(meteorology) A luminous halo, each side of the sun parallel to the horizon, caused by refraction of sunlight through ice crystals.

A supposed error in reasoning attributed to René Descartes. A form of circular argument.

plural of Cartesian circle

An area in a field of crops where the stalks have been flattened into a circle or other geometric shape.

(geography) The line which marks approximately the northernmost place in the Southern Hemisphere where the sun does not set on the summer solstice and does not rise on the winter solstice.

(US, Canada) an intersection with a circular shape and, usually, a central island

Instrument for measuring azimuths. It is a graduated circle on a sight, gun carriage, searchlight, etc.

(mathematics) The problem, proposed by ancient Greek geometers, of using a finite ruler-and-compass construction to make a square with the same area as a given circle.

plural of crop circle

(projection) a circle whose plane is oblique to the axis of the primitive plane