▁method ▁is ▁a ▁method ▁for ▁determ ining ▁the ▁distance ▁of ▁a ▁C ep heid ▁variable ▁star ▁suggested ▁by ▁Walter ▁Ba ade ▁in ▁ 1 9 2 6 ▁and ▁further ▁developed ▁by ▁Ad ria an ▁W essel ink ▁in ▁ 1 9 4 6 . ▁In ▁the ▁original ▁method ▁the ▁color ▁of ▁the ▁star ▁at ▁various ▁points ▁during ▁its ▁period ▁of ▁variation ▁is ▁used ▁to ▁determine ▁its ▁surface ▁bright ness . ▁Then , ▁knowing ▁the ▁apparent ▁magnitude ▁at ▁these ▁points ▁in ▁time ▁the ▁angular ▁diameter ▁can ▁be ▁calculated . ▁Me asure ments ▁are ▁also ▁taken ▁of ▁the ▁radial ▁velocity ▁using ▁Do pp ler ▁spect ro sc opy . ▁This ▁allows ▁one ▁to ▁determine ▁the ▁speed ▁at ▁which ▁the ▁front ▁surface ▁of ▁the ▁star ▁moves ▁toward ▁or ▁away ▁from ▁us ▁at ▁various ▁points ▁in ▁the ▁cycle . ▁Since ▁the ▁difference ▁between ▁this ▁and ▁the ▁average ▁speed ▁is ▁the ▁derivative ▁of ▁the ▁radius , ▁one ▁obtain s ▁the ▁variation ▁in ▁radius . ▁Com bin ing ▁this ▁with ▁the ▁change ▁in ▁angular ▁diameter ▁gives ▁the ▁distance . ▁It ▁is ▁now ▁possible ▁to ▁measure ▁the ▁angular ▁diameter ▁of ▁the ▁p uls ating ▁star ▁directly ▁using ▁optical ▁inter fer om eters , ▁allowing ▁a ▁more ▁accurate ▁measurement ▁of ▁the ▁star ' s ▁distance . ▁This ▁newer ▁technique ▁is ▁known ▁as ▁the ▁geometric ▁Ba ade – W essel ink ▁method . ▁ ▁A ▁closely ▁related ▁technique ▁is ▁the ▁expand ing ▁photos phere ▁method , ▁which ▁can ▁be ▁used ▁to ▁determine ▁the ▁distance ▁to ▁Type ▁II ▁super nov ae . ▁