dose not allow light energy to pass through it
optics - the branch of Physics that deals with light
optical activity - the rotation of the plane of polarisation when light passes through some materials
optical bench - a linear piece of apparatus used to mount optical components
optical binary - two stars that look as if they are close together when viewed from Earth
OR - a form of logic gate with a high output when either input A or input B is high
orbit - curved path of a planet around a star, or a satellite round a planet
Orion - prominent winter constellation in the Northern hemisphere
Orionids - meteor shower, peak 22^nd October
orion nebula - large gaseous nebula in the Orion's dagger (M42)
orthoscopic - type of telescope eyepiece with a fairly wide, distortion free field
oscillation - vibration. Regular movement backwards and forwards about a point
oscilloscope - instrument for showing wave forms
overtone - a component of a musical note with a frequency above the fundamental
ozone hole - hole in the atmospheric ozone layer allowing more ultra violet radiation to reach the ground