Caesium Clock

A very accurate clock used to define the second. The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.


The SI unit of luminous intensity. The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Cassegrainian Telescope

A type of reflecting telescope where the image is viewed through a hole in the main mirror. The light is reflected first by a convex mirror and then by a hyperbolic primary mirror. Invented by French physicist N. Cassegrain in 1672.

Celestial Sphere

Imaginary sphere of the heavens, with the earth as its center. Ancient civilizations believed that the stars were attached to the surface of a huge sphere centered on the Earth and rotated on a daily basis. This is a useful concept to understand planetary motion.

Centre of Gravity

The point on an object where the force of gravity is seen to act.

Centripetal Force

A force that causes a body to move in a circular path. The word “centripetal” means center-seeking, for object’s moving in circular motion, there is a net force acting towards the center which causes the object to seek the center.


The layer of gas above the visible surface (called the photosphere) of the sun. The temperature of this layer is between 6,000°C to 20,000°C.