added _EQUINOX_B1950 constant at @astrofrog's suggestion
added _EQUINOX_B1950 constant at @astrofrog's suggestion

StuartLittlefair/astropy,joergdietrich/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,AustereCuriosity/astropy,larrybradley/astropy,tbabej/astropy,AustereCuriosity/astropy,DougBurke/astropy,MSeifert04/astropy,astropy/astropy,lpsinger/astropy,StuartLittlefair/astropy,bsipocz/astropy,kelle/astropy,tbabej/astropy,mhvk/astropy,saimn/astropy,pllim/astropy,kelle/astropy,DougBurke/astropy,MSeifert04/astropy,bsipocz/astropy,funbaker/astropy,saimn/astropy,stargaser/astropy,kelle/astropy,pllim/astropy,lpsinger/astropy,larrybradley/astropy,bsipocz/astropy,larrybradley/astropy,larrybradley/astropy,DougBurke/astropy,saimn/astropy,MSeifert04/astropy,saimn/astropy,mhvk/astropy,joergdietrich/astropy,larrybradley/astropy,stargaser/astropy,joergdietrich/astropy,stargaser/astropy,pllim/astropy,dhomeier/astropy,dhomeier/astropy,saimn/astropy,stargaser/astropy,lpsinger/astropy,tbabej/astropy,astropy/astropy,DougBurke/astropy,pllim/astropy,bsipocz/astropy,MSeifert04/astropy,mhvk/astropy,aleksandr-bakanov/astropy,tbabej/astropy,aleksandr-bakanov/astropy,pllim/astropy,AustereCuriosity/astropy,lpsinger/astropy,joergdietrich/astropy,astropy/astropy,dhomeier/astropy,funbaker/astropy,tbabej/astropy,mhvk/astropy,joergdietrich/astropy,aleksandr-bakanov/astropy,astropy/astropy,dhomeier/astropy,funbaker/astropy,astropy/astropy,funbaker/astropy,mhvk/astropy,StuartLittlefair/astropy,kelle/astropy,AustereCuriosity/astropy,kelle/astropy,dhomeier/astropy,StuartLittlefair/astropy,lpsinger/astropy,AustereCuriosity/astropy