Celestial Equator: A great circle on the celestial sphere that is a projection of the Earth’s own equator onto the celestial sphere.
Celestial Poles: the two points at which the line of the Earth’s axis, extended outwards, meets the celestial sphere around which the stars appear to revolve.
Celestial Sphere: An imaginary sphere that surrounds the Earth, the inside surface of which the stars are seemingly attached.
Cepheid Variable: A variable star that decreases and increases in brightness in a regular periodic way.
Chromosphere: A thin layer of the Sun’s atmosphere that lies between the photosphere and the corona.
Circumpolar: A term used to describe a celestial body that remains above the horizon at all times when viewed from a particular place on the Earth’s surface.
Coma: A cloud of gas and dust that surrounds the nucleus of a comet.
Comet: A small body composed mainly of dust-laden ice that revolves around the Sun, usually in a highly elongated orbit.
Conjunction: A close alignment in the sky of two celestial bodies, which occurs when both lie in the same direction as viewed from Earth.
Constellation: One of 88 regions of the night sky containing groupings of stars joined together by imaginary lines to represent a figure.
Corona: The outermost region of the atmosphere of the Sun or a star.
Cosmology: The study of the nature, structure, origin and evolution of the Universe.