Object;Body or illusion in the sky.
Objective;Main lens of a refracting telescope.
Observatories;Astronomical, meteorological or geophysical station for observation.
Observatory;Observation station for the recording and evaluation of phenomena and processes in space.
Occultation;Passage of a celestial body in front of another one.
Oort cloud;Hypothetical sphere containing billions of comets outside the solar system.
Opposition;Relative position of a planet, a planet is in opposition when it is on the opposite side of the Sun from the Earth.
Optical telescope;Refractor.
Orbit;Path of a celestial body around another one, determined by the gravitational forces between the objects.
Orbit period;The time it takes a celestial body to complete a full orbit around another.
Ozone;Special molecular form of oxygen, where three rather than the usual two atoms form a molecule.
Ozone layer;Layer in the atmosphere of the Earth that absorbs the harmful ultraviolet radiation from the Sun.
Parallax;Angle of the apparent displacement of a celestial object when viewed from two different positions.
Parsec;Distance of a star with a parallax of one arc second per year.
Particle;Extremely small piece of matter.
Penumbra;Semi-shadow found around the cone of the core shadow during an eclipse.
Perihelion;Point nearest the Sun in the orbit of a planet.
Periodic;A sequence of events that occur at regular intervals is a periodic cycle.
Phase;Changing appearance of a non-luminescent body in the celestial sphere.
Phases of the Moon;Changing shape of that part of the Moon, which is visible from the Earth.
Photon;Small unit of electromagnetic radiation. 
Photosphere;Glowing areas in the atmosphere of the Sun.
Photosynthesis;Conversion of the energy of light into chemical energy and subsequent storage in the form of glucose.
Planet;Large rotating body, which is held in an orbit by the gravitational force of a star.
Planetesimals;Celestial bodies in the solar system with a size of between a few millimeters and that of an asteroid.
Planetoids;Small celestial bodies in the solar system located in a belt between the orbits of Mars and Jupiter.
Plasma;Ionized state of matter.
Plate tectonics;Large-scale geological movements.
Polar axis;Axis linking the two poles of the rotating globe of the Earth.
Polar light;Luminous phenomenon in the upper layers of the atmosphere, also known as aurora borealis and aurora australis respectively.
Polestar;Main star in the constellation of Ursa Major.
Precession;Gradual change in the inclination of the Earth's axis.
Pressure;Measure for the force on a surface.
Primordial nebula;Nebula of matter that originated from the early history of the universe (Big Bang).
Prism;Wedge-shaped piece of glass refracting light into the colors of the spectrum.
Probe;Measuring and testing instrument that is able to reach areas inaccessible to man.
Prominences;Large, bright eruptions in the chromosphere of the Sun.
Proton;Positively charged elementary particle.
Protostar;State of development of a star shortly before nuclear fusion actions begin to take place.
Ptolemy;Ptolemy, c. 90-150, astronomer, mathematician and geographer from Alexandria.
Pulsar;Rapidly spinning neutron star emitting a beam or radiation.