Object;0; 
Objective;37; 
Observatories;85; 
Observatory;169; 
Occultation;272; 
Oort cloud;338; 
Opposition;426; 
Optical telescope;554; 
Orbit period;696; 
Orbit;584; 
Ozone layer;888; 
Ozone;786; 
Parallax;999; 
Parsec;1108; 
Particle;1179; 
Penumbra;1222; 
Perihelion;1304; 
Periodic;1364; 
Phase;1448; 
Phases of the Moon;1526; 
Photon;1620; 
Photosphere;1670; 
Photosynthesis;1727; 
Planet;1845; 
Planetesimals;1938; 
Planetoids;2056; 
Plasma;2169; 
Plate tectonics;2202; 
Polar axis;2253; 
Polar light;2328; 
Polestar;2465; 
Precession;2521; 
Pressure;2588; 
Primordial nebula;2634; 
Prism;2737; 
Probe;2822; 
Prominences;2911; 
Proton;2980; 
Protostar;3028; 
Ptolemy;3129; 
Pulsar;3216;