(astronomy) The great circle through an object whose location is to be determined and the two celestial poles.

(of a black hole) Synonym of photon sphere

celestial equator

(algebra) The group of all complex numbers of unit modulus, with the group operation being complex multiplication.

(birdwatching) A designated radius in which bird populations are measured as part of a census.

plural of count circle

Synonym of fairy ring (“ring of fungi”)

plural of elf circle

(geometry) A circle that cuts three other given circles at right angles.

plural of orthotomic circle