â¢ Single Land Owner. The Scott AFB (St. Clair) Site is currently owned by a single land owner (St. Clair County) and is available to the NGA immediately at no cost. All other sites are owned by private entities, have associated significant costs, and some have no interest in selling their property to the NGA.
The parsec (symbol: pc) is a unit of length used to measure large distances to objects outside the Solar System. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. A parsec is equal to about 3.26 light-years (31 trillion kilometres or 19 trillion miles) in length. The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun. Most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun.
For a negligible extinction, one can compute the absolute magnitude of an object given its apparent magnitude and luminosity distance : where is the star's actual distance in parsecs (1 parsec is 206,265 astronomical units, approximately 3.2616 light-years).he absolute magnitude is similar to visual magnitude (correctly called apparent magnitude) where a difference of 5 in absolute magnitude corresponds to a factor of 100 in brightness, whereas for apparent magnitude a difference of one in magnitude corresponds to a factor of two in apparent brightness.