Core shadow;Central dark area of the shadow cone during an eclipse.
Corona;Outermost, extremely thin and hot layer of the atmosphere of the Sun.
Cosmology;Study of the origin, development and general structure and composition of the universe.
Cosmos;General term describing the world as an entity, synonym to universe and space.
COSTAR;Corrective Optics Space Telescope Axial Replacement: correcting device for the incorrectly cut main mirror of the HST.
Crab nebula;Gaseous nebula located in the constellation of Taurus (object M1 in the Messier catalogue): example of an object consisting of remains of a supernova.
Crust;Solid surface layer of a planet or other solid body.
Culmination;Highest position on the celestial sphere that stars can reach relative to the observer.
Cycle;Sequence of events that occur at regular intervals.
Dark matter;Any type of matter that is not visible, whether directly or indirectly, is referred to as dark matter.
Dark nebula;Cold cloud made of dust particles and gas, not emitting or reflecting any light.
Declination;Angular distance of a celestial object from the celestial equator.
Deformation;Deviation of an (rotating) object from spherical shape (flattening).
Density;Measure for the mass within a given volume.
Diffraction;Refraction of light, for example at the edges of concave mirrors.
Distance module;Difference between apparent magnitude (m) and absolute magnitude (M) of a star.
Doppler effect;Apparent variation in frequency of sound and electromagnetic waves resulting from the relative movement of the source and the observers to each other.