Position angle a hypothetical binary star with a PA of 135 degrees, that means an imaginary line in the eyepiece drawn from the north celestial pole to the primary (P) would be offset from the secondary (S) such that the NCP-P-S angle would be 135 degrees. When graphing visual binary orbits, the NCP line is traditionally drawn downward—that is, with north at bottom—and PA is measured counterclockwise. Also the direction of the proper motion can for example be given by its position angle. The definition of position angle is also applied to extended objects like galaxies, where it refers to the angle
Borel regular measure Borel regular measure In mathematics, an outer measure "μ" on "n"-dimensional Euclidean space R is called a Borel regular measure if the following two conditions hold: An outer measure satisfying only the first of these two requirements is called a "Borel measure", while an outer measure satisfying only the second requirement (with the Borel set B replaced by a measurable set B) is called a "regular measure". The Lebesgue outer measure on R is an example of a Borel regular measure. It can be proved that a Borel regular measure, although introduced here as an "outer" measure (only countably "sub"additive),